Book - Outline of Comparative Embryology 2-5

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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.

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Chapter V Parthenogenesis

I. NORMAL PARTHENOGENESIS

A. GENERAL DISCUSSION

As an embryological problem the phenomenon of parthenogcnesis has been of interest since its discovery in 1762 when Bonnet found that the summer generations of aphids reproduce by means of unfertilized eggs. The literature dealing with the subject is a vast one, for parthenogencsis is of wide occurrence in the animal kingdom and is also known among the plants. In spite of the vast amount of research on the problem, however, it has only recently become possible to bring the known cases into any kind of systematic order. The problem also has a great deal 01 cytological and genetical interest for we are here dealing with uniparcntal inheritance and it is obvious that the chromosomal mechanism which explains the inheritance of bisexual forms must undergo some modification to account for the appearances in unisexual organisms. While the cytological and genetieal aspects of parthenogenesis are closely related to the embryological, we can treat them only briefly and must devote ourselves chiefly to the embryological problems.

Confusion sometimes develops in the mind of the student in relation to parthenogencsis for at first thought one is apt to class all types of uniparental inheritance together and think of parthenogcnesis as a modified asexual type. The fallacy of this conclusion is easily seen, however, for parthenogenesis deals with the development of an egg although without the cooperation of a spermatozoon. It is clearly a gametic form of reproduction; its relationships are perhaps more easily understood if we speak consistently of unisexual and bisexual reproduction. It would perhaps be still clearer to use the terms monogametic and digametie, although these are not commonly found in the literature. The critical point in the evidence that we are here dealing with a second type of sexual reproduction is seen in the formation of polar bodies and the behavior of the chromosome during the maturation division. In some cases of parthenogenesis polar bodies are often seen to be formed and in other cases the behavior of the chromosomes in reduction is well known. In addition to this evidence, the sporadic occurrence of parthenogenesis in diverse classes of the animal kingdom, the occasional occurrence of parthenogenesis along with the bisexual method in the same species of animal, as for example the bee, and the not uncommon alternation of the parthenogenetic mode with typical sexual reproduction in the course of several generations within the same animal all bear out the same conclusion. Furthermore, the fact of artificial parthenogenesis, that is, the activation of an egg which would otherwise develop only by fertilization, shows that we are dealing with what must be a modification of the usual type of bisexual reproduction. Artificial parthenogenesis means the initiation of development on the part of the egg by chemical or physical stimulation artificially induced to take the place of the type of initiation usually to be seen only when the sperm enters the egg. It is really a study in experimental embryology which has been of the greatest value in giving an insight into the true nature of the process of fertilization.

a. Terminology. Throughout the animal kingdom parthenogenesis occurs in a wide variety of types and under many different conditions. Owing to this fact the literature concerning it contains many descriptive terms that are of special application and about which some confusion of meaning has grown up, as has been the case in certain other processes that are familiar but not clearly understood by the majority of students. In some species parthenogenesis is of rare occurrence and the eggs simply possess the power of developing without the intervention of the sperm, although they usually develop after fertilization. Such cases are known as facultative or optional and are of such irregular occurrence in the series as to be of no great embryological significance. Obligatory parthenogenesis occurs regularly in the life cycle of organisms which show it as a constant feature or only at certain intervals. Its importance is manifested by the fact that parthenogenetic eggs are often structurally different from those that are to be fertilized and there are morphological differences between the animals which are produced parthenogenetically and those which are produced bisexually. For instance, the eggs of daphnids which are fertilized usually develop only during the colder seasons of the year; they are known as winter eggs and have a protective shell that is very different from the summer eggs. Oftentimes cases of obligatory parthenogenesis show an alternation of generations although not of an asexual generation with a sexual, but of a unisexual generation or a series of them, with a bisexual generation. Where this type of development is firmly established a series of par thenogenetic generations may be developed to such an extent that it becomes the predominant method of reproduction and sexual generations appear only at infrequent intervals. Many of the more familiar cases of parthenogenesis are of this type.

Even in the extreme obligatory type it is often possible by experimental means to induce the sexual process, for instance, by a change of the environmental conditions. The rotifer, Hydatina senta, as shown by Shull and by Whitney, in old culture infusions will continue parthenogenetically for long periods of time, but if a fresh culture fluid is used bisexual animals soon make their appearance. Conversely, it is possible by maintaining appropriate conditions to postpone or even entirely to suppress the sexual process, as in the case of the rose aphid when cultivated through a series of years in a greenhouse.

In cases where structural differences are to be observed between the parthenogenetic and the bisexually developed eggs, a functional difference also is frequently seen in that the eggs of certain individuals are exclusively male producing, that is arrhenotokous, and of others female producing, or thelytokous. Where both males and females are produced in one brood, parthenogenesis is said to be amphoterotokous. Where the number of parthenogenetic generations which occurs before the sexual animals are produced is indefinite we say that the species has an “open life cycle.” Such cases occur in the aphids and in some species of phylloxerans. In others the female which produces the parthenogenetic eggs and hence is known as the stem mother must develop directly from the previous fertilized egg. This is the “ closed ” type, and it occurs in certain other phylloxerans and in the gall flies.

Considering the embryological aspects we should classify known cases according to the grade or degree of parthenogenesis shown. It is possible to distinguish seven or eight different grades of parthenogenesis as follows: (a) Pathological. In the ova of birds it is frequently observed that a few cleavages take place resembling the normal but not leading to the formation of the blastula. Similar phenomena have been observed in some other eggs as well. (b) Casual parthenogenesis. In the case of the silk moths under exceptional conditions aspermic development takes place. (0) Occasional parthenogenesis. In the ants, bees, and wasps, the development of the egg without fertilization always produces a male. (d) Partial parthenogenesis. The queen bee by the copulation on the nuptial flight receives all of the sperm which she will ever use, and afterwards she can fertilize the egg to produce a female, that is, a worker or a queen, or leave it to develop parthenogenetically into a male, since she has control over the process. (e) Seasonal parthenogenesis. In the case of Daphnia already mentioned and other Entomostraca and also the aphids the summer eggs are parthenogenetic and the winter fertilized. (f) Larval parthenogenesis. The case of Miastor is described in the chapter on paedogenesis. Some larvae are produced parthenogenetically and inside of them others through a number of generations. each eating its way out and producing yet others. finally the last larvae pupate, undergo metamorphosis, and develop bisexual forms. (g) Total

parthenogenesis. Only parthenogenetic eggs occur through many generations as is especially illustrated by the rotifers.

b. Cytological Relationships. A consideration of the cases of parthenogenesis upon the basis of their cytological conditions leads to the common classification of haploid and diploid parthenogenesis. By haploid parthenogenesis is meant the development without fertilization of an egg which has gone through the process of chromosome reduction. Haploid eggs in general are sexual, and in most cases in which aspermic development is possible the eggs are found to be capable of development either with or without fertilization. Hymenoptera, some of the Hemiptera, and the arachnids often exhibit haploid parthenogenesis. One case, the nematode Rhabdites (R. aberrans according to Kruger and R. pellio according to P. Hertwig), is worthy of note, however. Here the eggs develop parthcnogcnetically and are of the haploid type but the penetration of the sperm is necessary to initiate the development although it takes no further part and no true fertilization is accomplished. After it has penetrated the cortex of the egg, and thus set going the developmental processes, the sperm degenerates and the further development of the egg is as much parthenogenetic as if no sperm had been involved.

Diploid parthenogenesis is much more frequent, occurring in most of the other groups where parthenogenesis is found at all. Copepods, ostraeods, and the Hemiptera-Homoptera have this type characteristically whereas trematodes, nematodes, echinoderms, the phyllopods, the Orthoptera, and the Lepidoptera exhibit it in certain forms. An intermediate condition is found in the rotifers, both diploid and haploid parthenogenesis having been described for these forms. Triploid parthenogenesis is also known but is not common. The best understood case is that of the plant genus Hieracium which has been described by Rosenberg. There are two sections of this genus which differ in their cytological behavior. The sexual form of H. umbulatum has a diploid chromosome number of 18 whereas the variety linearifolium has a diploid number of 27. There are also some in which the diploid number is 36. The parthenogenetic Archieracium, which is closely related to this species, shows a chromosome number of 27. Rosenberg explains these facts upon the basis of hybridization. The triploid races, Rosenberg maintains, cannot easily beproduced by the usual bisexual processes. Triploid parthenogenesis, however, is of rare occurrence, and the triploidy in animals which result from fertilization of a diploid egg, if that can occur, certainly produces only an abnormal zygote.

Usually the number of polar bodies which is given off by the animal egg permits, the observer to distinguish whether he is dealing with the haploid or diploid mode of development; in the haploid case two polar bodies are produced in the haploid manner, whereas in the diploid a single maturation division produces only one polar body and no reduction of chromosomes can take place. There is one special case of parthenogenesis to which was early ascribed theoretical significance in working out a theory of fertilization. O. Hertwig in 1890 described a process in the maturation of the starfish Astropecten which has been many times referred to since, namely, the reunion of the nucleus of the second polar body with that of the egg. A similar condition was reported by Lefevre for the egg of Thalessema in which the second polar body is not extruded, the spindle remaining “submerged” in the cytoplasm of the egg. Upon the basis of these observations parthenogenesis was for a long time supposed to be explained as a kind of fertilization by the polar body. It is now known, however, that such cases seldom occur. A somewhat analogous case is that of the wasp H abrobracon in which a cytological study shows parthenogcnetieally produced males to be haploid on account of the fact that the first spermatocyte division is abortive while the second is equational (A. Whiting, 1927). Of course cases in which only one maturation division occurs are not rare.

c. Parthenogenesis and Sex. Much of the work upon parthenogenesis has been due to the bearing of the problem upon sex determination. Many experiments have been made for the purpose of discovering what factors can influence the production of males or females within the parthenogenetic strains. Especially have the rotifers been used for experiments of this kind. Punnett observed various kinds of females as follows: arrhenotokous females or those which produce males; thelytokous females (female producers) of three kinds, namely, those which produce, respectively, a large percentage, a small percentage, and those which produce no arrhenotokous females. In the attempt to influence the sex of the progeny of these forms conflicting results have been obtained. The studies of rotifers have been most extensively carried on by Whitney and by Shull. In general Shull’s experiments on the modification of sex have yielded positive results, for he has found a relationship between metabolism and sex determination. When the quantity of food given H ydatina senta is small the number of male producers is increased; old food reduces the number. Creatin, ammonium salts, beef extract, and low temperature prevent the formation of male producers. Longcontinued parthenogenesis results in a decrease in male production.

Oxygen in the water increases it, although osmotic pressure, alkalinity, and acidity were not found to be effective. Whether the egg is to develop into a male or a female producer is decided during the growth period but not in the early oogonial stages. Sex is determined a generation in advance.

Whitney, on the other hand, has found less evidence that male production is a response to external conditions, although he did find that crowding in H. senta permitted the development of fewer male producers in direct proportion to the degree of the crowding. He found, however, that neither oxygen, temperature, nor starvation is a factor. One of his strains when fed continuously upon a diet of Polytonia produced nearly all females for twenty-five generations, only three per cent of the daughters being male producing. But a change of diet to Dunaliella increased the percentage to 57. When they were fed again upon Polytonia, the percentage fell off. The influence of the food was found to be upon the grandmother.

In addition to the work of Shull and of Whitney, Noyes has reported a series of experiments on Proales decipiens lasting through 250 generations in which no males appeared although the animals were subjected to various environmental changes including changes of chemical composition, of temperature, and of feed. More found Brachionus to give increased arrhenotekous females upon treatment with chemicals such as ferric chloride (FeCl), whereas constant diet and temperature gave only thelytokous individuals.

Banta has studied the sex production in the parthenogenetic Cladocera, especially in M oina macrocopa. His studies with Brown upon sex control showed that experimental crowding brought about male production along with lowered metabolism in the females. The amount of food was not found to affect the results but a change in its character did have a profound effect. Male production was associated with an accumulation of excretory substances. Aeration with oxygen decreased the production of males even in crowded cultures. Similarly carbon dioxide, urea, and ammonium salts as well as other products related to excretion reduced the number of males.

d. Intersexes. Another problem in relation to sex has appeared in some of the parthenogenetie strains of Cladocera. It is the condition of intersexes or, as originally described by Banta, sex intergrades. The intersex condition manifests itself through the secondary sex characteristics in that a typical female has a series of eight secondary sex characters which are identified with her sex, and similarly for a typical male. The eight characters are as follows: (1) body size, the females are the larger at maturity; (2) size and position of the eye, the eyes of the females being smaller and crowding the margin of the head less than those of the males; (3) outline of the head, the angle of the anteroventral margins of the head being more acute in the male; (4) absence in the female of the nuchal protuberance; (5) and (6) character of first antennae, the males having swollen basal portions and two lateral stylets; and (7) and (8) the outline and armature of the lateral postabdominal margins, these being more concave and not serrated in the female. The intersex individuals may have the sex organs of either sex accompanied by from one to several of the secondary sex characters of the other sex. There are also hermaphroditic forms with various combinations of the male and female secondary characters. It is Banta’s opinion that these intersex conditions in Cladocera and other parthenogenetically reproducing forms are probably determined by environmental factors.

A relation of hermaphroditism to parthenogenesis appears in certain cases and it has been suggested by Winkler that it is one of transition, hermaphroditism leading over into parthenogenesis. At least the case of Rhabdites aberrans as described by Kruger points in that direction. These animals are free-living nematodes found in moist earth and are almost exclusively females, two counts showing four males out of 10,000 females and thirty-two males out of 2026 individuals, respectively. But the females are hermaphroditic and the entrance of sperm into the egg is necessary for the initiation of development, although it takes no further part in the process and the cleavage of the egg is as truly parthenogenetie as if no sperm had penetrated into it. In addition to this relationship, hermaphroditism may be superimposed upon parthenogenesis at least in one family of the Phyllopoda where it appears that Lepidurus which under favorable conditions reproduces parthenogcnetically “may become hermaphroditic when food is scarce ” (Bcrnard,1896).

e. Geographical Races. Another matter of interest is the occurrence of geographical races which are parthenogcnetic. The phyllopod, Artemia salina, according to Artom, reproduces differently in different localities. Races which are exclusively parthenogcnetic occur in Marseilles, at Capodistria and in certain other Italian localities. Bisexual races occur in Cagliari in Italy and in Utah; and a third class in which both bisexual and parthenogcnetic modes occur is found at Odessa in Russia. Vandel described the conditions in the isopod, Tr2‘chom'scus provisorius, in which there are two distinct races, one bisexual in the usual sense, and the other a female-producing parthenogcnetic race. These occupy distinct regions, the latter occurring in northern Europe, but at some points the two may co-exist. These two races are looked upon by Vandel as incipient species. Geographical parthenogenesis has also been observed CAUSES OF PARTHENOGENESIS 329

in the moth Solenobia by Seiler. Solenobza triquetrella and S. znfleti occur in both bisexual and parthenogenetic forms, the latter being permanent and thelytokous. Of S. triquetrella the parthenogenetic form is widely distributed in Germany, Austria, and Switzerland, but the bisexual form has been found in a single locality only. In S. pineti the bisexual form is common but the parthenogenetic is restricted to a very few localities. finally Trialeurodes vaporariorum, a white-fly has two races, according to Schrader, that are unlike in their parthenogenesis, in that the English race produces females with only three per cent males and the American parthenogenetic females produce males only.

f. Causes of Parthenogenesis. Of the causes of natural parthenogenesis little is known. It would seem that the developmental mechanism of the egg is capable of activation in more than one manner. Usually the entrance of a sperm into the egg sets off the processes which are involved. This initiatory action is capable of imitation by both chemical and physical means as in the cases of artificial parthenogenesis. There are also conditions in which, it would appear, development may go forward automatically, the participation of the sperm being entirely dispensed with. In the case of the normal fertilization of any egg there is no reason for regarding the developmental processes of the egg as brought to a. standstill before the sperm enters. Rather they are slowed down to a speed from which they will not recover unless external influences are operative, usually in the form of the sperm. Perhaps monogametie development is possible only where the inhibition is less intense, thus giving automatically normal parthenogenesis. On this hypothesis the failure to induce the development of sperm, by placing them in suitable media as has been reported by Locb and others, may mean nothing more than that the developmental processes in the sperm are reduced to a still lower speed from which no means thus far tried have been effective to return them to a functional level.

Other suggestions have also been ‘made as to the factors which are responsible for parthenogenesis. Issakowitsch held that in Cladoeera the alternation of bisexual and parthenogenetic stages was governed by changes in the nucleo-cytoplasmic ratio. Papanicolau reached similar conclusions. Shull, however, investigated the nucleo-cytoplasmic ratios in the rotifer, H ydatina senta, without finding any correlation between it and the mode of reproduction. Although it has been claimed that food, oxygen content, and other similar external factors influence the sex ratios of parthenogenetically produced animals, there is no good evidence that these factors are responsible. Banta thought the formation of an ephippium (winter egg, which develops upon fertilization) to be due either to unintentional change in the media, in which parthenogenetic daphnids had been developing, or to overcrowding.

Another suggestion (which is so far merely record of an observation rather than a cause) relates to the lagging chromosome of the phylloxerans which is observed to accompany parthenogenesis. Peacock and Harrison working upon lepidopteran crosses reached the conclusion that parthenogenesis is a result of hybridity and later supported this conclusion by a study of the data of other crosses including Nabours’ experiments on parthenogencsis in Apotettix. finally Nabours, working upon Parateltirc, studied genetically two strains one of which was highly parthenogenetic and the other had never so reproduced. He obtained genetic evidence that parthcnogenesis is a character which segregates in a Mendelian manner and believes it due to certain genes. Robertson’s cytological studies on Nabours’ material fall into line with this hypothesis. The entrance of the sperm is necessary for the second polocyte division, and when it is lacking the diploid chromosome number is retained. In this latter case if the specific genes for parthenogenesis are present development is initiated. It is to be noted that Agar working upon Daphnia and Whiting upon Habrobracon failed to find genetic evidence of the cause of parthenogenesis. But Punnett as long ago as 1906 had suggested the probability that the question is concerned with the zygotic constitution of the egg and thought that it might be a matter of Mendelian segregation. These different hypotheses show the need of much further study of this most elusive subject.

B. THE OCCURRENCE or NORMAL PARTHENOGENESIS

Natural parthenogenesis is of much wider occurrence in the animal kingdom than is generally recognized, and it is not rare among the plants. Examples are found in the following groups of animals: rotifers, nematodes, trematodes, Dinophilus, three orders of Crustacea, myriapods, arachnids, and ten orders of insects. It seems desirable to point out some of the important studies in each of these groups, although no attempt can be made toward inclusiveness. For references and a more nearly complete listing of cases the reader should consult Winkler.

a. Rotzfera. Among the rotifers, as already described, typical arrheno— tokous parthenogenesis occurs in the same groups of animals with thelytokous parthenogenesis. The studies on Hydatina senta are among the best known of all studies on the type of reproduction. Three types of females, as previously indicated, are found here, and by experimental means it is possible to change the strains from parthenogenetic to bisexual.

b. Nematode. For the nematodes the case of Rhabdites aberrans as described by Kriiger has already been mentioned. The trematodes are commonly described as giving rise to rediae from cells which are segregated at one end of the body of the sporocyst and become an ovary. The new rediae arise from these so-called “ova,” which must be thought to develop parthcnogenetically if the view of their origin is maintained that these are true eggs. This controversial subject is discussed in other chapters in this work.

c. Annelida. Among the annelids parthenogenesis is suspected in Dinophilus conklini from the preponderance of females. In the leech Ilemiclepsis marginata the early finding of parthenogenesis has failed of confirmation in more recent experiments (Brumpt, 1900).

d. Crustacea. Two orders of the Entomostraca exhibit parthenogenesis and include in their number some cases that have been given a great amount of study, while a third offers a case which is suspected to be parthenogenetic. Among the Phyllopoda examples may be cited from both the Branchiopoda and the Cladocera. Of the former, Apus, Lepidurus, and Artemia are well known. Reference has already been made to Lepidurus because of its hermaphroditic relationships. Thelytokous parthenogenesis occurs in these forms. Artemia salina likewise has been mentioned in connection with geographic parthenogenesis. On this form the most recent studies are those of Artom (1912). Two polocytes are formed in the development of the egg. Regularly a reduction of chromosomes from 42 to 21 takes place. The parthenogenetic forms are tetraploid and of a type called somatic. There are therefore two strains of Artemia, one diploid and bisexual, the other tetraploid and thelytokous parthcnogenetic. The following explanation has been upon the basis of the very slight evidence that in the parthenogenetic strain only one polocyte is formed (Brauer) and that without reduction. If the egg and polocyte nuclei, both of which retained their somatic number of 42 chromosomes, should fuse, then an individual with the tetraploid number would be produced. As yet this explanation must be looked upon as purely speculative. Numerous others of the true phyllo« pods, such as Branchipus streptocephalus, have been observed to repro duce parthenogenetically and still others are suspected because of the scarcity or absence of males.

Of the work on the Cladoeera, which dates from Weismann’s studies in 1879, the general facts are well known. Bisexual generations are followed by parthenogenetic which may continue indefinitely. The various races differ greatly in the number of generations produced within a year and in the frequency of the bisexual forms. Apparently most if not all forms are parthenogenetic at some stage of their life cycles, but those genera most studied have been Daphnia, Simocephalus, M oina, Bosmina, Chydorus, Polyphemus, and Leptodora, the first four being best known. The more recent students of the group include Olofsson (1918), Woltereck (1909-1911), Langhans (1911), Kuehn (1908), Chambers‘ (1913), Taylor (1914), Thiebaud (1913), and especially Banta and his co-workers, whose work is published in a series of papers beginning in 1914 and is not yet completed. In all forms the males are rarely seen and in some species are unknown. As already stated much of the work concerns the control of sex and the intersex conditions in these forms. There is good evidence that sex ratios and intersex conditions are both subject to environmental influences.

In the order Ostracoda, parthenogenesis was discovered in 1880 by W. Mueller and by Weismann for a number of genera, it having been shown that virgin females produced eggs that developed into females; that is, produced a thelytokous race. No reduction division was found to occur by Woltereck (1898) or by Schleip (1909) for a considerable number of forms which they studied and parthenogenesis for these animals is therefore of the diploid or somatic type. In 1914 Wohlgemuth studied carefully the reproduction of numerous fresh-water ostracods and thought they fall into two groups, one of a purely bisexual and the other of a purely parthenogenetic type, although there are transitional forms between the two. Other investigators of the Ostracoda have been Menzel (1911), Alm (1916), and Olofsson (1918). Their chief concerns have been to describe the facts of parthenogenesis, the relation of the bisexual generation to the parthenogenetic, the duration of each and the conditions under which they occur. Not much of importance to an embryological account is gained from these forms, although SchIeip’s work on the various species of C3/pris offers perhaps the best cytological description of parthenogenesis in any crustacean form. His observation of synapsis not followed by reduction is worthy of note. It may be remarked that some of the studies of ostracods have taken up the question, raised for many Crustacea, as to the possible impairment of vigor of a strain due to long-continued parthenogenesis. Here as elsewhere among the members of the class the general consensus of opinion seems to be that no loss of vigor can be noted.

Mention should be made of the possibility of parthenogenesis in the third order of the Entomostraca, the Rhizocephala. In the genus Sylon belonging to that suborder Smith reported his complete failure to find males and so suspected that parthenogenesis is the method of reproduction.

In one order of Malacostraca parthenogenesis has been reported, namely in the Isopoda. Vandel (1928) has given an account of geographical parthenogenesis, already mentioned in Trichoniscus. Two races occupying different localities were described, one bisexual, the other exclusively parthenogenetic. In some places they may exist side by side, but mating does not occur between the two races. Physiological factors tend to keep the two races distinct and to increase the divergence between them. The parthenogenetic race is found to be triploid in its chromosome relations. The eggs of the parthenogenetic female do not undergo synapsis.

For both of the two Inyriapod groups, the Chilopoda and the Diplopoda, cases of suspected parthenogenesis are recorded. Of the former Geophilus and Lomyctes, and of the latter N opoiulus and Polyxenus, have been studied. All of these include races which either lack males entirely or in which they are to be found in certain localities only. In addition observations were long ago made by Sograff (1882) upon Geophilus proximus of which unmated females laid eggs that began to develop.

Among the spiders accounts of parthenogenesis have been reported for nearly a century and it may still be suspected, but Montgomery in 1903 concluded that it is “very rare among spiders, and it is probable that most species do not show it at all.” Since Montgomery’s finding, it does not appear that any work has been done which successfully establishes parthenogenesis in these animals.

Among the mites there is no doubt that parthenogenesis occurs, for Ewing (1914) described the process in the bisexual Tetranychus telarius and showed that the unfertilized eggs give rise to males exclusively. It is arrhenotokous parthenogenesis. Among the ticks, the Brazilian form, Amblyomma agamum, has been shown by Aragas (1912) to multiply by thelytokous parthenogenesis.

It has been conjectured by Hennecke (1911) that the tardigrade. M acrobiotus macronyx, has an alternation between parthenogenesis and bisexual generations.

0. I nsecta. Parthenogenesis is of very general distribution in the insects, having been described in the ten orders. The following table, compiled in part from Winkler, summarizes the occurrence of the phenomenon. For references to most of these cases the reader is referred to Winkler, but a few of especial significance are mentioned following the table.

Aptera

M achilzls. Heymons, 1-905 ; doubtful but could find no males. Forbicina. Verhoeff, 1912; all females, so no doubt parthenogenesis occurs. 334 PARTHENOGENESIS

Orthoptera Mantidae

Mantis religiosa Przibram, 1909; induced experimental parthenogenesis, Sphodromantis not natural.

Phasmidad

Eurycnema. v. Wuelfing, 1899; three generations of parthenogenetic females in Java. Hanitsch, 1902; two generations of parthenogenetic females.

Bacillus. Dominique, 1896-99; two thelytokous generations of parthenogenesis. Males are rare. Also Stadelmann, v. Bachr, Cameron, Daiber.

Dixippus. Pantel and de Sinety, 1908. Parthenogenesis established. Schmitz, 1906. Four generations of thelytokous partheno genesis.

Hammerschmidt, 1910. five generations of thelytokous parthenogenesis.

Jeziorski, 1918. Four generations of thelytokous parthenogenesis.

Phasma. Thurau, 1899. Only females.

M onandroptera P . Raphidems }Bordoz, 1913. arthenogenetic. Leptinia. de Sinety, 1900. Thelytokous parthenogenesis. Pantel and

Ucles also found parthenogenesis. Phyllium. Bordas, 1898. Suspected parthenogenesis.

Locustidae Saga pedo. Claus-Grobben, 1917. Report parthenogenesis. Gryllidae M yrmecophila acervorum. Schimmer, 1909, suspected parthenogenesis. Tettigidae fig Nabour and colleagues, 1919-1929. Thelytokous parthenogen' . B . Telmatemx esis est understood cases

Coleoptera. Parthenogenesis is of rare occurrence and most of the older accounts are not trustworthy.

Tropiphorus carinatus. Calwer, 1916, reported parthenogenesis. Otiorrhynchus turca. Ssilantjew, 1906 0. ligustici. Wassiliew, 1909 Thelytokous parthenogenesis. 0. cribricollis. Grandi, 1913 Calandra oryzae. Hinds and Turner, 1911, report parthenogenesis common. Eggs produce both sexes.

Strepsiptera Stylops}Brues, 1903 Xenos Nassonov, 1910 Elenchus. Muir, 1906

Parthenogenesis probable due to morphology of female genitalia. IN SECTA 335

Thysanoptera

Most species are bisexual. Many may multiply parthenogenetically. The1ytokous parthenogenesis is established for Parthenothrips, Anaphothrips, Heliothrips, Taeniothrips, and Liothrips; for a number of others it is probable. Arrhenotokous parthenogenesis is experimentally shown for Antlwthrips verbasci.

Corrodentia Ectopsocus. Ribaga, 1904. Probably parthenogenetic.

Hemiptera

Aleurodes. Morrill and Back, 1911. Arrhenotokous parthenogenesis.

Trialeurodes. Schrader, 1920. English race thelytokous. American race ar rhenotokous.

Aphididae. Many species of Aphis are known to be parthenogenetic and types vary. In north they reproduce bisexually during colder seasons but in south may be exclusively parthenogenetic.

Other genera include Callipterus, Ceratophis, M acrosiphum, M yzus, Pemphigus, Rhopalosiphum, Schizoneura, Toxoptera; Chermcs, Pincus,Phyllozera.

Coccidae. Many genera of scale insects are described as parthenogenetic although some cases are not known. Included are Aspidiotus, Ceroplastes, Cryptococcus, Diaspis, Eriopeltis, Lecanium, Lep1.'dosaphes, Orthezia, Parthenolecanium, Pseudococcus, Pulvinaria, Saissetia.

Lepidoptera All proven cases belong to family Psychidae or are closely related to it. Acanthopsyche, Pachythelia, Psyche, Sterrhopteryx, Phalacropteryx, Apterona, Cochlophora, and Lufia are included. The best known is Solerwbia liclwnella. Hoffmann, 1859.

Solenobia triquetrella. Hofimann, 1859-1869. Usually bisexual. Both bisexual

and parthenogenetic modes described. Geographical parthenogenesis. v. Seibold, 1871; Rolph, 1884; Rebel, _1906; Dampf, 1907; Seiler, 1918. Solenobia pineti

Diptera. Parthenogenesis seldom found in this group.

Chirmwmus. Godlewski, 1914. Paedogenesis and parthenogenesis intermingled. Corynoneura. Goetghebeur, 1913. Rarely parthenogenetic. M etriocnemus. Picado, 1913. Males few in number.

Hymenoptera. Six families show parthenogenesis.

Tenthredinoidea The following show thelytokous parthenogenesis: Albia fasciala ' Allantus pallipes

Allantus canadensis Amauronematus puniceus

Amauronematus semilacteus Phyllotoma aceris Caliroa limaama Phyllotoma nemorata Cimbex connata Pontania capreae Croesus varus Pontania viminalis Emprfu abdominalis Pristiphora pallipes Empria pulverata Pristiphora fulvipes H emichroa alml Pteronidea spiraeae Hemichroa crocea Pteronidea tibialis Nematus erichsoni Thrimax mixta

Pachynematus obductus The following are probably thelytokous:

Caliroa aethiops Selandria stramineipes Phyllotoma vagans Strzmglylogaster lineata Producing both males and females are: Ametastegia equiseti Pseudoclavellaria amerinae The following are arrhenotokous:

Albia nitens N ematus luteus

Allantus cinetus Periclista albida Allantus viennensis Phymatocera aterrima Ametastigia glabrata Priophorus padi

Arge, all species Pristiphora conjugate Caliora annulipes Pristophora crassicornis Cladius Pristiphora geniculata Croesus brischkei Pristiphora testacea Croesus litipes Pristiphora alnivom Croesus septentrionalis Pteronidea, 15 species Lophyrus, all species Trichiocampus viminalis Nematus coeruleocarpus Trichiocampus lucorum

Cynipoidea.

The following genera contain species which are parthenogenetic and in most cases the males are unknown, so they are probably thelytokousz

Andricus ' Cynips Rhodites

Aulacidea Drastrophus Dryophania Ceroptres Phanacis N euroterus

Ichneumonoidea Chalcids

The following are arrhenotokous:

Ageniaspis Entedon Paracopidoswnopsis Anaphmldea Litomastix Pentarthron Copidosoma M elittobia Pteromalws Encarsia M icrtmzelus Schedius

Encyrtus M icroterys Tropidopria IN SECTA 337

The following are thelytokous:

Asphclinus Coccophagus Tetrastichus Aspidiotiphagus Odtetrastichus Tripoctenus

The following contain species that produce both sexes or are uncertain as to sex although parthenogenetic:

Eupelmus I sosoma Scutellista H abrocytus Paniscus Trichogramma

Braconids: All parthenogenetic species are described as arrhenotokous.

Ichneumonids: All parthenogenetic species produce mixed broods.

Proctrotrupids: Anagrus, Gonatopus are thelytokous. Balus, Paranagrus, Phanurus produce mixed broods, or first females then later males.

Doubtless many more parasitic hymenoptera are parthcnogenetic.

Formicoidea

Arrhenotokous parthenogenesis is very widespread among the ants. Many workers are fertile and lay unfertilized eggs from which males develop. Some hold the view that both males and females may develop from parthenogenetic eggs. A few cases are recorded in which thelytoky is claimed to be established. Certainly it is much less common than arrhenotoky.

Vespoidea

Parthenogenetic eggs of queens and workers develop into males only. Examples are Polistes gallicus, v. Seibold, 1871 Vespa germanica, Marchal, 1896

Apoidea For many bees arrhenotokous parthenogenesis is conclusively established. Haploid males are so produced, and diploid males probably do not occur, that is, males produced from fertilized eggs, although that possibility has not yet been excluded. In a few genera both males and females are produced parthenogenetically but this is not usual. Queens and workers are females produced bisexually.

As is to be seen from the preceding table, a great deal of work has been done upon insect parthenogenesis. It has, however, been to a large extent concerned either with the general life cycle or with the cytological and genetical aspects of the problem. Many studies of very fine character have been made upon gametogenesis and sex determination, and upon the factors involved in the control of sex. Of recent years the problems which are bound up with uniparental inheritance have attracted much attention. It must be admitted that much of this work has not advanced our general embryological knowledge of the insects to any great extent.

Among the many investigations may be mentioned a few to which the reader is referred if he would pursue further these aspects of the problem. The work of Nabours and his collaborators upon the various forms of grouse locut promises much in the cytological analysis since grasshopper chromosomes are among the most favorable for study. The saw flies have been studied especially by Doncaster and by Peacock and Harrison. Of the aphids and phylloxerans a very careful cytological study was made by Morgan. An earlier study by Tannreuther is a good account of the general embryology of the aphids. Parthenogenesis in the moths is most recently made known to us by the studies of Goldschmidt and of Seiler. Of the Hymenoptera the extensive work done by Patterson on Paracopidosomopsis will serve to introduce the student to the relation of parthenogenesis and polyembryony, and the best detailed account of the cytology of the bee is undoubtedly that of Nachtsheim. Other investigations have already been mentioned in connection with other aspects of the general parthenogenesis problems.

f. Plants. Among plants a considerable number of cases of parthenogenesis are already known and it is thought of interest to mention the conditions found there in this discussion although the problems involved are outside the scope of this work. It is difficult without careful study among the plants to distinguish between cases of parthenogenesis and those of the simple vegetative apogamy. Among higher plants haploid parthenogenesis is exceedingly rare, although several cases have been described for the lower forms. Haploid parthenogenesis is also described as generative parthenogenesis by Winkler and as true parthenogenesis by Strasburger. Diploid parthenogenesis is less common among the lower forms but more so among the vascular plants. It is due to the failure of the maturation process to occur to completion. The forms which are known to be parthenogenetic are as follows:

Haploid parthenogenesis: Spirogyra, Ernst, 1918; Vaucheria, von Wettstein, 1920; Ectocarpus, Kylin, 1918; Fucus, Overton, 1912 (probably haploid); Gastrodia, Kusans, 1915; Oenothera, Haberlandt, 1921, 1922; Datura, Blakeslee and Belling, 1922, evidence from breeding experiments; Nicotiana, Clausen and Mann, 1924.

Diploid parthenogenesis: Chara crinita, Ernst, 1918; Athyrium fel2':cfoemina, var. clarissima, Farmer and Digby, 1907; Scolopendrium vulgare, Farmer and Digby, 1907; Marsilia Drummondii, Strasburger, 1907 ; Allium odomm, Haberlandt, 1923 ; Atamosco texamz, Pace, 1913; Calycanthus, Schiirhofi, 1923; Alchemilla, Murbeck, 1901, Strasburger, 1905, Boos, 1917; Wikstroemia, Winkler, 1906; Eupatorium glandulosum, Holmgren, 1919 ; Erigeron annuus, Tahara, 1921; Antennaria alpina, Juel, 1900; Chondrilla, Rosenberg, 1912; Taraxacum, Juel, 1904, Osswa, 1913, Sears, 1922; Archieracium, Rosenberg, 1917.

II. ARTIfiCIAL PARTHENOGENESIS

Of great embryological significance is the phenomenon of artificial or experimental parthenogenesis. It has been the subject of much of the work on experimental embryology of the present century and its chief consideration should be a matter for a treatise on experimental embryology. However, its importance for an understanding of matters which have to do with the normal embryological process is so great that a brief account should be included here. For details and bibliography the reader is referred to the extensive accounts which have been published by Loeb (see his “Artificial Parthenogenesis and Fertilization”) and by Morgan in his “Experimental Embryology.” Wilson has also critically considered the cytological aspects of the problems concerned with artificial parthenogenesis in “The Cell.”

The student should be reminded of the double function which fertilization serves in normal development; the one is hereditary in character in that fertilization provides the mechanism for conveying the contribution of the male to the offspring. The other is strictly developmental in that it sets in motion those processes which have been inhibited or are latent in the egg at the time of maturation. It is with this second aspect of fertilization that experimental or artificial parthenogenesis deals. These experiments have sought to imitate and to interpret the normal processes of development as initiated by fertilization, and because of that a much clearer understanding of the normal process has been reached. The experiments have shown that it is possible to cause the unfertilized egg of a great many animals to develop into larvae largely under the influence of chemical treatment, although physical means have also been used to accomplish the same end less perfectly.

The work on parthenogenesis is chiefly a monument to the insight of Jacques Loeb and his analytical experiments have been of the most importance in solving the many problems involved. However, numerous other students have contributed to the progress of the work both before and since his announcement of the discovery that it was possible to induce eggs to develop artificially. Among the preliminary studies which antedated Loeb’s discovery in 1899 may be mentioned the paper of Loeb himself in 1892 in which he studied the effect of addition of sodium chloride to the sea water in which the eggs of Arbacia developed; Richard Hertwig’s observation (1896) on the effect of treating unfertilized sea-urchin eggs with strychnine; a series of experiments by Morgan (1896, 1899, 1900) on the effect of salt solutions on the segmentation of eggs and the formation of artificial asters; the work of Mead (1896-98) showing that unfertilized eggs of Chaetopterus will form polar bodies in certain solutions in which the amount of sodium chloride is increased. Loeb’s papers upon this subject are many, and to him is due the chief credit for the developing of our knowledge of the subject. Ampng those who early worked upon the problem was Delage, who in a series of papers from 1900 to 1913 added a great deal to the understanding of this difficult subject; he succeeded in rearing through metamorphosis the larvae of both sea-urchins and starfish which had been induced to develop by artificial parthenogenesis. Loeb and Bancroft (1913) brought a parthenogenetic frog through metamorphosis and found that its sex glands contained eggs. Shearer and Lloyd (1913) succeeded even better than Delage in bringing parthenogenetic larvae of Echinus through metamorphosis, and more recently (1918) Loeb has brought a considerable number of parthenogenetic tadpoles to the adult stage. It is to be noted, however, that only in the case of frogs has it been possible to imitate in the laboratory the conditions of nature sufficiently to bring animals developed by parthenogenesis to sexual maturity. Since Loeb’s work, Bataillon, R. S. and F. R. Lillie, Goldschmidt, Gray, Harvey, Heilbrunn, Herbst, Herlant, and Just are among the many who have made extensive studies into the embryological phases of artificial parthenogenesis.

VVhen one begins to consider the significance of artificial parthenogenesis he is at once led to inquire as to how general the phenomenon is or how generally it may be expected to be possible in the animal kingdom. It may well be that eggs are so constituted as to permit their development without fertilization, at least into the cleavage stages, and that we are merely unable to understand the requirements in each case and to devise the correct procedure. A broad generalization of this character is hardly safe, however, for up to the present time the only forms in which it has been possible to induce experimental parthenogenesis are those in which the experiment is relatively easy to perform. In every case the eggs are of the type which is shed freely into the water. The difficulties of the experimental work which is involved may be the reason for this limitation or it may be that a deeper limitation is operative so that eggs which are adapted for other kinds of development may not be stimulated to cleave artificially. Thus far the following groups have responded to the treatments that have been devised to induce parthenogenesis artificially. By far the most work has been done on the sea-urchins. Arbacia, Strongylocentrotus, T0:copneus'es, and Paracentrotus have all shown themselves excellently adapted to experiments of this type and they have been used for much important work, especially the first two mentioned. The sand dollar, Echinarachnius, has proved adaptable for similar experiments. Among the starfishes the eggs of the commoner species of Asterias as well as some of the more unusual starfishes, as Asterina gibbosa, have been used successfully. It is to be noted, however, that the methods which have proven most successful with the sea—urchin give but very poor results with the starfish, and conversely little success has been obtained by applying the two best methods of inducement for the development of the starfish egg, namely, carbon dioxide and mechanical shaking, to eggs of the sea-urchins. The eggs of the frog, Rana fusca, have often been induced to develop by parthenogenesis, and the most successful treatment has been again a mechanical one, namely the simple pricking of the surface of the egg. Various annelids have been studied, especially Chaetopterus, Amphitrite, Thalessema, but the best results in this group have unquestionably been gotten for the eggs of Nereis. Here again the means of inducing parthenogenesis is physical rather than chemical, namely, the subjecting of the egg to heat.

A second question arises in attempting an embryological interpretation of the experiments in artificial parthenogenesis, namely, do these experiments produce animals that are fairly normal. As already pointed out Loeb’s own studies led him to the belief that “parthenogenetic larvae may be normal and apparently healthy,” and indeed, he said, “if the raising of the larvae was not such a tedious process parthenogenetic animals would exist today in large numbers.” His methods have certainly produced, both in his own hands and in those of other investigators as well, larvae which resemble the normal in every particular, and Delage, Shearer, and also Fuchs were able to bring parthenogenetic larvae to adulthood. The chromosome relations of the individuals as produced, however, have not been made out with certainty as yet and it is still doubtful whether haploid larvae produce normal adults. Parmenter (1925) has shown that in the larval and adult stages Loeb’s parthenogenetic frogs have the diploid chromosome number. The direct evidence is entirely lacking as to whether the adults produced from diploid larvae can themselves produce offspring. It should be said that most investigators are of the opinion that the parthenogenetic larvae which are experimentally produced are not merely normal in appearance but that they really are in every respect what they seem, that is, normal animals.

‘Loeb developed a theory of parthenogenesis as a result of his experiments, and what he spoke of as his improved method is based upon that theory. According to his View the formation of the egg membrane is the deciding criterion by which the initiation of development may be recognized. Indeed he traced a causal relation between the formation of the egg membrane and the subsequent development of the egg and attached much more importance to this process than had previously been done. Membrane formation is the deciding condition of development. Not all later workers have agreed with Loeb in this view, but unquestionably his explanations have not been out of harmony with the facts which he observed.

The so-called improved method of Loeb for inducing artificial parthenogenesis consists of two steps. Its details as to concentration and length of exposure must be worked out for the particular species of sea—urchin eggs used and indeed different individuals will respond differently so far as the duration of the exposures are concerned. With regard to these points the method is entirely an empirical one. The procedure as worked out for Arbacia eggs is as follows: Unfertilized

. . . . N . eggs are placed in a sea water mixture containing 2 cc. of T6 butyric

acid to 50 cc. of sea water. Apparently any monobasic fatty acid would serve equally well for this purpose but Loeb found butyric to be satis N factory for the experiments (2.8 cc. of I5 butyric were necessary in the case of Strongylocentrotus purpuratus eggs). The duration of the exposure must be brief, from 1% to 3 mintues. The eggs are then transferred to normal sea water and after 10 or 15 minutes to hypertonic sea water consisting of 8 cc. of 2% M NaC1 to 50 cc. of sea water. At a temperature of 23° the eggs must remain in this solution from 17% to 25 minutes after which they are transferred to normal sea water. The effect of this double treatment has been shown many times and in many different localities. A very large percentage, usually about as great as is obtained from the development of fertilized eggs, go ahead with the cleavage process and develop to larvae; doubtless the inability to rear the larvae is not in any way due to the fact that they were parthenogenetically produced.

The explanation of this double procedure has been a matter of some discussion. Loeb believed that membrane formation sets going certain chemical reactions upon which the future development of the egg depends. These chemical reactions, however, leave the egg in a condition from which it will not itself recover, and unless a second factor is employed in the process it will disintegrate rapidly. These reactions are on the nature of oxidations and are at least to a certain extent cytolytic in nature. The second factor is therefore necessary as a corrective to regulate the extensive oxidization which is produced by the exposure to the first solution. Loeb’s conclusion that cytolysis must take place unless corrected by the hypotonic solution has been questioned by some later experimenters, notably by Just, who has been able to induce the development of normal plutei of Arbacia by the use of hypertonic solutions alone. The exact preparations of the salts which are used to render the sea water hypertonic and the durations of the exposure to this mixture must be regulated with a great exactness if successful results are obtained. His optimum solution is made up of 22 parts of 2% M NaCl or KCI plus 78 parts of sea water, although variations from this mixture were also used. Of course great care was used to control the experiment in every way necessary. Eggs subjected to a treatment with this solution form membranes while still in the solution. Subsequently the eggs are returned to sea water and their development proceeds. Upon the basis of these experiments Just holds that the treatment with the fatty acid is not necessary and therefore that the hypertonic sea water is not serving as a corrective to stop the cytolysis induced by the unusual step in Loeb’s procedure. He is disposed to relate the activation of the sea-urchin eggs by this treatment to the egg secretion, fertilizin, which is given off by the unfertilized eggs into the sea water and which has been shown to be a necessary intermediary to normal fertilization with sperm. Just suggests that the activating agent which will accomplish experimental parthenogenesis serves to bind the fertilizin produced by the egg and thus to complete the necessary cortical changes which must take place if development is to proceed. The full explanation or the artificially induced changes has perhaps not yet been offered with regard to the experimental facts. However, there is no question that the entire subject has been one of the most successfully studied of all problems of experimental embryology.

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