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League BB. The chromosomes of the guinea-pig. (1928) J Morphol. 40(1):

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The Cheomosomes Of The Guinea Pig

Bessie Beakley League

University of Texas

Two Plates (twenty-pour FIGURES) author’s abstract

  • Contribution no. 208 from the Department of Zoology, University of Texas. The writer has been aided in this work by grants from the Committee for Eesearch on Sex Problems of the National Eesearch Council. The work has been done under the direction of Prof. T. S. Painter.
  • The essential results of the present paper were given in a brief note (by Painter). Science, vol. 64, p. 336, 1926.

Introduction

The spermatogenesis of five guinea-pigs was studied. The spermatogonial chromosome number is approximately sixty-two plus or minus two. The primary spermatocyte number is approximately thirty-one. The spermatogonial number in the early prophase is lower than it is in later stages. This condition is due to late fragmentation of the large chromosome^ found in the earlier stage. A possible sex chromosome of the X-Y type may be identified. Its components segregate during the first maturation division.

The present investigation is the third of a series of studies being carried, on at this laboratory dealing with the chromo- somes of the common rodents. The practical genetical ap- plication of these studies and the broader theoretical aspects have been discussed by Painter ( ’26) in the first of this series of papers. The guinea-pig differs from the other rodents so far investigated in its very high chromosome number. If my observations have been correctly interpreted, this high num- ber is, however, a late phylogenetic acquisition arid is brought about by a fragmentation of larger chromosomes, as will be shown below.^

The first extensive work dealing with the chromosomes of guinea-pigs was that of Stevens (’ll), in which she gave the diploid number as approximately fifty-six and the sex chro- mosomes as of the X-Y type. Eecently, Harmon and Root ( ’26) have described the diploid number as thirty-eight, but agree that the sex chromosomes are of the X-Y type.


Mateeial And Technique

Five mature male guinea-pigs were used for this study. After the animals were stunned by a blow on the head, the testes were removed, cut into small pieces, and the tubules teased apart very quickly and fixed in Allen’s modification of Bonin’s fluid. The tissue was subsequently treated ac- cording to the technique outlined by Painter (’24). Sections cut at 8 ij were found to be most satisfactory. Divisions of all orders were numerous in the testes of adult guinea-pigs.

Spermatogonia

Six equatorial plate views of dividing spermatogonial cells are given (figs. 1 to 6). The chromosome counts in these cells vary slightly, the number identified being sixty-two (figs. 3 and 6), sixty-three (fig. 4), and sixty-four (figs. 1, 2, and 5). A number of other spermatogonial plates were counted, giv- ing essentially the same results as in the accompanying figures. In no case was the number found to be less than sixty or greater than sixty-four.

There are few spermatogonial chromosomes of striking form or size. Two chromosomes, labeled a in the drawings are sometimes conspicuous because of their great size. Un- doubtedly, they are the large chromosomes identified by Miss Stevens. The remaining elements are much smaller and range in shape from bent rods to somewhat oblong blocks. The presence of such a large number of small elements makes counting difiicult and may account in part for the apparent variation in numbers observed.

In late prophases of dhdding spermatogonia a very inter- esting condition is encountered (figs. 7 and 9). (For the sake of clearness some of the chromosomes in these cells have been omitted in the figures.) The chromosome number at this time is clearly lower than in the equatorial plate stage. Although the exact number has not been determined, due to confusion with regard to the interpretation of transition stages in chromosomes, it is approximately forty. A close study of individual chromosomes reveals one or more cross constrictions in many of the elements (figs. 7 and 9). Later, these constrictions break through, but the fragments are held together by chromatic strands and give somewhat the ap- pearance of a string of sausages. In figure 7d and figure 9e this process is especially clearly shown. Subsequently, these chromosome segments apparently separate entirely, for there is no visible connection between the elements in later stages and little or no suggestion of the arrangement found in prophases. By the equatorial plate stage, the number is clearly over sixty.


Primary Spermatocytes

During the early growth period an acidophilic element and a chromosome nucleolus are associated together (figs. 22, 23), but, as in the case of the mouse (Grutherz, ’22; Cox, ’26), these components separate later, the acidophilic body disappear- ing and the chromosome nucleolus persisting and entering the first maturation spindle.

In equatorial plate views the elements are well separated (figs. 10 to 13 and 16). Counts made at this time show that at least thirty separate elements are present (figs. 11, 12, and 16), and in the clearest cells thirty-one elements are found (figs. 10 and 13). The latter number is regarded as probably the correct reduced number.

First maturation spindles viewed from the side are inter- esting for, frequently, one can easily make an approximate count of the haploid number of elements. In figure 14, for example, at least twenty-nine elements can be recognized. (This figure and the one following are ‘spindle dissections’ in which the individual elements are separated.) A number of similar counts have made it certain that the elements seen in the equatorial plate view have been correctly interpreted. One tetrad, decidedly larger than the rest of the chromosomes (labeled a in figs. 12 and 16), has obviously been derived from the pair of large spermatogonial chromosomes, as Stevens, and Harmon and Boot have shovm. Frequently, in telophase stages of this division, one large element lags behind in the spindles, and then aa division occurs it apparently breaks up into segments, suggesting that we are dealing with a com- pound chromosome (figs. 18, 19). This condition may be inter- preted as an explanation of the apparent discrepancy between the possible diploid number of. sixty-four and the haploid number of thirty-one. As in the case uf the spermatogonial elements, most of the tetrads are much alike in form' and size.

It has not been possible to identify with ‘ any' degree of • certainty a sex chromosome in the earlier phases of the first maturation spindle, although here and there cells are found' in which one element is suggestive of an X-Y complex. In telophase stages it is not unusual to observe, lagging in the spindle, two bodies, one of which is large and the other small (figs. 17, 20, 21, and 24). Both Stevens, and Harmon and Root have observed this structure and have identified it as being made up of a large X and a small Y component. Stevens describes the separation of these elements as coming either early or late. The figures indicate that the X and Y are segregated in this division just as in the case of other mam- mals (Painter, ’24).

Segondaey Spermatocyte Divisions

Few secondary spermatocytes were found in the material studied, and these were not very favorable for counting. One or two counts were made in which not less than thirty ele- ments could be distinguished.


Discussion

The present work has shown that the chromosome number in the guinea-pigs studied is very high and may be safely taken as sixty-two plus or minus two. ■ Sixty-two is the aver- age of the spermatogonial counts, which ranged from sixty to sixty-four, and thirty-one is the haploid number most fre- quently observed. It is not implied, of course, that there is really any variation in the chromosome number in the germ cells, but the small size of the cells, and the small size of many of the chromosomes with the attendant absence of distinctive shapes, greatly increases the difficulty of counting and, con- sequently, . the liability to error. Considering the nature of the material, the writer is probably not justified in fixing the number within closer limits than sixty-two plus or minus two. These results are in general accord with the work of Miss Stevens, who reported approximately fifty-six as the diploid chromosome number. On the other hand, my results are in marked disagreement with the low number of thirty-eight reported recently by Harmon and Boot.'"’

The lower chromosome number of Harmon and Root may, perhaps, be accounted for in part through the use of a technique which has not entirely prevented the tendency for adjacent elements to coalesce. Experience at this laboratory has shown that the method used by them (cold Flemming) is not quite as favorable for mammalian chromosome study as the modified Bonin technique. Thus, in figure 6 of their work, which is a polar view of the first maturation division, they have counted nineteen elements, many of which are obviously compound in nature. Normally preserved mam- malian tetrads appear much as bivalent elements do in insect spermatogenesis and do not show the tendency to throw off buds, such as we see in figure 6 of Harmon and Root. In view of the counts made in the present study (figs. 10 to 13 and 16) it seems probable that, in the material of Harmon and Root, some adjacent tetrads have fused together, form- ing compound masses which they have interpreted as one bivalent chromosome. As the writer interprets figure 6 of Harmon and Root, there are about thirty elements present, a number which is very close to the haploid count of the present work.

The spermatogonial count of Harmon and Root, as shown in figure 2 of their paper, cannot be accounted for on the basis of inadequate preservation, but is due in large measure to another cause. The present work shows that in prophase stages of spermatogonia, the chromosome number is much lower than in fully formed spindles and that many of these larger elements break up into smaller bodies (tigs. 7 and 9) before the equatorial plate stage. Figure 2 of Harmon and Boot is clearly a cell in which this breaking up of the chromo- somes has not yet taken place. Close observation of the indi- vidual elements reveals a number of constrictions in the chromosomes, indicating that this breaking, -up process was under way, and perhaps was masked by the technique em- ployed. It is possible that in the race of guinea-pigs employed by Harmon and Root the chromosomes do not break up in the spermatogonial metaphases, as they do in the Texas strain.


  • For review of earlier work see Harmon and Root (1. e.).


All recent workers on spermatogenesis in guinea-pigs have observed in the first maturation division a lagging element apparently made up of a large and a small part. This struc- ture has been interpreted as an X-Y sex chromosome complex, and the probabilities are that this interpretation is correct. However, it should be realized that the evidence for this con- clusion is neither critical nor complete, but rests largely on the similarity of this element to the X-Y sex chromosomes of the other mammals and of the insects.

The most interesting feature of the present work is the high chromosome number of the guinea-pig as compared to the other rodents and to mammals in general, and the method by means of which this high number is derived from a rela- tively simple condition. In all the other rodents studied in this laboratory (mouse, rat, rabbit, and squirrel^) the chromo- some number ranges from forty to forty-four. In view of the work on insects, especially that of McClung and his students on Orthoptera, we should expect that, within small groups such as the rodents, there would be a close similarity in the chromosomes, both as to number and morphology. If the interpretation given to my observations is correct, this conclusion is essentially true in the present case, the guinea- pig having originally possessed the typical rodent complex. This number later has broken up into the larger number of smaller more uniform elements.

The results of the study of the squirrel are as yet unpublished.


From the standpoint of the geneticist, in view of the large number of tetrads, linkage gronps should be much rarer in the guinea-pig than in the other rodents. This conclusion is based upon the supposition that the change has been due solely to fragmentation of the chromosomes, and not to any important rearrangement within the elements themselves or to the acquisition of new elements.

Summary

  1. The spermatogonial number is approximately sixty-two plus or minus two.
  2. The primary spermatocyte number is approximately thirty-one.
  3. The spermatogonial number in the early prophase is lower than it is in later stages. This condition is due to late fragmentation of the large chromosomes found in the earlier stage.
  4. A possible sex chromosome of the X-Y type may be identified. Its components segregate during the first matura- tion division.


Bibliography

Allen, Ezra 1919 A technique which preserves the normal cytological condi- tions in the testis of the albino rat. Anat. Eee., vol. 16.

Cox, Elizabeth K. 1926 The chromosomes of the house mouse. Jour. Morph, and Physiol., vol. 43, no. 1.

Gutherz, S. 1922 Das Heteroehromosomen-Problem bei den Vertebraten. Arch. f. mikr. anat., Bd. 96.

Harmon, Mary T., and Root, Prank 1926 Number and behavior of the chromosomes in Cavia cobaya. Biol. Bull., vol. 51.

Painter, T. S. 1924 A technique for the study of mammalian chromosomes. Anat. Rec., vol. 27.

1924 The sex chromosomes of man. Amer. Nat., vol. 58.

1925 A comparative study of the chromosomes of mammals. Amer. Nat., vol. 59.

1926 The chromosomes of the rabbit. Jour. Morph, and Physiol., vol. 43.

Stevens, N, M. 1911 Preliminary note on heteroehromosomes in the guinea-pig. Biol. Bull., vol. 20.

1911 Heteroehromosomes of the guinea-pig. Biol. Bull., vol. 21.


PLATE 1

explanation : OF PIGTJEES-

Figures 1 to 6 represent spermatogonia! cells of the guinea-pig.

1, 2, and o Equatorial plate stage, showing sixty-four chromosomes,

3 Equatorial plate ■ stage, showing sixty-two chromosomes.

4 Equatorial plate stage, showing sixty-three chromosomes.

6 Prophase stage, showing sixty-two chromosomes.

7 and 9 Parts of early prophase stages, showing fragmenting of chromosomes.

8 Side view of a primary spermatocyte spindle with division of a possible X-Y tetrad.

10 Late diakinesis with thirty-one tetrads.

11 and 12 Polar views of equatorial plate stages of primary spermatocytes, showing thirty tetrads.


PLATE 2

EXPLANATION OF FIGURES

13 Late diakinesis stage, showing thirty-one tetrads.

14 and 15 'Spindle dissections’ of side views of primary spermatocyte spindles.

14a and 15a Large tetrad.

16 Polar view of equatorial plate stage of primary spermatocyte, showing thirty tetrads.

17, 20, 21, and 24 Side views of primary spermatocyte spindles with division of possible X-Y elements.

18 and 19 Side views of primary spermatocyte spindles, showing the oetad dividing into four chromosomes.

22 and 23 Growth stages, showing separation of acidophilic and basophilic elements in heterochromosomes.


Cite this page: Hill, M.A. (2020, June 7) Embryology Paper - The chromosomes of the guinea-pig (1928). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_chromosomes_of_the_guinea-pig_(1928)

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