Contributions to the Knowledge of Non-Disjunction of the Sex-Chromosomes in Drosophila virilis. I. General Problems

Transcription

1 340 Cytologia 3 Contributions to the Knowledge of Non-Disjunction of the Sex-Chromosomes in Drosophila virilis. I. General Problems Received April 18, 1932 By Hideo Kikkawa Zoological Institute Kyoto Imperial University Introduction A study on non-disjunction of the X-chromosome in D. virilis was first published by WEINSTEIN ('22) and, afterwards, by ARAI ('30) and DEMEREC and FARROW ('30). Here I am going to report a similar study which I have made on the same material. Also I should like to mention some points which deserve special notice concerning the problem. Before going further, I wish to express my hearty thanks to Pro fessor TAKU KOMAI for his kind advice and criticism, and also to Dr. MITSUSHIGE CHINO who furnished me some stocks of the material. Primary Non-disjunction The results obtained from crosses involving several sex-linked characters are summarized in Table 1. In this table, the small number of the regular males as compared with the number of females in the mating, put on the first line, is due to a lethal factor which is involved in many of the cultures used. Of the total 8537 regular females, only one exceptional female has been secured, while in the males, one exception is found in about 500 regular males, if the effect of a lethal factor is taken into calculation.

2 1932 Contributions to the knowledge of non-disjunction 341 Table 1 Primary non-disjunctions. The female to male ratio1) in exceptional individuals is 1:15, which is in agreement with that obtained by DEMEREC and FARROW. Besides what is shown in this table, I was able to obtain one ec, v, cv, sg, mt and one yellow equational exception among the offspring of the following mating, +, ec, v, cv, sg, mt, Bx, w/ ~ +.2) The appearance y, +, +, +, +, +, +. +, of such exceptions seems to indicate that the spindle-fibre attachment of the X-chromosome in D. virilis is not on the yellow side but on the other side. This assumption is strongly supported by the data on the effect of temperature on crossing-over and also by fact concerning the locus of the bobbed gene (unpublished data). 1) The results obtained by ARAI ('30) and DEMEREC and FARROW ('30) may be summarized as follows: Regular Excep. (XXY) Excep. (XO) Female: male ARAI X-rayed :2.5 Control :1.7 DEMEREC & X-rayed :20 FARROw Control :11 The female-male ratio in primary exceptions obtained by ARAI is very much different from that obtained by the present writer and also from that given by DEMEREC and FARROW. This is apparently due to the stock ARAI used; the yellow stock which he used gives rise to exceptional females more frequently than stocks involving other sex-linked factors. However, there is no doubt that this abnormality is not due to the secondary non-disjunction, because the exceptional males obtained in his experiment were all sterile. Thus, the yellow gene or other genes involved in that stock seem to be connected with this abnormality, although further detail is left for future study. 2) A list of the genes used in this paper: y, yellow (2.4); ec, echinus (8.5); v, vermilion (26.0); cv, crossveinless (28.0); sg, singed (52.0); mt, miniature (76.5); Bx, Beadex (94.0); w, white (108.5); we, eosin (Allel. of white); bb, bobbed (182.0). Cf. CHINO, '29, '30.

3 342 H. KIKKAWA Cytologia 3 Secondary Non-disjunction The frequency of secondary non-disjunction in D. virilis is markedly less than that in D. melanogaster; for instance, WEINSTEIN ('22) obtained 1.3% in his material; DEMEREC and FARROW ('30) found in their more extensive works that the frequency was only 0.67 per cent. My result obtained from several XXY females found in various experiments shows a much similar feature (Table 2). In a total of 2066 flies, 11 exceptionals were found, the frequency being only 0.53 per cent. The female to male ratio in exceptional individuals is 1:4.5 in my ex periment, although the ratio obtained by other workers is close to 1:1. Table 2 Secondary non-disjunctions. Cytological evidence Because of the rarity in producing secondary exceptions, it is very difficult to determine on a genetical ground whether a given female is of the XXY or of the XX type. But, among the daughters of one XXY female, there should be found the same number of XX and XXY females (BRIDGES,'16; STERN, '30). Cytological test was then applied to these daughters. Microtome sections and acetocarmine smears were used, but primarily the latter, which is simpler and more convenient. As METZ ('16) stated, six pairs of the chromosomes are fouud in the normal oogonial metaphase of the material, consisting of 10 rod shaped ones which are hardly distinguishable from one another, and two dot-like or micro-chromosomes. METZ states that a pair of these rod-shaped chromosomes are slightly longer than the others and are presumably the X-chromosomes. But according to my observation, on both section and acetocarmine smear materials, 3 longer rod-shaped,

4 1932 Contributions to the knowledge of non-disjunction shorter rod-shaped and one dot-like pairs of chromosomes were always found, though these pairs, strictly speaking, show some grada tions in length. Two homologous chromosomes of these pairs lie side by side in the gonial metaphase, as is generally found in Diptera, but it is often observed that two homologous chromosomes are placed vis-a-vis (Fig. 1; a, d, h). The same tendency was also found in the presence of an extra Y. Judging from this fact, sex chromosomes in D. virilis seem to be one of the shorter rod-shaped pairs1) instead of the longer pairs. But no difference was found between the length of X and Y (Fig. 1; g, h, i). In a mass culture of a non-disjunctional strain, one XXXY female was found (Fig. 1; i); but no male with XO or XYY constitution has Fig. 1. Camera lucida drawings of chromosomes of Drosophila virilis. a-c, oogonial metaphase of XX females (S); d, oogonial metaphase of an XX female (A); e, sper matogonial metaphase (A); f, a chromosomal aberration found in an ovary of an XXY female (A); g-h, oogonial metaphase of XXY females (A); i, oogonial metaphase of an XXYY female (A). (S), section material ; (A), acetocarmine material. 1) This assumption has recently been confirmed by the chromosomal configuration found in I-V translocations.

5 344 H. KIKKAWA Cytologia 3 been detected cytologically. While examining an XXY female, 18 chromosomes were seen in several oogonia in a certain tract of an ovary (Fig. 1; f). These can hardly be called polyploids, but are rather heteroploids judging from the combination of the different kinds of chromosomes. At any rate, the occurrence of such cell sguggests that polyploidy and heteroploidy may be realized not only in maturation divisions, but also in gonial divisions. Crossing-over in XX and XXY females One equational female with ec, v, cv, sg, mt combination obtained +, ec, v, cv, sg, mt, Bx, w/ from a cross of ~ +, was mated to wild y, +, +, +, +, +, +, + type males. Among the daughters of this mating the same number of XX and XXY females should be expected. But, as stated before, it is difficult to determine genetically whether a given female is of the XXY or of the XX type. So cytological tests were applied to those females. If no XXY type female was found with the cytological method (aceto carmine smear method) among the ten daughters originated from one mother, the mother was assumed to be an XX type female. Because the frequency with which a female, in spite of having XXY constitu tion, produces XX females only among her ten daughters, would be (1/2)10_??_1/1000, and such a case is hardly expected. Even if the ratio of XX to XXY be 5:3 as shown later, the frequency would still be (5/8)10_??_9/1000. and the probability of the occurrence of such a case is also negligible. Thus, among the ten daughters of the equational exception tested by the above method, six females were of the XX type, while four were of the XXY type, the ratio being 3:2. Exceptional males were found in every ca. 100 males in the XXY strain (secondary exceptions), and 1 in every ca. 400 in the XX strain (primary excep tions); these frequencies are well in accordance with those given in Tables 1 and 2 (Table 3). The frequency of crossing-over was examined for the following regions only: ec-cv, cv-sg, and sg-mt, and the results are summarized in Tables 3 and 4. These indicate that the frequency of crossing-over in XXY females is not different from that in XX females; in other words, Y-chromosome exerts no influence on the frequency of crossing-over in the XXY females as has been confirmed also in D. melanogaster (ANDERSON,'26, '29; BRIDGES and OLBRYCHT, '26).

6 Table 3 Progeny of ec, cv, sg, mt, (Bx), (w)/+. +, +, +, +, +, ~y 1) All fertile. 2) All sterile.

7 346 H. KIKKAWA Cytologia 3 Table 4 Recombination values in XX and XXY females. XX to XXY ratio among daughters of XXY females From BRIDGES' extensive work in D. melanogaster, we know that XXY and XX females are produced in an equal number among the regular offspring of one XXY females. In D. virilis, however, my experiment shows an abnormal ratio as given in Table 5, the difference being 2.2 times the standard deviation. Table 5 XX to XXY ratio among daughters of XXY females. ƒð4.06 Diff. 9 Diff./ƒÐ=2.2 Discussion A role of the Y-chromosome in the XXY females: A characteristic feature of the secondary exceptions is that the two X-chromosomes derived from the mother are always non-crossovers except some rare cases of equational exceptions. From this fact, BRIDGES ('16) has assumed that the Y in the XXY will occasionally synapse with one of the X's in the growth period (heterosynapsis), and that, if one X is in

8 1932 Contributions to the knowledge of non-disjunction 347 synapsis with Y, the X will be prevented from undergoing crossing-over. When the X and the Y that have synapsed with each other, disjoin and pass to opposite poles, the other free X should pass to the pole with the disjoined X or to the pole with the disjoined Y. ANDERSON ('29) who subsequently studied a case of high non-dis junction, has expressed the view that the Y in the XXY is not concerned directly with the phenomenon of synapsis, but acts as a regulator at the reduction division. He states: "If the percentage of crossing-over be taken as an index of the closeness of synapsis, then we may say that those X chromosomes which synapse most intimately, especially in the region near the spindle fiber, are distributed most regularly to poles at the reduction division. The presence of a Y chro mosome acts as a distribuing element in the distribution chiefly of those X chromosomes which synapse or pair most loosely with each other." This assumption seems to be applicable to the normal case of the secondary non-disjunction also. As to the question which of these two assumptions is more adequate, we can not say anything until more data on this problem, especially the cytological data, accumulate. However, BRIDGES' assumption is very simple, and it can explain various phenomena on the secondary excep tions, though it can not be applied to the ANDERSON'S case mentioned above. As stated before, the secondary non-disjunction is very rare in D. virilis, and the heterosynapsis, even if it occurs at all, certainly has a frequency which is very small (about 3%). A question remains on the ratio of XX females to XXY females among the regular offspring of one XXY mother (cf. Table 5). It may be that this abnormal ratio is due to mere experimental error or else to the peculiarity of the females used. However, the following explanation seems also plausible. If one XXY type female is considered, she would show normally in the growth period the XX-Y synapsis. When these two X's separate at the reduc tion division and enter two daughter cells, the free Y would follow either of these X's. But, if it is left by chance on the equatorial plate or somewhere else, eggs involving one X may be produced, and these will become normal females (XX) by receiving another X from the male. A similar conception has already been advanced by MORGAN and his co-workers ('25) and DEMEREC and FARROW ('30) for the explanation of an abnormal sex ratio in the primary exceptions. The diagram illus trating all such possible cases in primary and secondary exceptions is given below.

9 348 H. KIKKAWA Cytologia 3 (Here, for convenience's sake, I have assumed that two types of synapsis (XX-Y and XY-X) are possible). Primary Always Diagram 1 non-disjunctions XX synapsis Non-disjunctional females may be produced from the case (c) only, while such males may be produced from either the cases (b) or (e). (A) Secondary XX-Y synapsis non-disjunction (B) XY-X synapsis (heterosynapsis) Regular XX females may be produced from any of the cases (a), (c), (g) and (h), while regular XXY females may result either from the case (b) or (f). Non-disjunc tional females may be produced from the case (d), while such males may result either from the case (e) or (i). The above diagram shows that females are fewer than males among both the primary and secondary exceptions, and that the XXY females are fewer than the XX females among the regular progenies of XXY females. These assumptions seem to accord well with my experimental results in D. virilis. Summary (1) The frequency of primary exceptions in D. virilis is about 1:1000 ; that is somewhat higher than that in D. melanogaster (1:2000). The frequency of primary exceptional females is much lower than that

10 1932 Contributions to the knowledge of non-disjunction 349 of the males, being 1:15 in my experiment. (2) The secondary exceptions are very rare as previous workers have pointed out, the frequency being only 0.53 per cent in my ex periment. (3) The non-disjunction was verified on cytological ground also. There are 3 longer rod-shaped, 2 shorter rod-shaped and one dot-like pairs of chromosomes in the normal oogonial metaphase. The X-chro mosomes are probably one of these two shorter pairs. No difference in length is found between X and Y. (4) Crossing-over in XXY females is not different from that in XX females. (5) XX to XXY ratio among regular daughters of XXY females has been found to be about 5:3, instead of 1:1. Such an abnormal ratio can be easily accounted for by assuming that one of the sex chromosomes (X or Y) lags on the spindle and does not enter the nucleus of either of the two newly formed cells at the first maturation division. Literature cited Anderson, E. G., The proportion of exceptions in the offspring of exceptional females from X-ray treatment in Drosophila. Michigan Acad. Sci., Arts and Letters. 5: Studies on a case of high non-disjunction in Drosophila melanogaster. Zs. f. ind. Abst. u. Vererb. 51: Arai, Y., The production of non-disjunction by X-rays in Drosophila virilis. (Japanese) Japanese Journ. Genet. 6 : Bridges, C. B., Non-disjunction as proof of the chromosome theory of heredity. Genetics 1: 1-52, and Olbrycht, T. M., The multiple stock "X-ple " and its use. Genetics 11: Chino, M., Genetic studies on the Japanese stock of Drosophila virilis. I. (Japanese) Japanese Journ. Genet. 4: Genetic studies on the Japanese stock of Drosophila virilis. II. (Japanese) Ibid. 5: Demerec, M., and Farrow, J. G., Non-disjunction of the X-chromosome in Drosophila virilis. Proc. Nat. Acad. Sci. 16: Metz, C. W Chromosome studies in the Diptera. Journ. Exp. Zool. 21: Morgan, T. H., Sturtevant, A. H., and Bridges, C. B., 1925, The genetics of Droso phila. Bibliog. Genetica. 11: Stern, C. 1930, fiber Reduktionstypen der Heterocbromosomen von Drosophila melanogaster. Zs. f. ind. Abst. u. Vererb. 51: Weinstein, A Crossing-over, non-disjunction, and mutation in Drosophila virilis. Sigma Xi Quart 10: