A Possible Case of Negative Segregation Distortion in the SD System of Drosophila melanogaster

Transcription

1 Copyright 1989 by the Genetics Society of America A Possible Case of Negative Segregation Distortion in the SD System of Drosophila melanogaster Yuichiro Hiraizumi Department of Zoology, The University of Texas, Austin, Texas ' Manuscript received July 6, 1988 Accepted for publication October 8, 1988 ABSTRACT Models proposed to explain segregation distortion in Drosophila melanogaster are based partly upon the observation that, in the Sd heterozygous Rsp'/Rsp' male, the chromosome carrying the sensitive Rsp" allele is distorted, but the chromosome carrying the insensitive Rsp' allele is not. The results of the present study suggest that this may not always be the case. Under a certain genotypic condition, the chromosome carrying the Rsp' allele can be transmitted to the progeny, in frequencies of more than.5 (about.6), or correspondingly, the chromosome carrying the Rsp' allele may be distorted with respect to the one with the Rsp' allele. Thus, the relative sensitivity and insensitivity of the two Rsp alleles in a male are not absolute, but relative; and they may be reversed depending upon the residual genotype. If this is true, a major modification of the current models or a proposal of an entirelv new model mav becomenecessary to explain the mechanism of segregationdistortion satisfactorily. HE segregation distorter (SD) system of Drosoph- T ila melanogaster is one of the typical examples of meiotic drive. It was first discovered in a natural population of this species in Madison, Wisconsin (SAN- DLER, HIRAIZUMI and SANDLER 1959). Subsequent surveys have revealed that, except for populations in Texas (HIRAIZUMI and THOMAS 1984), the SD second chromosome (on which the main elements of this system are located) is spread worldwide among natural populations of this species, although the frequency of this chromosome within each population is only a few percent. When heterozygous SD/SD+ males are crossed to normal SD+/SD+ females, they produce progeny containing the SD chromosome in excess of the Mendelian expectation of 5%, usually 9% or higher. This distorted transmission frequency is due to dysfunction of sperm containing the normal SD+ chromosome (HARTL, HIRAIZUMI and CROW 1967; NICOLETTI, TRIPPA and DE MARCO 1967; TOKUYASU, PEACOCK and HARDY 1972). The SD system consists of two distinct major elements (HARTL 1973): the Sd element is located to the left of and very close to the pr locus (2L-.4) on the left arm of chromosome 2, and the Rsp element is located within the centromeric heterochromatin of the right arm of chromosome 2, proximal to the stw locus (2R-55.1) (GANETZKY 1977). There are two allelic alternatives of Rsp, sensitive (Rsp") and insensitive (Rsp'). As so far observed, a chromosome carrying Rsp' is not distorted by the SD activity. The first model of the mechanism of segregation distortion was proposed by HARTL (1973). He assumed that the Sd and Rsp elements interact in the Genetics 121: (February, 1989) following way: as a necessary condition for sperm maturation, it is proposed that the Rsp locus must complex with the product of the Sd locus. This product is presumed to be a multimeric regulatory protein of which three types may be distinguished: Sd+/Sd+ homomultimers, which are able to interact with both Rsps and Rsp'; Sd+/Sd heteromultimers, which complex preferentially with Rsp'; and Sd/Sd homomultimers, which are not able to complex with either Rsps or Rsp'. The majority of regulatory protein in the Sd+/Sd male is assumed to be Sd+/Sd heteromultimers. In 1977, GANETZKY found that a deletion of the Rsps locus led this chromosome to behave as if it carried an Rsp' allele, and that a deletion of the Sd locus from an Sd Rsp'/Sd Rsp' sterile male increased its fecundity (GANETZKY 1977). Based upon these observations, he proposed the following modifications of HARTL'S model. Rather than the binding of an Sd product at the Rsp locus being necessary condition for normal spermiogenesis, it is this binding that causes sperm dysfunction. It is assumed that the Sd product complexes more readily with Rsp" than with Rsp' and that the amount of Sd product is limited with respect to the number of binding sites available. In order to explain the low male fertility of some genotypes, GA- NETZKY further assumes that the Sd product, when not competed for by an Rsp" locus, can bind to an Rsp' locus. Further modification of HARTL'S model was proposed by HIRAIZUMI, MARTIN and ECKSTRAND (198). Briefly, their model is as follows. A modifier of SD, M(SD), found to be located close to the cn locus (2R-57.5), produces a multimeric repressor protein which binds to the Rsp locus as a necessary condition

2 264 Y. Hiraizumi for normal spermiogenesis. The Sd locus produces a certain product that tends to bind with the repressor complexed to the Rsp locus. This binding, similar to the lactose system in E. coli, disrupts the repressor- Rsp complex, causing the Rsp locus to be turned on. The product of the Rsp transcript, in turn, results in sperm dysfunction. The Rspi allele has a strong complexing affinity with the repressor such that the Rsp*- repressor complex is resistant to the inducing activity of the Sd product, whereas the Rsp allele has a weaker complexing affinity with the repressor such that the Rsp -repressor complex is more readily disrupted. Although these models differ somewhat from each other, there is one common assumption among them, i.e., in an Sd Rsp /Sd+ Rsp male, the chromosome carrying the Rsp allele is distorted, but the chromosome carrying the Rsp allele is not. All observations so far reported support this assumption, although the degree of distortion may differ depending upon the degree of sensitivity of the Rsp allele. The purpose of the present paper is to report a possible instance where the Rspi chromosome can be distorted in the Sd Rsp /Sd+ Rsp heterozygous male. This phenomenon, which should not be confused with so-called suicide distortion (SANDLER and HIRAIZUMI 196; HARTL 1974), may be termed negative segregation distortion. MATERIALS AND METHODS For purposes of simplification, two parameters are defined as follows: k2 is the frequency of the second chromosome (x) recovered among progeny of a mating, +/+ female x x/+ male and, similarly, kj is the frequency of the third chromosome (y) recovered among progeny of a mating, +/ + female X y/+ male. The values of k2 and kj presented in Tables of this report are those which were not adjusted for differential viabilities of segregating progeny genotypes. Strains and lines: Drosophila melanogaster strains and chromosome lines used in this study are listed below. Explanations and symbols for mutants are given in LINDSLEY and GRELL (1968). (1) : a standard Sd+ Rsp strain with chromosome 2 marked by two recessive eye color mutants, en and bw. (2) a1 b sp; ve st ea: a strain with chromosome 2 marked by three recessive mutants, al, b and sp; and chromosome 3 marked by three recessive mutants, ve, st and ea. The second chromosome carries Sd+ and presumably a partially insensitive Rsp. (3) It stw3: an Sd+ Rsp chromosome 2 line which carries two recessive mutants, It and sku. This chromosome carries an RspSs allele which is super sensitive to SD activity. This line has been maintained in this laboratory by repeated backcrosses to standard en bw females. (4) SD-72: an Sd Rsp chromosome 2 line isolated in 1956 from a natural population in Madison, Wisconsin, and showing a kz value of.99 in SD-72/ males. It carries one para- and one pericentric inversion. (5) R(SD)(cn- 14): a chromosome 2 line, Sd Rsp, which shows a kz value of.98 in R(SD)(cn-I#)/ males. It was derived by crossing-over from SD-36, and presumably consists of 2L and very proximal 2R from SD-36 (including Sd and Rsp ) and 2R from en on from a pr cn chromosome. It carries the marker en and a recessive lethal gene or genes. No structural TABLE 1 List of the average kz values for the second chromosome, x, from female X x/ male matings No. of Chromosome x matings kz SE SD R(SD)(cn-14) R(SD)(cn-14)-~ R(SD)(cn-14)-~ R(SD)(~n-14)-~ R(SD)(cn-14)-~ abnormality has been found in this chromosome. When this chromosome was constructed (HIRAIZUMI and NAKAZIMA 1967), the kz value was about.85. The two SD lines have been kept in two ways:by backcrossing to females and, after such backcrosses, as a balanced lethal strain with an Zn(2LR)Cy or an Zn(2LR)SMS chromosome 2 balancer. These balanced lethal strains were used in the present study. (6) T47/ln(ZLR)Cy, bw: a balanced lethal strain in which T-7 is a typical, strong P type chromosome in the P-M system of hybrid dysgenesis (KIDWELL, KIDWELL and SVED 1977; HIRAIZUMI 1971, 1977; MATTHEWS 1981). This strain has the P cytotype. Zn(ZLR)Cy, bw is a chromosome 2 balancer carrying the recessive marker bw. Mating scheme: Unless otherwise stated, all of the matings in this study were made in a culture vial by crossing a single male, two days or younger in age, to two virgin females, two to seven days in age. Parents were kept in the culture vials for three days and then discarded. Progeny were counted on the 14th day after the mating was initiated and again on the 18th or 19th day. By this time, practically all of the F1 progeny had eclosed without the possibility of including any progeny of the next generation. Unless otherwise stated, all the culture vials were kept at a room temperature of 25 O C. RESULTS In late 1985, a1 6 sp; ve st ca homozygous females were crossed to R(SD)(cn-I#)/Zn(2LR)Cy males, and the F1 progeny of the genotype R(SD)(cn-I#)/al 6 sp; + + +/ve st ca were collected. Forty-five such males were backcrossed individually to two a1 6 sp; ve st ca homozygous females, and the resulting progeny were scored for each of the four segregating classes separately. The Kz values calculated for the R(SD)(cn-I#) chromosomes recovered among progeny were generally low, with an average of about.6 (kg for this chromosome is.98 with the standard cn 6w strain, see Table 1). This result suggested that the a1 6 sp; ve st ca strain carried, when compared with the standard en 6w strain, either a suppressor of SD or an Rsps allele which was partially insensitive to SD activity. The distribution of kz values among 45 matings ranged widely from.37 to.9, and nine of them (2%) showed k, values smaller than.5. Since the number of progeny produced in each mating was small (average of about 33), these low k2 values could simply be due to sampling chance. In order to examine this point further, four low k2 lines were isolated and re-

3 genative Segregation Distortion 265 TABLE 2 List of the average ka values for the second chromosome, x, from en bw female X x/zt stw3 male matings Chromosome x No. of matings progeny Total R(SD)(cn-Z4) R(SD)(cn-Z4)-x R(SD)(cn-Z4)-x tested. The low values were indeed heritable, giving averages of consistently less than.5. Suppressors of SD so far reported can bring the kz value for the SD chromosome down towards.5 but not below.5 (reviewed by HARTL and HIRAIZUMI 1976). Therefore, if the kz values less than.5 are due to suppressors present in the al b sp; ve st ca strain, they have novel properties. It was first thought that either the insensitive Rsp allele in the original R(SD)(cn-I4) chromosome was changed towards sensitive such that it became more sensitive than the Rsp allele in the al b sp chromosome, or the Sd allele itself was changed in such a way that the direction of interaction between the product of Sd and the Rsp locus was reversed. There were also the possibilities that either a specific event such as a deletion occurring somewhere in the R(SD)(cn-I4) chromosome was causing a drastic reduction in the viability of its carrier, or the P element in the P-M system of hybrid dysgenesis (KIDWELL, KID- WELL and SVED 1977) was introduced into the R(SD)(cn-I4) chromosome, perhaps by some accidental event. This could result in a P-type R(SD)(cn-I4) chromosome showing, when its SD activity was suppressed, kz values smaller than.5. It has been shown that P-M dysgenic males cause kz values for the P-type second chromosomes of smaller than.5 (HIRAIZUMI 1971, 1977; MATTHEWS 1981). Whatever the cause, I felt that it was worthwhile to investigate these chromosomes further. The low k2 chromosome lines which were analyzed in the present study will be symbolized as R(SD)(cn-14)-~4, R(SD)(cn-14)-~5, R(SD)(cn-I4)-x37 and R(SD)(cn-l4)-x39. The kz values for the R(SD)(cn-Il)-x chromosomes when heterozygous with the standard chromosome: Table 1 summarizes the kz value distributions for various SD/ heterozygous males. It is clear that the R(SD)(cn-I4)-x chromosomes show considerably lower average kz values than that of the original R(SD)(cn-I4) chromosome, although all of them still retain the ability to distort the chromosome. The kz values for the R(SD)(cn-I4)-x chromosomes when heterozygous with the super sensitive It stw3 chromosome: Table 2 shows the mean k2 values for the R(SD)(cn-I4), R(SD)(cn-I4)-x and the standard cn bw chromosomes when heterozygous, in males, with It kz SE TABLE 3 List of the average k,(a) and k3(b) values for the wild-type third chromosome (+ + +) from az b sp; ve st ea female X x/az b sp; + + +/ne st ea male matings No. of Chromosomex matings LAA) SE KAB)* SE SD R(SD)(cn-14) R(SD)(cn-14)-~ R(SD)(cn-Z4)-xS R(SD)(cn-Z4)-~ R(SD)(cn-14)-~ * Frequency of the third chromosomes recovered among the F, progeny of al b sp; ve st ca and al b sp; phenotypic classes only. Frequency of the third chromosomes recovered among the F1 progeny of + + +; ve st ca and + + +; phenotypic classes only. Twenty-two of them showed both kda) and kz(b) equal to 1. and the value of kj(a) could not be calculated for these matings. Therefore, the actual number of matings for kj(a) was 667. stwj. As expected, the kz values are very high for the SD chromosomes because of the supersensitivity of the Rsp allele carried by the It stw chromosome. Clearly, the R(SD)(cn-I4)-x chromosomes are strongly distorting the It stw chromosomes, although the degrees of distortion are somewhat lower than that of the original R(SD)(cn-I4) chromosome. The kz and LS values for the R(SD)(cn-I4)-x/uZ b sp; + + +/ve st cu males: R(SD)(cn-Z4)-x/Zn(2LR)SM5 males were crossed to al b sp; ve st ca females and the F1 progeny males, R(SD)(cn-I4)-x/al b cp; + + +/ve st ca, were individually backcrossed to two al b st; ve st ca females in order to measure the kz as well as the k3 values. Crosses were also performed with other second chromosomes used in place of R(SD)(cn-I4)-x. Since four classes were segregating among the progeny, the k values were calculated in the following way. The kz(a) values for the non-(al b sp) second chromosome were calculated based upon the a1 b sp; ve st ca and + + +; ve st ca phenotypic classes only; kz(b) values were calculated based upon the a1 b sp; and + + +; phenotypic classes only. The k3(a) values for the non-(ve st ca) third chromosome were calculated based upon the a1 b sp; ve st ca and a1 b sp; phenotypic classes only; and the kg(b) values were calculated based upon the + + +; ve st ca and + + +; phenotypic classes only. Although the main purpose of this study was to investigate the segregation frequency of the second chromosome, the analysis was made first for the third chromosome to see if the segregation frequency from the ve st ca heterozygous male was normal. Table 3 shows the average kj(a) and kj(b) values for the genotypes listed. In all cases the k3 values were close to the Mendelian.5, although they were slightly larger than.5, perhaps due to differential viabilities of segregating phenotypes.

4 266 Y. Hiraizumi TABLE 4 List of the average kz(a) and &(B) values for the second chromosome, x, from a1 b sp; ve st ca female X x/al b sp; + + +/ ve st ea male matings No. of Total Chromosomex matings progeny kz(a)" SE kz(b)' SE SD , R(SD)(cn-l4) 63 21, R(SD)(cn-ll)-~l 2, R(SD)(cn-14)-~5 5 13, R(SD)(cn-I4)-~ , R(SD)(~n-l4)-~ , , Frequency of the non-(a1 b sp) second chromosomes recovered among the F1 progeny of al b sp; ve st ca and + + +; ve st ca phenotypic classes only. Frequency of the non-(al b sp) second chromosomes recovered among the F, progeny of al b sp; and + + +; phenotypic classes only. There were no differences in k) values among different genotypes (nl = 6, n2 = 7796, F = 1.16, P >.5), indicating that the segregation frequency in the third chromosome from the ue st ca heterozygous male parents is not affected by the genotype of the second chromosome. The average k3(a) values appeared to be consistently larger than those of k3(b) (nl = 1, n2 = 7796, F = 37.25, P<.1). This suggests the presence of synergistic interaction inviability between chromosomes 2 and 3. The kj(a), K4B) X genotype interaction was not significant (nl = 6, n2 = 7796, F =.75, P >.1). Table 4 summarizes the average k2(a) and k2(b) values for several different male genotypes. It can be seen that the k2 values for the R(SD)(cn-I4)- x chromosomes are very close to the theoretical value of.5. In particular, the k4b) values are consistently smaller than.5 for all of the four lines examined. Considering that the phenotype of the progeny carrying the R(SD)(cn-I4)-x chromosomes is wild type, whereas that of the non-r(sd)(cn-i4)-x progeny isa1 b sp (three mutant phenotypes which may show somewhat reduced viability), the observed k4b) values of even slightly less than.5 seem to suggest that a considerable degree of segregation distortion may be taking place against the R(SD)(cn-Z4)-x chromosomes. The kz values for the SD-72/R(SD)(en-I4)-x males: SD-72/R(SD)(cn-14)-~ heterozygous males were individually crossed to females, and the Kz values for the R(SD)(cn-l4)-x chromosomes were calculated. If the change from the R(SD)(cn-I4) to R(SD)(cn-I4)-x chromosome was at the Rsp locus such that the Rspi allele in the R(SD)(cn-14) chromosome changed to a partially sensitive allele, say Rsp", the genotype of the SD-72/R(SD)(cn-l4)-x males may be written as Sd Rsp'/ Sd Rsp", and such males will give an average k2 value for the Sd Rsp" chromosome [R(SD)(cn-I4)-x] of smaller than Mendelian.5 (MARTIN and HIRAIZUMI 1979). It might also be that the change was at the Sd TABLE 5 List of the average RZ values for the second chromosome, x, from en bw female X x/sd-72 male matings Progeny No. 21 All progeny pooled No. of No. of Total mat- mat- prog- Chromosomex ings kz SE ings eny kz R(SD)(cn-14) R(SD)(cn-14)-~ R(SD)(cn-ll)-xS R(SD)(cn-14)~ R(SD)(~n-14)-~ locus, or at a locus other than the Sd and the Rsp loci. In both cases, the tested male genotype will be written as Rsp' homozygote and therefore the present type of mating cannot distinguish these two possibilities by simply measuring the k2 values. Mendelian 1 : 1 segregation will be expected in both cases. Table 5 shows the average k2 values for the genotypes listed. Since male fertility in this type of mating was much reduced, the values of k2 were calculated in two ways: the unweighted mean k2 based upon the males producing at least ten progeny and the overall k2 based upon the total number of progeny pooled for all replications for each genotypes shown. From this table, it can safely be concluded that the segregation frequency from the SD-72/R(SD)(cn-l4)-x male is Mendelian one to one. This indicates that the R(SD)(cn-I4)-x chromosome carries an Rsp' allele which is as insensitive as that carried by the SD-72 chromosome. The kz values for the R(so)(cn-I4)-x/Z~(2LR)SM5 males: Additional matings similar to those presented in the previous section were conducted by replacing the SD-72 with the Zn(2LR)SMS chromosome. As the Zn(2LR)SM5 balancer chromosome 2 was shown to be Sd+ Rspi (HARTL 1975; HIRAIZUMI, MARTIN and ECK- STRAND 198), the genotype of males in this section can be written as Sd+ Rsp'lSd Rsp". Results, together with those of R(SD)(cn-l4)/Zn(2LR)SM5 males, are summarized in Table 6. As the k2 values are close to the Mendelian.5 in all cases, there is no indication of suicide distortion for the R(SD)(cn-l4)-x4 chromosome. These results confirm the conclusion that the Rsp allele in the R(SD)(cn-I4)-x4 chromosome is as insensitive as that carried by the original R(SD)(cn-14) or the Zn(2LR)SM5 chromosome. The kz and k3 values from the R(SD)(cn-Il)-x/al b sp; + + +/ve st ea females: Although unlikely, the possibility still remained that the k2 values for the R(SD)(cn-I4)-x4 chromosomes smaller than.5 were in fact due to reduced viability of individuals carrying these chromosome in the heterozygous condition. This could be the case if the change from the R(SD)(cn- 14) to the R(SD)(cn-I4)-x chromosomes was due to

5 Segregation genative Distortion 267 TABLE 6 List of the average ka values for the R(SD)(cn-I4) or the R(SD)(cn-I4)-x4 chromosomes from the reciprocal matings of en bw X R(SD)(cn-Z4)/Zn(2LR)SM5 or en bw X R(SD)(C~-I~)-X~/Z~(~LR)SM~ parent Female No. of Total progeny matings Male parent kz SE R(SD)(cn-I4)/ln(2LR)SM5 3, R(SD)(cn-I4)/In(2LR)SM5 bw.498 2, Mean , R(SD)(C~-I~)-~~/I~(~LR)SM~ , R(SD)(cn-I4)-x4/In(2LR)SM ,8. Mean 35, TABLE 7 Average kz(a), kz(b), R3(A) and k3(b) values from the reciprocal matings, xlal b sp; + + +/ve st ca X a1 b sp; ve st ca No. of ma- Chromosome x tings" kxa) kdb) R(SD)(cn-14) 11 (63).598 (.7).586 (.677) R(SD)(cn-ll)-xl 182 (67).589 (.516).564 (.482) 147 (57).564 (.5).9 (.4) k m R(SD)(cn-I4) 11 (63).1 (.5).527 (.) R(SD)(cn-Z4)-x4 182 (67).3 (.8).519 (.55) 147 (57).6 (.523).59 (.) a The figures outside of parentheses refer to matings where the chromosome x is transmitted from the mother; the figures within parentheses refer to matings where the chromosome x is transmitted from the father. The data are from Table 3 and Table 4 for k3 and k2, respectively. gross chromosomal structural changes such as dele- tions. The possibility of reduced viability was tested by crossing two young R(SD)(cn-l4)-x/al b sp; + + +/ ve st ca females to three a1 b sf; ve st ca males, and by scoring the F1 progeny that were not recombinant phenotypes with respect to both the second and the third chromosome markers; ie., by scoring only those F1 progeny with the phenotypes (a1 b sp; ve st ca), (a1 b sp; + + +), (+ + +; ve st ca) or (+ + +; + + +). Since the map distances between any of the two markers are approximately 5%, there had to be some undetectable double crossovers, but such chances might be relatively small. In fact, old data from the crosses of a1 dp b pr c px sp/ females X a1 dp b pr c p x sp males indicated that the frequency of double crossovers between the a1 and b loci was 25/ 2168 or.115, and it was 96/2168 or.443 be- tween the b and sp loci. For practical purposes, such a small fraction of undetectable double crossovers in the second chromosome may be ignored. Similar matings were performed in which the R(SD)(cn-I4)-x4 chromosome was replaced by the original R(SD)(cn- 14) and, as a control, by the standard chromosome. Results are summarized in Table 7 and the results of variance analyses in Table 8. As described earlier for males, the value of kz(a) for females was TABLE 8 Variance analyses of the kz and k, values in the reciprocal matings, A, B and C as shown A: a1 b sp; ve st ca X lal b sp; + + +/ve st ca Source of DF variance MS F P Between reciprocals Between kaa) and h(b) co.1 Interaction Error Between reciprocals Between kaa) and kj(b) <.1 Interaction Error B: a1 b sp; ve st ca X R(SD)(cn-I4)-x4/al b sp; + + +/ve st ca Between reciprocals co.1 Between kz(a) and kz(b) co.1 Interaction Error Between reciprocals Between kj(a) and kj(b) co.1 Interaction Error C: a1 b sp; ve st ca X R(SD)(cn-I4)/al b sp; + + +/ve st ca Between reciprocals co.1 Between kz(a) and kz(b) <.1 Interaction Error Between reciprocals Between kxa) and k;t(b) CO.1 Interaction Error larger than that of the corresponding kz(b) and, similarly, the k3(a) value for females was larger than that of the corresponding k9(b) for all of the chromosomes examined. These results again suggest the presence of synergistic interactions in viability between chromosome 2 and 3 phenotypes. The control chromosome (category A in Table 8) showed homogeneous kz and k3 values between the reciprocal matings. The R(SD)(cn-I4)-x4 and the R(SD)(cn-I4) chromosomes (categories B and C, respectively, in Table 8) also showed homogeneous k3 values between recipro-

6 268 Y. Hiraizumi cals, but both showed highly significant differences in kz values between the reciprocal matings. The mean k~ value for the R(SD)(cn-I4) chromosome was much larger when it was transmitted from the heterozygous male than from the heterozygous female parents, whereas this situation was completely reversed for the R(SD)(cn-I4)-x4 chromosome. Considering that the kz values from the female parents represent the relative extent of differential viabilities of segregating phenotypes (in all cases, the wild phenotype individuals appear to be more viable than those with the a1 b sp mutant phenotypes, since the k2 values are larger than.5), it may be concluded that a positive segregation distortion takes place in the R(SD)(cn-I4)/al b sp males; whereas, there appears to be a negative segregation distortion in the R(SD)(cn-I4)-~4/al b sp males. The kz(b) values from the R(SD)(cn-l4)-x4/uZ b sp; + + +/+ + + males: In order to examine if the third and the X chromosomes from the a1 b sp; ve st ca strain were involved in the induction of possible negative distortion, males of the genotype R(SD)(cn-I4)-x4/al b sp; + + +/+ + + were constructed, where the wild type third and the X chromosomes were from the standard strain. These were mated individually to two a1 b sp; ve st ca females, and the resulting kz(b) values were measured. From a total of 391 matings, an average k2(b) value of.7 f.5 was obtained. Although this value was homogeneous to the mean kz(b) value of.564 f.1 1 from the R(SD)(cn-14)- x4/al b sf; + + +/ve st ca females (nl = 1, nz = 573, F = 2.8, P >. lo), it was significantly larger than that of.482 f.7 from the R(SD)(cn-l4)-~4/al b sp; + + +/ve st ca males (nl = 1, n2 = 998, F = , P <.1). Thus, the presence of the ve st ca or the X or both chromosomes from the a1 b sp; ve st ca strain appeared to be a necessary condition for negative distortion, although clearly the a2 b sp second chromosome itself has the major effect. This result leads to the question: can the third chromosome alone or the X chromosome alone or both together from the a1 b sp; ve st ca strain induce negative distortion without the a1 b sp second chromosome? The test for the X chromosome has not yet been completed, but it was done for the third chromosome alone. Eighty- three R(SD)(cn-I4)-x4/; + + +/ue st ca males, with the X chromosome from the standard strain, were individually mated to two females, giving an average kz value of.78 f.12. This value appeared to be somewhat smaller than that of.727 k.5 from the R(SD)(cn-l4)-x4/; + + +/+ + + males (Table l), although the difference was not statistically significant (nl = 1,122 = 486, F = 2.44, P >.1). A similar mating was made for the original R(SD)(cn-I4) chromosome. A total of 572 replications gave an average k2 value of.96 &.4. This value was significantly smaller than that of.978 &.1 (Table 1) from the R(SD)(cn-l4)/; + + +/+ + + males (nl = 1, nz = 2325, F > 1, P <.1). Thus, although the ue st ca chromosome appears to cause a weak suppression of segregation distortion, it alone cannot cause negative distortion. Possible effects of the P-M system of hybrid dysgenesis: One other possibility to account for kz values smaller than.5 might be that the a1 b sp; ve st ca is an M-type strain, whereas the R(SD)(cn-I4)-x chromosomes are P-type chromosomes. HIRAIZUMI (197 1, 1977) and MATTHEWS (1981) showed that dysgenic males for this system caused k2 values for the P-type second chromosomes of smaller than.5. Although the a1 b sp; ve st ca strain and the R(SD)(cn-I4) chromosome line have been classified as typically M-type, a re-typing was performed as the possibility existed that the derivation of the R(SD)(cn-I4)-x chromosomes might be due to the introduction of the P element into the original R(SD)(cn-I4) stock. Accordingly, various parental reciprocal matings involving the, a1 b sp; ve st ca, R(SD)(cn-I4)-x4/In(ZLR)SM5 and the T-O7/Zn(ZLR)Cy, bw strains were made. Progeny were raised at 28.5 C, and the F1 progeny females were tested for their ability to produce eggs. Results are shown in Table 9. The T-O7/Zn(ZLR)Cy, bw strain behaved as a P strain in which the cytotype and the T-7 chromosome were typical P, whereas the Zn(ZLR)Cy, bw was an M type chromosome. All the cn bw, a1 b sf; ve st ca and the In R(SD)(cn-I4)- x4/(2lr)sm5 strains had M chromosomes and M cytotype. Therefore, the kz values smaller than.5 for the R(SD)(cn-l4)-x4 chromosome were not caused by the P-M system of hybrid dysgenesis. How was the R(SD)(cn-ll)-x chromosome generated? When the R(SD)(cn-I4)-x chromosomes were isolated from the R(SD)(cn-14)/Zn(ZLR)Cy strain, it was first thought that the induction of the instabilities in their k2 values occurred through some mechanism associated with the hybridization between the R (So)(cn-I4)/Zn(ZLR)Cy and the a1 6 sp; ue st ca strains. Since the number of matings made at that time was relatively small (only 45 matings), a large scale repetition was deemed valuable. Accordingly, a large number of the crosses, a1 b sp; ue st ca females X R(SD)(cn- 14)/Zn(ZLR)Cy males, were made. Their F1 progeny males, R(SD)(cn-I4)/al b sp; + + +/ue st ca, were individually backcrossed to a1 b sp; ve st ca females in the expectation that, if hybridization between the two strains was the cause, the results originally observed in 1985 should be repeated. The results, however, were unlike the original set. Among the 45 matings tested in 1985, nine (2%) showed k2 values (when calculated based upon all four progeny phenotype classes) of smaller than.5. Among the 63 repeated matings (data are presented in Table 4), only 24 (4%) of them gave k2 values less than.5. Since each k2

7 Segregation genative Distortion 269 TABLE 9 Fertility of FI females from the matings shown, when raised at 28.5 Parental genotypes F, progeny females Male Female Genotypes F Sb %F a1 b sp; ue st ca R(SD)(cn-Z#)-x#/In(2LR)SM5 R(SD)(cn-l4)-~4/ln(2LR)SM5 a1 b sp; ue st ca T-O7 IIn(ZLR)Cy, bwd T-O7/Zn(2LR)Cy bw a1 b sp; ue st ca T-O7/In(ZLR)Cy bw a1 b sp/r(sd)(cn-i#)-x# a1 b sp/in(zlr)sm5 a1 b sp; ve st ca al b sp/r(sd)(cn-z#)-x# a1 b sp/ln(zlr)sm5 R(SD)(C~-Z#)-X~/I~(~LR)SM~ /R(SD)(cn-Z4)-x# a1 b sp; ve st ca R(SD)(cn-I4)-x#/In(2LR)SM5 T-O7/Zn(ZLR)Cy bw a1 b sp; ve st ca T-O7/Zn(ZLR)Cy bw T-O7/In(ZLR)Cy bw /In(ZLR)SM5 /R(SD)(cn-I4)-~4 lin(2lr)sms a1 b spl a1 b spl T-O7/R(SD)(cn-Z4)-x# T-O7/In(2LR)SM5 R(SD)(cn-I#)-x#/In(ZLR)Cy, bw T-OO7/R(SD)(cn-Z4)-x4 T-O7/In(2LR)SM5 R(sD)(cn-I4)-~4/In(2LR)Cy bw a1 b SPIT-7 a1 b splin(2lr)cy bw a1 b SPIT-7 a1 b sp/in(2lr)cy bw lt-7 lin(2lr)cy bw lt-7 lzn(2lr)cy bw Fertile: number of females which produced eggs. Sterile: number of females which did not produce eggs. T-7 is known to be a typical P-type second chromosome. * In(ZLR)Cy, bw is known to be a typical type second chromosome. The R(SD)(cn-Z#)-x# and the In(ZLR)Cy, bw chromosome carried allelic lethals and therefore could not be tested Lethal 47 Lethal value was calculated based on the relatively small number of progeny per mating (average of about 41), a considerable fraction of these low kz values could simply be due to sampling chances. In fact, progeny tests performed for these low kz lines indicated that they were not heritable. Therefore, there is no evidence that the change from R(SD)(cn-I4) to R(SD)(cn- 14)-x was caused by the hybridization between the two strains. When the R(SD)(cn-14) chromosomes from this strain are made heterozygous in males with the standard chromosome, the males give k2 values for the R(SD)(cn-14) chromosomes of about.98 (see Table 1). Although no records have been kept, occasional males showing k2 values of.7 or so (comparable to those of the R(SD)(cn-l4)-x chromosomes) were found during the course of routine checks of the R(SD)(cn-I4)/Zn(2LR)Cy strain. At least in several cases, these reduced k2 values were heritable. It is therefore likely that the R(SD)(cn-I4)-x chromosomes were generated once in the R(So)(cn-14)/Zn(2LR)C~ strain, and, when the crosses were made in 1985 from which the present R(SD)(cn-I4)-x chromosomes were isolated, a single male of the R(So)(cn-I4)-x/Zn(2LR)Cy genotype was included by chance which caused a clustering recovery of the R(SD)(cn-I4)-x chromosomes. How these R(SD)(cn-I4)-x chromosomes were generated in the R(SD)(cn-I4)/Zn(2LR)Cy strain is not yet understood and awaits future studies. DISCUSSION Although the process which caused the change from the R(SD)(cn-14) to the R(SD)(cn-I4)-x chromosomes is unknown at this time, the present investigation suggests the possibility that, in Sd Rsp /Sd+ Rsps heterozygous males, the chromosome carrying the insensitive Rsp allele can be distorted by its homolog bearing the sensitive Rsp5 allele, at least for the genotypic condition presented in this study. The relative viability of the R(SD)(cn-I4)-x4/aZ b sp genotype with respect to the az b sp mutant homozygous genotype can be b sp; + + +/ve estimated from the R(SD)(cn-I4)-x4/aZ st ca female X a1 b sp; ve st ca male matings (see Table 7). It is calculated to be approximately from kz(a) and from k2(b). Using these estimates of relative viabilities, the degree of distortion, d (HIRAI- ZUMI 1977), can be estimated from the corresponding k2 values shown in Table 4:.256 for k2(a) and.281 for kz(b). Using these estimates of d, the true k2

8 27 Y. Hiraizumi values (after adjusting to the differential viabilities) are calculated to be.427 and.418 for kz(a) and kz(b), respectively. Thus, a considerable degree of segregation distortion may be taking place in the R(SD)(cn-I4)-x4/aZ b sp; + + +/ve st ca males against the R(SD)(cn-I4)4 chromosome, or probably for the R(SD)(cn-I#)-x chromosomes in general. As mentioned earlier, both HARTL S and GANETZ- KY S models predict that the chromosome carrying the Rsp allele is transmitted to the progeny in excess of the Mendelian expectation of.5 from the Sd Rspi/ Sd+ Rsp male parent. In the present study, this is true for the R(SD)(cn-l#)-x/ (Sd Rsp /Sd+ Rsp ) and the R(SD)(cn-I4)-x/Zt stw3 (Sd Rspi/Sd+ RspSS), but not for the R(SD)(cn-l4)-x/aZ b sp (Sd Rsp /Sd+, probably partially insensitive Rsp ) male hemizygous for the X and heterozygous for the third chromosome from the a1 b sp; ve st ca strain. For such a male, the results are opposite: the R(SD)(cn-I4)-x chromosome carrying the Rsp allele is transmitted in a frequency of less than Mendelian.5. Several important possibilities for this phenomenon, such as P-M hybrid dysgenesis and differential viabil- ities of segregating progeny genotypes, have been ruled out in this study, but the evidence for the negative segregation distortion has not yet been firmly established. There still remains a possibility that it is due to an unknown element or elements which is quite independent of the SD system. However, if it is proved that the observed phenomenon is in fact associated with the SD system, it will become necessary to make a major modification of the models such that the direction of interaction between the Sd product and the Rsp locus can be reversed depending upon the residual genotype. This implies the presence of a third element (or elements) which is involved in determining the direction of the Sd product-rsp locus interaction. This situation is also true for the modified HARTL s model proposed by HIRAIZUMI, MARTIN and ECKSTRAND (198), although, in their model, the reversion in the direction of interaction may occur either between the M(SD) product and Rsp locus or between the Sd product and Rsp-M(SD) complex. Since it was shown earlier that, besides the az b sp second chromosome, the presence of the ve st ca third and/or the X chromosomes from the a1 b sp; ve st ca strain appeared necessary to induce negative distortion, a complex mechanism should be involved in this phenomenon. This, as well as a proposal of a new model of segregation distortion, is to be the subject of future studies. Finally, it is worth commenting on the observation recently made by Wu et az. (1988). They reported that there was a clear correlation between the sensitiv- ity of the second chromosome and the amount of repeated copies at the Rsp region such that when a chromosome carries more copies, it is more sensitive to segregation distortion. On first sight, their observation may appear to be in favor of GANETZKY S and against HARTL S (and modified) model. However, the nature and function of the repeated sequence are not yet known. If the repeated copies are not of the Rsp locus or if they do not include the crucial part of Rsp (such as the binding site), then their observations are not necessarily contradictory to HARTL S (and modified) model. The present study has been supported by a research grant, 5 R1 GM 1977, from the National lnstitutesof Health. The author wishes to express his heartfelt thanks for their kind support. LITERATURE CITED GANETZKY, B., 1977 On the components of segregation distortion in Drosophila melanogaster. Genetics 86: HARTL, D. 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9 Distortion Segregation genative 27 1 SANDLER, L., and Y. HIRAIZUMI, 196 Meiotic drive in natural Dynamics of spermiogenesis in Drosophila melanogaster. I. populations of Drosophila melanogaster. V. On the nature of the Individualization process. Z. Zellforsch. 124: SD region. Genetics45: Wu, C. I., T. W. LYITLE, M. L. Wu and G. F. L1~,1988 SANDLER, L., Y. HIRAIZUMI and I. SANDLER, 1959 Meiotic drive Association between a satellite DNA sequence and the Rein natural populations of Drosophila melanogaster. I. The cyto- sponder of segregation distorter in D. melunoguster. Cell: genetic basis of segregation distortion. Genetics 44: TOKUYASU, K. T., W. J. PEACOCK and R. W. HARDY,1972 Communicating editor: A. T. C. CARPENTER