The sibling species Drosophila pseudoobscura and D. persimilis each carry two gene arrangements in the right arm of the X chromosome; Standard (ST).
GENE DIFFERENCES BETWEEN THE SEX RATIO AND STANDARD GENE ARRANGEMENTS OF THE X CHROMOSOME IN DROSOPHILA PERSIMILIS DAVID POLICANSKY
AND
E. ZOUROS
Department of Biology, Uniuersity of Massuchuseits, Boston, Massachusetts 02125 AND
Depariment of Biology, Dalhousie Uniuersity, Halifax, Nova Scoiia B3H4Jl Manuscript received March 24, 1976 Revised copy received August 25,1976 ABSTRACT
The sibling species Drosophila pseudoobscura and D . persimilis each carry two gene arrangements i n the right arm of the X chromosome; Standard (ST) and Sex Ratio (SR). The SR sequence of D . persimilis and the S T sequence of D.pseudoobscura have the same banding pattern. These cytologically identical arrangements carry different alleles a t the esterase-5 (Est-5) and phosphoglucomutase-I (Pgm-I) loci. All Ihe alleles on the SR arrangement of D . persimilis are also present on the ST arrangement of that species, and the same allele was most common in all cases. However, the allele frequencies at the Pgm-I locus are significantly different between the two arrangements. Within the ST arrangement of D. persimilis, the alleles a t the Est5 and Pgm-I loci are in linkage equilibrium.
ATURAL populations of Drosophila persimilis and D. pseudoobscura are Npolymorphic for inversions on the third chromosome and to a lesser extent in the right arm of the X chromosome. The species share two inversions on each of these chromosomes: the Standard (ST) sequence in the third chromosome is cytologically the same for both species, and the Sex Ratio (SR) sequence of the X chromosome of D. persimilis is the same as the Standard (ST) sequence of the X chromosome of D. pseudoobscura (DOBZHANSKY and EPLING1944). PRAKASH and LEWONTIN ( 1968, 1971) have compared the allele frequencies at three loci occurring in different arrangements of the third chromosome in both D.persimilis and D . pseudoobscura. There is considerable genic differentiation between the various arrangements, but the allele frequencies in the ST arrangement shared by the two species are not very different. Pt-IO is fixed for the same allele in both species and the most common allele at the a-amylase locus is the same in both (1 .OO; more than 50% in both species). The pattern is somewhat different for the Pt-12 locus. There are two alleles segregating in both species, but the more common allele in D.pseudoobscura (1.20; 80%) is the less common in D.persimilis (40%). This work was supported in part by a Faculty Research Grant from the University of Massachusetts and by a grant from the National Research Council of Canada Genetics 85: 507-511 March, 1977.
508
D. POLICANSKY A N D E. ZOUROS
Unlike the ST arrangement of the third chromolsome, the gene arrangement of the X chromosome shared by the two species produces strikingly different phenotypes in each. Males of D.persimilis produce mostly female progeny, while the progeny of D.pseudoobscura males have a normal sex ratio. The selective values of this strange phenotype is not understood, but it is likely that it has been present f o r a long time (POLICANSKY 1974). It is with the aim of learning more about the evolutionary relationships of the inversions involving this trait that we have compared allele frequencies at two loci on the right arm of X chromosomes of D.persimilis bearing SR and ST arrangements with allele frequencies on the two arrangements of D.pseudoobscura chromosomes. MATERIALS A N D METHODS
Flies were trapped near Johnson City, California, in the summer of 1974 and a t Big Stump Campground, California, in the summer of 1975. The first location is a t about 1850m elevation in the Kern River Valley north of Bakersfield, and the second is at about 1650m in the Coast Range west of Paskenta. Males and one son from each female were mated to a laboratory strain of D.persimizis homozygous for known alleles at the esterase-5 (Est-5) and phosphoglucomutase-I (Pgm-I) loci. Electrophoresis was done on the progeny of these matings. The species of each male was determined as follows: if the male produced fertile sons it was D.persimilis. If its sons were sterile it was ST D.pseudoobscura. If no or very few sons were produced, the fly was SR and the daughters were mated again to the laboratory strain. If the SR fly had been D.pseudoobscura, its daughters were hybrids and produced very few or no progeny on mating with the laboratory strain, and the few males which were produced were sterile. In this way SR males could be identified as well as the species to which they belonged. Electrophoresis of the two loci was performed according to the techniques of PRAKASH, LEWONTIN and HUBBY(1969) for Est-5. We used the technique of AYALA et al. (1972) for Pgm-I. We also attempted to use the technique described by PRAKASH (1974) for Odh-3, but we were able to obtain bands for Odh-1 only. RESULTS A N D DISCUSSION
We collected over 400 flies in the Kern River Valley, and of 400 chromosomes tested, 12 were pseudoobscura ST and 388 were persimilis ST. Of 600 chromosomes tested from Big Stump, 35 were pseudoobscura ST and 10 were pseudoobscura SR, and of the 555 persimilis chromosomes only 23, or 4%, were SR, and one of these was lost. The allele frequencies at the Est-5 and Pgm-l loci are presented in Table 1 with the allele frequencies in D.pseudoobscura for comparison. All but 27 of the chromosomes tested for those two loci were from Big Stump; the only difference in allele frequencies in the chromosomes from the Kern River Valley was the presence of the 1.29 allele of Est-5 in three cases (11%). Since the allele frequencies were almost identical in all other respects, we have pooled the data. There is no evidence of differentiation between SR and ST at the Est-5 locus in D.persimilis. Dividing the alleles into two classes, 1.20 and others, we find exact test that the observed deviation from the expected associations by FISHER’S is not significant ( P = 0.212). There is differentiation at the Pgm-l locus; by FISHER’S exact test P = 0.025 (Table 2). Analysis of the frequencies of gametic types shows no significant association of alleles at the Est-5 and Pgm-1 loci within the persimilis ST arrangement.
GENES AND INVERSIONS
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D.persimilis
TABLE 1 Comparison of allele frequencies at two loci on the X chromosomes of D. persimilis and D. pseudoobscural Allele
null 0.85 0.90 0.95 0.97 1.oo 1.02 1.04 1.07 1.12 1.16 1.20 1.29
n
useudo. SR
Esterase-5 pseudo. ST persim. SR
persim. ST
.01
0
0
.02
0 0
0 0
.08 .Ol .50
0 0
.02 26 .10 .01 0 0
0 0 0 0 0 ,05 0 0 .95 0
0 0 0 0 0 .04. .07 .01 .87
99
750
22
182
0 0 0 0
.06 .03 0 .91 0 0 0
*
*
.os
Phosphoglucomutase-l
0.7 1 .o 1.5 2.0
n _____~
.02 .95 .03 0
.65 .34 .01
0 .05 .95 0
99
527
22
0
_____________
0 .25 .75 0
____
114
__
from PRAKASH (1974). * Less than .005. Classification of loci and alleles follows that of PRAKASH (1974).
1 Data
There are three main points to be made in summary of the results. The first is that the allele frequencies at the Est-5 and Pgm-I loci reported here are similar to those reported by PRAKASH (1969) for Est-5 and by DOBZHANSKY and AYALA(1973) for Pgm-I. Secondly, the SR arrangement appears to be poorer than the ST arrangement in allozyme polymorphism; this is in agreement with the situation in D.pseudoobscura. However, due to the rarity of the persimilis SR arrangement in our large samples, it is possible that this apparent lack of variation is due to sampling error. The third and most important point is illustrated in Table 3, where we present NEI’S (1972) index of genetic similarity between the four arrangements with respect to the two loci we have examined. There is considerable genic differentiation between the persimilis SR arrangement and the cytologically indistinguishable pseudoobscura ST arrangement, and even more between the prsimilis SR and the phenotypically similar pseudoobscura SR arrangements. In other words, the alleles are more species-specific than they are inversion- or phenotype-specific.
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D. POLICANSKY A N D E. ZOUROS
TABLE 2 Test of homogeneity between SR and ST gene arrangements at the Pgm-I locus in D. persimilis Pgm-l allele
Arrangement
ST SR Total
11
1.5 86
1.0 28 1 29
114
22 136
21 107
P = ,025 by Fisher’s exact test.
We conclude that the identical gene arrangements persimilis SR and pseudoobscura ST have undergone considerable genic differentiation in the two species. This differentiation may or may not be related to the fact that this arrangement causes a highly abnormal sex ratio among the progeny of the males of one of the species. The cytochemical basis of this distorted sex ratio is not understood, but the present comparison, which shows that the persimitis SR arrangement is much more similar genetically to the persimilis ST than to the pseudoobscura ST arrangement, and that the two SR arrangements are least similar, at least with respect to these two loci, makes it clear that the loci themselves are not directly involved in the phenotypic effect. In spite of recent evidence that certain regions of the chromosome are preferentially involved in inversions, and that inversions may arise more often than previously thought (YAMAGUCHI and MUKAI1974), the sequence of events required f o r the same arrangement to arise independently in two species and then become incorporated is a very improbable one. It seems more likely that the persimilis SR-pseudoobscura ST arrangement predates the divergence of the two species. If this is the case, then the present genic constitution of SR in D. persimilis has apparently been strongly influenced by continuous recombination pressure from the (now) much more common ST arrangement, in spite of the inverted segments. This influence is not apparent in D.pseudoobscura, in which the inversions overlap. In view of our results, we feel that PRAKASH’S (1974) explanation of the marked differences between the alleles associated with pseudoobscura SR and ST as being due to selection is only one possibility. It is easy to believe that random TABLE 3 Nei’s index of genetic similarity between any two gene arrangements of the X chromosome of D. persimilis and D. pseudoobscura
pseudo. SR pseudo. ST pers. SR pers. ST
pseudo. SR
pseudo. ST
persimilis SR
persimilis ST
-
.477
.038
-
.256
.170 .374
-
.981 -
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events produced the difference in D. pseudoobscura, especially since there is little crossover between these inversions (STURTEVANT and DOBZHANSKY 1936). We thank J. COYNEand J. SOHNfor helpful discussion and critical review of this paper. LITERATURE CITED
AYALA,F. J., J. R. POWELL, M. L. TRACEY, C. A. M o u G o and S. PBREZ-SALAS, 1972 Enzyme variability in the Drosophila willistoni group. IV. Genic variation in natural populations of Drosophila willistoni. Genetics 70 : 113-139. DOBZHANSKY, TH. and F. J. AYALA,1973 Genetics of natural populations. IX. Temporal frequency changes of enzyme and chromosomal polymorphisms in natural populations of Drosophila. Proc. Natl. Acad. Sci. U.S. 70: 680-683. DOBZHANSKY, TH. and C. EPLING,1944 Contributions to the genetics, taxonomy and ecology of Drosophila pseudoobscura and its relatives. Carnegie Inst. Wash. Publ. 554, Washington, D.C. NEI, M., 1972 Genetic distance between populations. Am. Naturalist 106: 283-292. POLICANSKY, D., 1974 5 e x ratio,” meiotic drive and group selection in Drosophila pseudoobscura. Am. Naturalist 108: 75-90. PRAKASH, S., 1969 Genic variation in a natural population of Drosophila persimilis. Proc. Natl. Acad. Sci. U.S. 62: 778-784. -, 1974 Gene differences between the Sex Ratio and Standard gene arrangements of the X chromosome and linkage disequilibrium between loci in the Standard gene arrangement of the X chromosome in Drosophila pseudoobscura. Genetics 77: 795-804.
PRAKASH, S., and R. C. LEWONTIN,1968 A molecular approach to the study of genic heterozygosity in natural populations. 111. Direct evidence of coadaptation in gene arrangements of Drosophila. Proc. Natl. Acad. Sci. U.S. 59: 398-450. --, 1971 A molecular appreach to the study of genic heterozygosity in natural populations. V. Further direct evidence of coadaptation in inversions of Drosophila. Genetics 60: 405-408.
PRAKASH, S., R. C. LEWONTINand J. L. HUBBY, 1969 A molecular approach to the study of genic heterozygosity in natural populations. IV. Patterns of genic variation in central, marginal and isolated populations of Drosophila pseudoobscura. Genetics 61: 841-858. 1936 Geographical distribution and cytology of “sex STURTEVANT, A. H. and TH.DOBZHANSKY, ratio” in Drosophila pseudoobscura and related species. Genetics 21 : 473-490. O., and T. MUKAI,1974 Variation of spontaneous occurrence rates of chromosomal YAMAGUCHI, aberrations in the second chromosome of Drosophila melanogaster. Genetics 78: 1209-1221. Corresponding editor: R. C. LEWONTIN
