Evidence for Extensive Genetic Differentiation Arrangement of Drosophila Pseudoobscura and D. Persimilis and Identification of Hybrid Sterility Factors between T H E Sex-ratio and T H E Standard

This study deals with sex-ratio genes tightly linked within the Sex-Ratio inversion. By taking advantage of the fact that the Sex-Ratio chromosome of Drosophila persimilis [SR(B)] is homosequential to the Standard chromosome of D. pseudoobscura [ST(A)], we carried out two reciprocal introgression experiments. Individual segments of SR(B) or ST(A) were introgressed into the genome of D. pseudoobscura or D. perszmilis, respectively. Males possessing a hybrid SR(B)-ST(A) X chromosome and a genetic background derived from either of the two species were tested for fertility and sex-ratio expression.-It was found that, in terms of the meiotic drive genes, the Sex-Ratio chromosome differs extensively from the Standard chromosome. Because recombinations of these genes result in a complete loss of sex-ratio expression, this finding lends strong support to the hypothesis of gene coadaptation. Coadaptation, in this context, is the advantage of being transmitted preferentially. In light of this finding, the evolution of the sex-ratio system in these two sibling species is discussed.-Introgression experiments also yielded information about hybrid sterility. With reciprocal introgression, sterility interactions were found to be “asymmetric.” The asymmetry is fully expected from the viewpoint of evolution of postmating reproductive isolation. 0 explain the extensive polymorphisms of chromosomal inversions in DroT sophila pseudoobscura, Dobzhansky (1 970 and references cited therein) invoked the concept of a complex of coadapted genes, or supergene, which behaves like a single well-adapted Mendelian gene. The concept of coadaptation includes “both selection of alleles at dgerent loci within gene arrangements to produce a haploid genome that is physiologically balanced, and selection of alleles of the same loci between inversions to produce heterosis in heterokary’ Present address: Center for Demographic and Population Genetics, The University of Texas, Houston, Texas 77025. Genetics 105: 71-86 September, 1983 72 C.-I. WU AND A. T . BECKENBACH otypes” (YRAKASH and LEWONTIN 1968). It is, however, a formidable task to demonstrate the nature of genes within an inversion; in particular, to identify the number and locations of these genes and the mechanisms by which they together confer high fitness on their carriers. The difficulty is best illustrated by the disagreement over the interpretation of nonrandom associations between arrangements of the third chromosome in D. pseudoobscura and the electrophoretic alleles they carry. PRAKASH and LEWONTIN (1968, 1971) suggested that nonrandom associations are evidence that the structural genes coding for the proteins they examined are part of the coadapted gene complex. NEI and L r (1 975, 1980) stressed that the observed patterns could be explained without reference to any adaptive difference among electrophoretic alleles. To support the “supergene” hypothesis, it is sufficient, as well as necessary, to break the tightly linked gene complex and then score for fitness differences. One approach is to bring together the same chromosomal arrangement from different localities (DOBZHANSKY and PAVLOVSKY 1958). If each arrangement represents a different adaptive gene complex, recombination could disrupt the integrity of these supergenes. T o further elucidate the nature of coadapted genes in any inversion, it is, however, necessary to have chromosomal arrangements that are associated with different fitness-related phenotypes. One such system is the Sex-Ratio (SR) inversion polymorphisms present in each of the sibling species, D. pseudoobscura and D. perwnzhs. The right arm of the X chromosome (XR) of each species has two types of arrangements, referred to as Standard (ST) and Sex-Ratio (SR) arrangement, respectively. Male carriers of the SR chromosome transmit primarily (95-99%) X-bearing sperm and produce predominantly daughters. The SR chromosomes, therefore, enjoy a great advantage through meiotic drive. Previous studies showed that the inversions between the SR and ST arrangements in D. pseudoohscurn almost completely suppress recombination on XR (STURTEVANT and DORZHANSKY 1936). Since genes that jointly produce the meiotic drive effect double their chance of being transmitted by paternal carriers, it is conceivable that any inversion which completely or partially binds such genes together will increase in the population. The genes within such a new inversion would be coadapted in the sense of meiotic drive, not of viability or fertility. To verify this coadaptation hypothesis, it is necessary to show that, within the SR inversion, there are two or more genes that are indispensible for the complete expression of the sexratio trait. Tests of this hypothesis are possible due mainly to several phenomena. First, the F1 hybrid females of D. pseudoohsruru and D. perszrnzlis are both viable and fertile, although the F1 hybrid males are completely sterile (LANCEFIELD 1929; and many later studies). Second, studies of recombination frequencies between the SR chromosome of D. p~rrzinzlzr [SR(B)] and the ST chromosome of D. psPudoohctuin [ST(A)] together with studies of the banding pattern of these two chromosomes suggest that ST(A) and SR(B) are homosequential (STURTEVANT and DORZHANSKY 1936; DOBZHANSKY 1944; and this study). There are, thereSEX-RATIO AND STERILITY GENES 73 fore, three XR arrangements among the two sibling species. (For an explanation of notation, please refer to the MATERIALS AND METHODS, Notation; also, see Figure 1 for graphic representation.) Third, biochemical and visible markers are available on these chromosomes. The fact that ST(A) is homosequential to SR(B) is evidence against the hypothesis that the inversion per se (e.g., the position effect) is the cause of meiotic drive. Rather, it supports the hypothesis that the SR inversions only serve to bind the “sex-ratio” (hereafter referred to as sr to distinguish them from the chromosomal arrangements) genes tightly together by suppressing any recombination between the SR and ST arrangement. By introgressing SR(B) into D. pseudoobscura by repeated backcrossing or, reciprocally, by introgressing ST(A) into D. persimilis, the gene complexes bound together by the inversions can be broken up. The chromosomal segments of SR(B) can then be studied either individually or in various combinations for the expression of the sr trait. Reciprocal introgression procedures are necessary because they are complementary for these reasons: It was hypothesized (and confirmed in this study) that several hybrid sterility factors are distributed on the X chromosome. If the sr trait is controlled by a large number of genes dispersed on SR(B), a substantial portion of it has to be introgressed to D. pseudoobscura before sr genes can be mapped. This will tend to give rise to sterile rather than sr males. It would be more fruitful in this case to introgress a small segment (or small segments) of ST(A) into D. pel-sitnilis. On the other hand, introgression of SR(B) into D. pseudoobscura would be quite feasible for mapping sr genes if sr expression is controlled by only one or two loci. This procedure, in complement with the reciprocal introgression, also provides information on modification of sr expression by genes of the sibling species. In addition to information on the genetics of the sr trait, the introgression of segments of foreign X chromosome would also yield information about genetic interactions underlying sterility in hybrid males. Most studies on the relative effects of whole chromosomes of Drosophila on various properties of 201.9 Bh 135.7 se 105.7 Y w v eo b Bet-5 sp tt 111. B 176.7 204.2 n v D. pswdoobsurra 1