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Studies on the genetics of the sex-ratio trait in the two sibling species of Drosophila : D. Pseudoobscura and D. persimilis Wu, Chung-I

Abstract

Many species of Drosophila have an X chromosome inversion polymorphism: the Standard arrangement and the Sex-Ratio (SR) arrangement. Male carriers of SR transmit predominantly X-bearing sperm and hence produce nearly all-female progeny. In the absence of strong counteracting selection, SR would be meiotically driven to fixation, causing population extinction. In PART I, the role of virility selection in maintaining the SR polymorphism in D. pseudoobscura is examined by partitioning it into two components. The SR males are found to suffer substantial virility reduction which would not be detected if the differences between the two components are not heeded. The significance of this finding is discussed in light of two specific observations usually associated with the Sex-Ratio polymorphism in this species. One is the absence of suppressor modifiers of "sex-ratio" expression and the other is the temperature-dependent distribution of the sex-ratio trait. In PART II, a model is proposed to describe the behavior of the autosomal suppressor modifiers of "sex-ratio" meiotic drive. These modifiers, if neutral in other respects, should increase because they tend to be associated with the rare sex (males). However, selection operating on the sex-linked drive locus will sometimes act against autosomal modifiers as well because the two loci are always in gametic phase disequilibrium. Conditions under which modifiers will not increase are presented in terms of the fitnesses of different genotypes at the sex-linked drive locus. To prevent an increase in modifier alleles, the fitness of Sex-Ratio males relative to Standard males has to be no greater than 0.3. This result is in good agreement with that of PART I. PART III deals with "sex-ratio" genes, tightly linked within the Sex-Ratio inversion. By taking advantage of the fact that the Sex-Ratio chromosome of D. persimilis (SR(B)) is homosequential to the Standard chromosome of D. pseudoobscura (ST(A)), two reciprocal introgression experiments were carried out. Individual segments of SR(B) or ST(A) were introgressed into the genome of D. pseudoobscura or D. persimilis, 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, evolution of the sex-ratio system in these two sibling species is discussed. Introgression experiments also yielded information about hybrid sterility. Four types of sterility interactions were identified; one of them involved at least three genetic elements. With reciprocal introgression, sterility interactions were found to be "asymmetric". The asymmetry is fully expected from the viewpoint of evolution of postmating reproductive isolation.

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