Transmission Conflict in X - Y Chromosomes and Consequences to Heritable Variation in Fitness in Diopsid Dipterans

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9807937 Wilkinson The evolutionary significance of sexual selection has received considerable study in the past twenty years, but controversy remains over the form and magnitude of benefits inherited by offspring as a consequence of mate choice. The work proposed in this study is designed to determine if transmission conflict between the sex chromosomes, i.e. sex-linked meiotic drive, can provide heritable variation in fitness that could be exploited by female choice. The motivation for this novel hypothesis is threefold. First, sex-linked meiotic drive influences fitness because it distorts the sex ratio. If females choose males possessing modifying genes that suppress sex-linked drive, then they will produce more offspring of the limiting sex. Sex-linked drive should be more common than autosomal drive because genetic exchange between the sex chromosomes is reduced or absent. The second motivation comes from theoretical work on the maintenance of genetic variation among Y chromosomes in general and X-linked meiotic drive in particular. This work indicates that heritable variation can be maintained either by recurrent sequential evolution of driving and suppressing alleles, a balanced polymorphism, or cyclical changes in allele frequencies. Recent work on several fly species indicates that drive-modifying genes are common and evolve rapidly. Third, theory and experimental work indicate that traits which benefit males but cost females should accumulate on the sex chromosomes. Direct evidence for an association between meiotic drive suppression and a sexually selected ornament was recently obtained in the lab. In two dramatically sexually dimorphic southeast Asian species of stalk-eyed flies (Diopsidae) long eye span was both preferred by females and indicated the presence of a Y-linked factor that suppresses X-linked meiotic drive. The work proposed here will determine how sex chromosome drive is maintained in stalk-eyed flies and, therefore, how this drive could influence the evolution of a male ornament. The extent to which X-linked drive and Y-linked suppression influence male and female fitness will be quantified in two ways. Replicate captive populations will be used to follow experimental evolution of driver and suppressor chromosome frequencies, sex ratio, and eye stalk length caused by selection. The life history components responsible for evolutionary change will be identified in a series of experiments that will involve testing each male and female genotype to determine if they differ in mating success, longevity, fertility, sperm competitive ability, or fecundity. Results from this work have the potential to influence current thinking on sexual selection. If the evidence is consistent with sex chromosome conflict influencing male eye span evolution, then the possibility that meiotic drive has influenced sexual selection in other species must be considered. This includes species known to exhibit meiotic drive, such as guppies, as well as species currently not known to exhibit meiotic drive, since one outcome of drive-suppressor coevolution is a temporary absence of detectable chromosome drive within a population.