Fitness Analysis of the "Sex-Ratio" Polymorphism in Experimental Populations of Drosophila pseudoobscura

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Abstract
Sex-ratio (SR) is a naturally occurring X-linked trait which, in males, causes the elimination of Y-bearing sperm during spermiogenesis. Virtually all of the progeny of SR males carry the trait. Despite its natural tendency to increase, which is caused by meiotic drive, the trait is maintained at equilibrium in many natural populations. In experimental populations it is usually reduced or eliminated by the standard X chromosome (ST). A series of 6 replicated experimental population designs were studied, each differing from the others by 1 or more life history parameters. The simplest life history (discrete generation, single mating of females, low density) allowed complete analysis of the components of fitness. Other designs, by comparison, provided evidence concerning the effects of each of the life history complications on the dynamics of SR. In addition to the meiotic drive present in all populations, viability, fecundity (gamete production), and virility (male sexual) selection were operating in some of the populations. The adult components, particularly that of the SR males probably made the greatest contributions to the SR frequency changes. Viability of SR males was found to be density-dependent. If this result is general, then assessment of the role of viability selection will require some understanding of the population densities involved. It was noted that superficially similar gene frequency change trajectories can arise through very different mechanisms.