Fisherian Sex Ratio Evolution

Genetic stocks of platyfish, X. maculatus, with a three-factor sex determination system (X,Y,W) were used to investigate Fisherian sex ratio evolution.  Populations were established with known sex-determining alleles and biased sex ratios. Population sex ratios evolved towards 1/2 along predicted trajectories. In eight of ten cases, the alleles evolved to values predicted to be at Fisherian equilibrium. In two cases, observed frequencies differed from predicted values. A likely source of violation of the predictions was variation in P-alleles linked to the sex-determining alleles. The P-locus (pituitary locus) has a major affect on age at maturation. Fitness effects due to differences in age at maturation explain at least a portion of the deviation from predicted frequencies; the faster generation time of the early P-allele can explain the difference between observed and predicted allele values. Using the same genetic stocks, populations were established at values predicted by simulations to be at sex ratio equilibrium. Over time, sex ratios did not differ significantly from 0.5, however, they had traveled along a sex ratio equilibrium curve. This change likely reflects a fitness advantage realized by individuals carrying earlier maturation P-alleles. These results suggest that early maturation P-alleles can be favored under certain conditions, but do not explain the maintenance of late maturation P-alleles in natural populations. This work lead to the project discussed directly below, as well as plans to investigate whether there are post-maturation fitness consequences correlated with P-alleles.

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