Evolution of pleiotropic alleles for maturation and size as a consequence of predation

Abstract

Understanding life-history evolution requires knowledge about genetic interactions, physiological mechanisms and the nature of selection. For platyfish, Xiphophorus maculatus, extensive information is available about genetic and physiological mechanisms influencing life-history traits. In particular, alleles at the pituitary locus have large and antagonistic effects on age and size at sexual maturation. To examine how predation affects the evolution of these antagonistic traits, I examined pituitary allele evolution in experimental populations differing in predation risk. A smaller size, earlier maturation allele increased in frequency when predators were absent, while a larger size, later maturation allele increased in frequency when predators were present. Thus, predation favours alleles for larger size, at the cost of later maturation and reproduction. These findings are interesting for several reasons. First, predation is often predicted to favour early reproduction at smaller sizes. Second, few studies have shown how selection acts on alleles that affect age and size at sexual maturation. Finally, many studies assume that trade-offs between these life-history traits result from antagonistic pleiotropy, with alleles that positively affect one trait negatively affecting another, yet this is rarely known. This study unequivocally demonstrates that genetically based trade-offs affect life-history evolution in platyfish.

Figure 1

Antagonistic pleiotropy results in trade-offs between age and size at sexual maturation in platyfish, with alternate P-alleles affecting life histories differently.

Figure 2

Evolutionary change in the frequency of the large male genotype ($\overline{x}$±s.e.) in predator-present (grey line) and predator-absent (black line) populations. T0, population establishment; T1, genotype frequencies quantified, followed by the introduction of predators to four populations. Changes tracked from T1–T3 (T1, n=361; T2, n=271; T3, n=506).