Effects of Sexual Selection and Predation on a Genetic Polymorphism for Body Size

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

The Effects of Sexual Selection and Predation on a Genetic Polymorphism for Body Size Alexandra Basolo The long-term goal of this study is to understand the factors that maintain variation within populations in genes that affect adult body size, and the factors that cause differences between populations. In many animal systems, variation for adult body size is common. Both natural selection and sexual selection can contribute to the maintenance of body size variation. Intrasexual selection via male-male competition often favors large male size, as larger males more often win contests. However, small males may adopt alternative mating strategies, such as sneaking, to avoid direct contests as well as subvert female choice for larger males. Natural selection via predation may also favor the maintenance of certain size classes. In some species, the level of predation risk is positively correlated with body size, and in other species, it is negatively correlated with body size. In addition, generation time may favor smaller individuals if smaller individuals reproduce at an earlier age. Thus, sexual selection, predation and generation time may affect the evolution of body size. The traditional view for the evolution of sexually-selected traits is that such traits evolve until they are countered by some source of natural selection, usually predation. When sexual selection and predation both favor larger size, however, life history correlates of size, such as age at first reproduction, may constrain the evolution of body size. Variation between individuals in age and size at sexual maturation is commonly observed, but the underlying genetic basis of this variation is rarely known. However, such knowledge is important to understand the evolutionary mechanisms responsible for body size variation. This study proposes to investigate the factors affecting allelic variation at a locus (the P-locus) that strongly influences adult body size in the southern platyfish, Xiphophorus maculatus. Alleles at this locus vary within and across natural populations. A primary effect of these alleles is the initiation of sexual maturation. Individuals with different P-alleles become sexually mature at different times. Females continue to grow after sexual maturation, but males grow very little after sexual maturation. These alleles then affect both the size at sexual maturation and adult body size in males. Genetic stocks are available with linked color patterns. It is thus possible to use these color markers to track changes in P-allele frequencies in experimental populations. X. maculatus therefore provides a known genetic system with which to test hypotheses about body size polymorphism. A set of experiments will be conducted to examine the effect of sexual selection and natural selection via predation on the evolution of P-alleles. First, male-female mating interactions will be observed in the absence of direct male-male competition for both large and small male morphs. Second, the mating success of large and small male morphs will be assessed. In these experiments, large and small males will be tested with females to determine if male mating behavior changes in the presence of another male, e.g. if smaller males adopt an alternative mating strategy. Male mating success will be determined by progeny analysis. Third, the importance of natural selection via predation on body size will be investigated and P-allele evolution will be quantified in populations with predatory fish and populations without predatory fish. Color pattern markers will be used to track changes in P-allele frequencies over time. A comparison between these populations will determine to what level predation maintains P-alleles for larger male body size by tracking the change in P-allele frequencies in each replicate and comparing the trajectories.