Host-Size-Matching in a Sperm-Dependent Asexual Fish
Among vertebrates, there are approximately fifty sperm-dependent asexual species, all of which are of hybrid origin. Sperm-dependent asexual vertebrates use sperm from one, or both, of their parental species for reproduction. To address the hypothesis that selection has resulted in asexual phenotypes that resemble their host’s phenotype, I studied size, shape, and genetic variation (using six microsatellite loci) in nine populations of a sperm-dependent asexual fish, Chrosomus eos-neogaeus, in Alberta, Canada. These nine populations differed in the presence or absence of each of the two parental species: three populations coexist with C. eos, two populations coexist with C. neogaeus, and four populations coexist with both parental species. Consistent with my hypothesis, I found that C. eos-neogaeus tended to match the body size of the parental species with which they coexist, and that C. eos-neogaeus that coexist with only C. eos were genetically divergent from C. eos-neogaeus that are syntopic with only C. neogaeus. The genetic divergence among C. eos-neogaeus populations was independent of geographic distance among populations, and estimates of quantitative trait divergence among C. eos-neogaeus populations exceeded neutral expectations. These observations suggest that processes other than migration, mutation, and drift are likely shaping the diversity of C. eos-neogaeus in the lakes sampled for this study. For example, sexual selection leading to host mimicry, or natural selection leading to environmental adaptation, may explain the observed pattern of host-size matching.