Reproductive Success and Assortative Mating among Pomoxis Species and Their Hybrids

We investigated the patterns of reproductive isolation and gene flow between crappie species Pomoxis spp. by conducting a series of controlled breeding experiments using both pure parental species and various forms of interspecific hybrids (i.e., F 1 , F 2 ) as parents. To assess relative reproductive success of mating types, progeny from each of three different experimental treatments were assayed for genotypic composition at four diagnostic allozyme loci. In the first experimental treatment, we confirmed the viability and fertility of F 1 hybrids and observed that genotypic and allelic frequencies among the F 2 progeny departed from predicted Mendelian genotypic proportions. In the second experimental treatment, we confirmed that F 3 hybrids can arise from intermating F 2 hybrids. However, the small number of progeny recovered from experimental ponds suggests that either prezygotic or postzygotic barriers to gene flow had limited reproductive success. In the third experimental treatment, we evaluated the relative reproductive success of the parental species and both reciprocal F 1 hybrids. We rejected the null hypothesis of random mating from equal mixtures of parental and hybrid brood based on a maximum likelihood mixture model estimate of resulting progeny. Within each replicate, one or both parental species were most successful, although F 2 hybrid progeny also occurred more often than expected; first generation hybrids and backcrosses occurred less frequently than expected. These and previously reported observations concerning the viability of hybrids and backcrosses indicate that the two crappie species can interbreed but that they mate assortatively. Because reproductive isolation is imperfect, we caution against intentional mixing or creation of zones of secondary contact between these species.