Sexual Selection Among Clones of Unisexual Fish (Poeciliopsis: Poeciliidae): Genetic Factors and Rare-Female Advantage

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Abstract
Interspecific hybridization between the viviparous fishes Poeciliopsis monacha and P. lucida has produced two all-female forms: P. monacha-lucida, which is diploid, and P. monacha-2 lucida, a triploid. To reproduce, both must be inseminated by males of P. lucida. Expansion of unisexual populations is limited by the ability to attract males, which discriminate against hybrid forms. Unisexual clones in nature differ in their proportions of pregnant females, and laboratory studies comparing reproductive behavior and pregnancy rates demonstrate that some clones are more successful than others at acquiring sperm. When clones are at equal frequencies in a population, some attract males more readily than others. Learning by males of P. lucida to discriminate against one unisexual clone does not generalize completely to new clones; thus, unfamiliar clones are more likely to be courted than familiar clones (Keegan-Rogers 1984). Although all members of a triploid clone are genetically identical, members of a diploid hemiclone share only an identical monacha genome. This study shows that the unfamiliar-female advantage persists among diploid hemiclones even when the lucida genome of clonal members varies, as in natural populations. Pregnancy experiments confirm that, at unequal clonal frequencies, individuals of a clone that is rare in a population and unfamiliar to a given male are more likely to be impregnated than those of a common or familiar clone. Males of all five strains of P. lucida used in this study (two highly inbred, one less inbred, and two relatively wild) preferred conspecific females to unisexuals. No differences were found among the strains in the strength of their tendencies to choose either conspecifics over unisexuals or unfamiliar clones over familiar clones. We discuss how genetic differences in sexual attractiveness among clones and a rare-female advantage may help to explain the widespread establishment and continued coexistence of numerous clones.