Mate choice and the Amazon molly: how sexuality and unisexuality can coexist.

One of the most complex and important problems in modern evolutionary biology remains to explain the evolution and persistence of sexual reproduction given the costs of sexuality. One way to study the relative strengths and weaknesses of sexuality and asexuality is to study the dynamics of the coexistence of sexual and asexual organisms. An excellent model system for such an approach is the unisexual Amazon molly, Poecilia formosa. This livebearing fish is clonal, but requires sperm to trigger embryogenesis. Amazon mollies need to obtain sperm from males of closely related species. In an effort to understand the apparently paradoxical coexistence of the Amazon molly with its sexual hosts, many theories were tested, but most seem to have little relevance to understanding stability, whereas male mating behavior is potentially very important.