Although asexual species are very rare among plants and animals, biologists study these exceptions to understand the broader role of sexual reproduction and the diversity it engenders. The basic sexual cycle of meiosis (which produces recombinant gametes) and syngamy (fusion of gametes) arose in our single-celled ancestors and has undergone little change for a billion years. In contrast, asexual reproduction, or cloning, has arisen secondarily in most phyla of metazoan animals and cryptogamous plants. Most asexual lineages appear, however, to be evolutionary dead ends with a limited potential for further diversification. Overall, asexual species comprise only a few buds at the tips of diverse branches on a tree-of-life that is fundamentally sexual. Nevertheless, the lessons to be learned from natural clones are particularly relevant as we embark on an era when the artificial cloning of mammals, and potentially humans, are no longer fantasies.
How do clonal organisms arise from sexual ancestors? Are successful clones ecological generalists, i.e. "general purpose genotypes," or specialists? Are there any diversified and truly ancient clonal taxa? Work in this lab over the past 30 years with Mexican topminnows in the genus Poeciliopsis has explored some of these questions.
P. monacha and P. lucida reproduce sexually with their own species members. In regions in which the two species overlap however, hybridization has produced all-female biotypes that reproduce by cloning via hybridogenesis and gynogenesis. In hybridogenesis, the female mates with a male, forming a female offspring with both the maternal and paternal genomes. When that female offspring produces eggs however, the male genome is discarded. In gynogenesis, the females are triploid and also mate with males, but the male genome does not contribute to the offspring.