Sex and the single fish—or what good is genetic diversity?

Robert Vrijenhoek, Ph.D.
Monterey Bay Aquarium Research Institute

Wednesday, December 1, 1999
3:00 p.m.—Pacific Forum

Mary Hirsch, Artist

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.

Three main questions concerning asexual organisms are addressed: (1) How do clonal organisms arise from sexual ancestors? (2) Are successful clones ecological generalists, i.e. "general purpose genotypes," or specialists? and (3) Are there any diversified and truly ancient clonal taxa? Examples, and hopefully some answers, will come from our work during the past 30 years with Mexican topminnows in the genus Poeciliopsis.

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 Last updated: December 19, 2000