Part front matter for Part II Unisexual Clonality in Nature

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Clonality is often associated with unisexuality—the presence of only females within a species. Indeed, the observation of a strongly female-biased sex ratio often provides the initial hint that a taxon under investigation might be a clonal reproducer. In 1932, Carl and Laura Hubbs described the first all-female vertebrate species known to science. It was a fish, which they named the Amazon molly in honor of a fabled human tribe of all-female warriors. The seemingly outlandish notion that these creatures reproduce clonally was later confirmed in the laboratory (Hubbs and Hubbs, 1946). Since then, dozens of all-female clonal or quasi-clonal taxa have been identified in various groups of fish, amphibians, and reptiles from five continents (Dawley and Bogart, 1989; Vrijenhoek et al., 1989). Collectively, these constitute about 0.1% of all extant vertebrate species.

It might appear that an absence of males in any unisexual taxon would sentence females to sexual abstinence and strict clonality, and indeed this is sometimes true. In other cases, however, females in a unisexual taxon mate with males of related bisexual (gonochoristic, or two-sex) species and then use the sperm to facilitate their own clonal or hemiclonal reproduction. Sperm-independent unisexuality (parthenogenesis) will be the topic of chapter 3, and sperm-dependent forms of unisexuality (gynogenesis, hybridogenesis, and kleptogenesis) will be described in chapter 4. All unisexual lineages of vertebrate animals appear to have arisen via hybridization between bisexual species. Thus taxa that are now asexual or quasi-asexual had long prior evolutionary histories of sexual reproduction, indicating that vertebrate clonality is a derived (not ancestral) as well as a polyphyletic (multiorigin) condition. In recent decades, much has been learned about the genetics, evolution, and ecology of these recurring transitions from sexuality to clonality.

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