A Brief History of Asexual Cnemidophorus
Department of Biology
The following is a summary of the origins and current status of the asexual species of Cnemidophorus found in North America. References are included to indicate the sources of my facts, but these references represent only a small fraction of the studies on these species.
Lizards of the genus Cnemidophorus (whiptails) are generally thermophilic, highly active foragers. They are found through much of the Americas warmer regions, but are particularly speciose in the deserts of the southwestern U.S. and Mexico. The group consists of both asexual and sexual species, with the former comprising about 30% of the 45+ members of the genus .
The origin of all asexual Cnemidophorus species is attributed to hybridization between different sexual species in the genus (see figure). Both diploid and triploid species occur, the latter resulting from secondary hybridizations. [Note: Because of their asexual nature, the term "species" as applied to parthenogenetic Cnemidophorus is clearly not meaningful in the traditional biological sense. Most systematists classify an asexual Cnemidophorus "species" as all the individuals resulting from a particular sex-specific combination of sexual species . Such species may represent one or more independent clonal lineages. I follow that convention here.] While hybridization events between sexual species probably were and are not uncommon, only a small fraction appear to have resulted in the formation of asexual lineages, presumably because the particular genetic conditions necessary for parthenogenesis typically do not occur in the hybrids. The formation of all major asexual lineages is thought to have occurred on the order of a few to perhaps ten thousand years ago , with little indication of recent hybridization events having contributed substantially to current populations. While some asexual lineages may have died off, extant lineages have been very successful, particularly in the Sonoran and Chihuahuan deserts. Populations of asexual Cnemidophorus can be quite dense in appropriate habitats (pers. obs.), and range sizes of the asexual species are larger than those of some sexual species in the genus .
Asexual Cnemidophorus species are all-female and reproduce via parthenogenesis. "Fertile" diploid or triploid eggs are generated directly from oocytes via a pre-meiotic endomitosis , which occurs without cellular or nuclear division. During meiosis, identical rather than simply homologous chromosomes form tetrads, so that neither crossing-over nor segregation results in any effective recombination of genetic material. Thus all offspring are females genetically identical to their mothers (except for de novo mutations), and as a result populations of asexual Cnemidophorus species tend to have extremely low levels of genetic diversity.
Asexual Cnemidophorus also possess extremely high levels of heterozygosity as a result of their hybrid origins, with these heterozygous genomes being maintained by the parthenogenetic reproduction of the species. While sexual Cnemidophorus have allozyme heterozygosity indices of about 0.05 (i.e. 5% of loci are heterozygous) , a level typical for sexual vertebrates , the asexual species have heterozygosities of 0.24 to 0.44, among the highest values observed in vertebrates. Because most asexual species appear to consist of only a few independent clonal lineages, the variation in heterozygosity within species is very low. For the forty-plus allozymes examined, clones within species typically vary at only one or two loci, and in many cases the loci show the same degree of heterozygosity (e.g., abb rather than aab) .