Department of Biology
Alistair J. Cullum
Department of Ecology and Evolutionary Biology
University of California, Irvine
Irvine, CA 92697-2525
American Naturalist 150(1):24-47 (1997)
Abstract - Many asexual animal species are of hybrid origin, with consequent high levels of heterozygosity. Data from some studies suggest that increased heterozygosity may be functionally correlated with superior performance in a variety of fitness-related traits; thus hybrid asexual species could be expected to exhibit some degree of heterosis. This "spontaneous heterosis" hypothesis is tested in a comparative study of asexual and sexual species of the lizard genus Cnemidophorus. Asexual species of the genus are parthenogenetically-reproducing hybrids of the sexual species, and as a result have high levels of heterozygosity which have persisted since their origins. Five whole-organism physiological traits (burst speed, endurance, maximal exertion, standard metabolic rate and evaporative water loss rate) were examined in five asexual species and the sexual species that gave rise to them. Trait values for sexual and asexual species were compared using a non-phylogenetic approach and two phylogenetically-controlled approaches capable of dealing with reticulate phylogenies. In contrast to the predictions of the heterosis hypothesis, performance for four of the traits in asexual Cnemidophorus was not statistically different than in their sexual parental species, and asexuals had significantly worse endurance. On the whole, the overall trend appeared to be towards worse performance in asexuals. An obvious interpretation of these results is that heterozygosity and "vigor" need not be functionally related. However, other factors may be counterbalancing possible beneficial effects of heterozygosity, including detrimental epistatic effects resulting from the karyotypically mixed genome of these hybrids, and the accumulation of deleterious mutations in the asexual lineages via Mullers ratchet.