COMPARISONS OF PHYSIOLOGICAL PERFORMANCE IN SEXUAL AND ASEXUAL WHIPTAIL LIZARDS (GENUS CNEMIDOPHORUS) - IMPLICATIONS FOR THE ROLE OF HETEROZYGOSITY

Many asexual animal species are of hybrid origin, with consequent high levels of heterozygosity, Data from some studies suggest that increas ed heterozygosity may be functionally correlated with superior perform ance in a variety of fitness-related traits, Thus, hybrid asexual spec ies could be expected to exhibit some degree of heterosis. This sponta neous heterosis hypothesis is tested in a comparative study of asexual and sexual species of the lizard genus Cnemidaphorus. Asexual species of the genus are parthenogenetically reproducing hybrids of the sexua l species and as a result have high levels of heterozygosity that have persisted since their origins. Five whole-organism physiological trai ts (burst speed. endurance, maximal exertion, standard metabolic rate. and evaporative water loss rate) were examined in live asexual specie s and the sexual species that gave rise to them. Trait values for sexu al and asexual species were compared using a nonphylogenetic approach and two phylogenetically controlled approaches capable of dealing with reticulate phylogenies. Ln contrast to the predictions of the heteros is hypothesis, performance for four of the traits in asexual Cnemidoph orus was not statistically different than that in their sexual parenta l species, and asexuals bad significantly worse endurance. An the whol e, the overall trend appeared to be toward worse performance in asexua ls. 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 karyotypi cally mixed genome of these hybrids, and the accumulation of deleterio us mutations in the asexual lineages via Muller's ratchet.