Physiological Consequences of Asexuality in Cnemidophorus
|Alistair J. Cullum
Department of Biology
My dissertation work was a study of physiological performance in lizards of the genus Cnemidophorus. This genus is unusual in that it contains a large number of asexual species, all of which had their origin in relatively rare hybridization events occurring between sexual species in the genus. These hybrids produced subsequent generations by the process of parthenogenesis, resulting in strictly clonal, all-female lineages that have shown considerable ecological success. I was interested in discovering how asexual Cnemidophorus might differ from their sexual counterparts with regard to five whole-animal traits. Three of these were locomotor traits, each of which reflects some aspect of physiological capacity: burst running speed (reflecting maximal muscular and biomechanical performance), treadmill endurance (a measure of aerobic capacity) and maximal exertion (a measure of anaerobic capacity). I also measured standard metabolic rate and evaporative water loss rate. All of these traits are either known or believed to be important determinants of fitness in desert reptile species like Cnemidophorus.
The following questions were asked using these measures of physiological performance:
Many asexual animal species, in fact, 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. I tested this "spontaneous heterosis" hypothesis in five asexual species of Cnemidophorus and the sexual species that gave rise to them. For the five traits above, population means for females of sexual and asexual species were compared using phylogenetically-controlled approaches. In contrast to the predictions of the heterosis hypothesis, asexual Cnemidophorus had significantly worse endurance than predicted, and the overall trend among all traits appeared to be towards worse performance in asexuals. These results support other recent findings 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.
One of the major potential disadvantages to asexual reproduction is believed to be a reduction in phenotypic variability. If rare phenotypes are important in surviving occasional intense selection events, or if different phenotypes are favored at different times, populations with a greater variety of phenotypes may be more successful in the long term. The degree to which asexuality actually reduces population variability has rarely been studied, however. I examined the variance of the above traits in four asexual species of Cnemidophorus, and their parental species, again with females only and using phylogenetically-controlled methods. The analysis revealed lower levels of trait variance in asexual species for the first three traits, but no detectable differences between asexual and sexual species for the other two traits. A second analysis examined the influence of each populations maximum and minimum value on overall population variability, and revealed that the variances of sexual populations were more heavily influenced by outliers than were asexual ones. These results suggest that part of the reason for increased variance in sexual populations may be a greater tendency for these populations to produce extreme phenotypes.
One of the obvious differences between sexual and asexual species of Cnemidophorus is the presence of males in the former. Both of the above studies focused on females so that comparisons would be more conservative, but I became interested in knowing how males might influence population trait distributions. Additionally, I wished to determine the degree to which any observed gender differences in performance might be explained simply by differences in body size, rather than differences in underlying physiology. An examination of six sexual species revealed a strong trend towards higher trait values in males. At least some of this trend was explained by differences in body size (with males being longer and proportionately more massive than females), but a sizable fraction of the dimorphism in whole-animal performance could only be explained by differences in physiology, most likely due to the effects of testosterone and other androgens.