Fluctuating asymmetry in an extreme morphological adaptation in the Chilean bee Xeromelissa rozeni (Hymenoptera: Colletidae)

Abstract

Fluctuating asymmetry (FA), random deviations from perfect symmetry in bilateral traits, is a common inverse measure of developmental stability (DS), which is related to one’s ability to buffer against environmental and genetic perturbations. There is a widespread hypothesis that heterozygosity grants an increased ability to compensate for developmental errors caused by genetic and environmental factors, rendering homozygous individuals less symmetric than heterozygous ones. In addition, if natural selection on FA is common, nonessential traits should exhibit higher asymmetry than functionally essential traits. This is especially well tested in haplodiploid organisms, which present a clear distinction between “homo”zygosity (males) and heterozygosity (females). Relatively few FA studies looked at this relationship in hymenopterans or in haplodiploid organisms in general and the results are rather inconsistent. This study compares FA measurements of seven parts of the maxillary palpus, with sclerotized and membranous parts scored separately, and two wing venation characters for males and females of the Chilean bee Xeromelissa rozeni (Toro and Moldenke, 1979). The results of this study suggest that there is an equally strong selection force for maxillary palp symmetry in both males and females leading to a relatively low FA in both sexes, and that less functional traits exhibit higher FA due to relaxation of selection. Lastly, we stress the importance of testing a larger number of independent traits.