Tarsal asymmetry, trait size, and extreme phenotypes in a sexually size-dimorphic water boatman Callicorixa vulnerata

Abstract

Fluctuating asymmetry (FA) in morphological traits can vary with the size of characters and the mode of selection acting on them. In a previous study, mid-leg tarsal FA (tarsal spine number and tarsal length) in the water boatman Callicorixa vulnerata was inversely related to fitness in both sexes. The mid-legs of water boatmen are used to cling to bottom substrate during underwater feeding and if under stabilizing selection, extreme phenotypes are predicted to exhibit elevated FA. In this study, it is shown that water boatmen with large or small mid-leg tarsal traits tend to have higher levels of tarsal FA than modal phenotypes, possibly because of increased homozygosity, poor genomic balance, or greater stress during development in extreme phenotypes. This relationship suggests selection against asymmetry per se will indirectly impose stabilizing selection on trait size. Regression analyses revealed that the relationship between FA and trait size differed between the sexes and was best described by a U-shaped distribution in females but by a relatively flat, negative linear association in males. These results indicate possible directional selection on male tarsal traits. Alternatively, they suggest associations between FA and trait size do not always reflect the mode of selection acting on a trait.