Condition-dependent signalling of genetic variation in stalk-eyes flies

Handicap models of sexual selection predict that male sexual ornaments have strong condition-dependent expression and this allows females to evaluate male genetic quality(1-5). A number of previous experiments have demonstrat ed heightened condition-dependence of sexual ornaments in response to envir onmental stress(6-9). Here we show that genetic variation underlies the res ponse to environmental stress (variable food quality) of a sexual ornament (male eye span) in the stalk-eyed fly Cyrtodiopsis dalmanni. Some male geno types develop large eye span under all conditions, whereas other genotypes progressively reduce eye span as conditions deteriorate. Several non-sexual traits (female eye span, male and female wing length) also show genetic va riation in condition-dependent expression, but their genetic response is en tirely explained by scaling with body size. In contrast, the male sexual or nament still reveals genetic variation in the response to environmental str ess after accounting for differences in body size. These results strongly s upport the hypothesis that female mate choice yields genetic benefits for o ffspring.