Differential condition dependence among morphological traits inferred from responses to heat stress is predicted by sexual selection theory

Abstract

Secondary sexual traits that are condition-dependent are expected to reveal the physiological state and/or genetic quality of individuals, and therefore, should more often be found to be under sexual selection than (1) secondary sexual traits not currently condition-dependent, and (2) nonsecondary sexual traits. In the present study, we contrasted the degree of condition dependence in three morphological traits of male Drosophila bipectinata: two secondary sexual traits (distinct components of the sex comb), one of which significantly predicts mating success in nature (segment 2), whereas the other does not (segment 1), and a nonsecondary sexual trait (sternopleural bristle number). As predicted, comb segment 2 decreased significantly in size, in response to increasing temperature during development, whereas comb segment 1 and sternopleural bristle number either did not change significantly, or increased with increasing temperature. These results support the hypothesis that conditiondependence, inferred from stress-induced reductions in trait expression, engenders a trait to sexual selection. Small-combed genotypes did not exhibit disproportionate reductions in larva-to-adult survivorship and adult body size compared to large-combed genotypes, suggesting that comb size does not reveal genotypic quality, at least as revealed by sensitivity in body size and juvenile survivorship to thermal stress.

Genetic variation and phenotypic plasticity in circadian rhythms in an armed beetle, Gnatocerus cornutus (Tenebrionidae)

“…This could generate sexual difference in activity and circadian related traits. Furthermore, sex difference in circadian characters may be especially prevalent across different environments because male sexual traits tend to be condition dependent and thus males may be susceptible to stressors like elevated temperatures (Rashed & Polak 2010). To test these ideas, we measured the locomotor activity of a number of G. cornutus genotypes (isolines) at 25 o C and 29 o C, assessing the free-running period and power of the circadian rhythm, as well as total activity, across genotype, sex and temperature environments.…”

Size and symmetry of sex combs were not related to male mating success inDrosophila melanogasterreared at different temperatures

“…In the context of sexual selection, increased size of male sexual traits may reveal male vigor and lead to differential mating success through male–male competition and/or female preference for a larger sexual trait (Rashed & Polak ). Furthermore, sexual traits may be more sensitive to environmental variation than some other “ordinary” traits (Parsons ).…”

“…Furthermore, sexual traits may be more sensitive to environmental variation than some other “ordinary” traits (Parsons ). Rashed and Polak () suggested that the sex comb size may be sensitive to the thermal environment experienced during larval development because the potential allocation of resources varies depending on the length of the development period.…”

“…Such a result is not surprising since variable responses in FA have been observed even under stressful conditions, including thermal stress (Vishalakshi ). For example, D. bipectinata males reared at 29°C produced smaller and less symmetrical sex combs than males reared at 25°C (Polak & Starmer ); in addition, the comb segment 2 decreased significantly in size in response to increasing developmental temperature (Rashed & Polak ). Recently, Polak and Tomkins () revealed that the comb phenodeviance increased with thermal stress in this species.…”