Condition‐dependence and sexual ornamentation: Effects of immune challenges on a highly sexually dimorphic grasshopper

Abstract

Sexual ornaments contribute substantially to phenotypic diversity and it is particularly relevant to understand their evolution. Ornaments can assume the function of signals-of-quality that the choosy sex uses to evaluate potential mating partners. Often there are no obvious direct benefits and investment into mate choice is primarily rewarded by beneficial alleles that are inherited to the offspring. Inter-sexual communication via sexual ornaments requires honesty of the sexual signal, yet the question of what maintains honesty remains only partially solved. One solution is that honesty is maintained by trait expression being dependent on individual condition, since condition-dependent trait expression offers an effectively inexhaustible source of genetic variability. Here we test in the highly sexually dimorphic club-legged grasshopper Gomphocerus sibiricus if putative sexual ornaments, in particular the striking front-leg clubs, are more strongly affected by a lipopolysaccharide (LPS) immune challenge than putatively not sexually selected traits. Our results show overall little condition-dependent expression of morphological and song traits, with sexually selected traits exhibiting effects comparable to nonsexually selected traits (with the possible exception of stridulatory file length and syllable-to-pause ratio in advertisement songs). Interestingly, field observations of individuals of lethally parasitized individuals suggest that a very strong environmental challenge can specifically affect the expression of the front-leg clubs. The presence of 1% of males in natural populations with missing or heavily deformed clubs plus 5% with minor club deformations furthermore indicate that there are risks associated with club development during final ecdysis and this might act as a filter against deleterious alleles.

Recent advances in understanding the mechanisms of sexually dimorphic plasticity: insights from beetle weapons and future directions

“…On the other hand, many traits that exhibit sexually dimorphic plasticity in insects are ornaments or signals, not beetle weapons, and are not expected to play by the same rules [54]. Examples of condition-dependent ornaments include wing pigmentation in damselflies [55,56], wing melanization in dragonflies [57], forelimbs in grasshoppers [58], calling songs in crickets [59], and pheromone production in beetles [60]. Unfortunately, the function of dsx itself, much less the effect of this gene on sexually dimorphic plasticity has been little studied outside of the context of condition-dependent weaponry and the evolution of insect sexual differentiation.…”

Transcriptome assembly for a colour-polymorphic grasshopper (Gomphocerus sibiricus) with a very large genome size

“…Here, we present a de novo transcriptome assembly for the club-legged grasshopper Gomphocerus sibiricus , an alpine-dwelling species that is unusual for its striking sexual dimorphism in front leg morphology [18] (see Fig. 1) and is also characterized by the widespread occurrence of a balanced green-brown polymorphism [17].…”