Assessing anatomical changes in male reproductive organs in response to larval crowding using micro-computed tomography imaging

Abstract

Selection favour males that anticipate intraspecific competition levels and develop appropriate competitive traits. In holometabolous insects, larval density can act as an ecological cue of males future intraspecific competition levels while also limiting access to food allocated to sexually selected traits (e.g., testes). To date, however, most studies have been conducted in larval densities that may have little or no relevance to species ecology. Here, we manipulated Drosophila melanogaster larval density based on the natural history of the species and used 3D micro-CT imaging to show a strong modulation of testes volume based on larval density: males from high larval densities generally had larger testes. Conversely, males from high densities tended (albeit not statistically significantly) to have the smallest accessory glands and ejaculatory bulb. Overall, we used micro-CT to measure male reproductive morphology in an ecologically relevant design for which the findings broadly support predictions from sexual selection theory.

The transcriptomic signature of physiological trade-offs caused by larval overcrowding in Drosophila melanogaster

“…Larval crowding is also expected to affect reproductive processes, for example by increasing testis sizes relative to body mass [27,41,42]. Males and females developing in crowded larval environments are smaller and with lower mating and reproductive success, displaying higher investment in mating opportunities [27,75].…”

“…Literature Downregulation of metabolic pathways linked to dietary protein (e.g., TOR) due to protein limitation caused by high larval density (Prediction 1) [26] Upregulation of immunity due to density-dependent prophylaxis observed in other insects as well as upregulation of pathways related to survival (e.g., autophagy) if density-dependent responses increase survival (Prediction 2) [20,[37][38][39] Upregulation of stress-related pathways (e.g., heat-shock proteins) as an hormesis-like response to overcrowding (Prediction 3) [18,19] No change or downregulation of pathways involved in reproduction (e.g., spermatogenesis) although some insects respond to high larval density by increasing testis sizes relative to body mass. [27,[40][41][42]…”