Investment in adult reproductive tissues is affected by larval growth conditions but not by evolution under poor larval growth conditions in Drosophila melanogaster

In many insects, the larval environment is confined to the egg-laying
site, which often leads to crowded larval conditions, exposing the
developing larvae to poor resource availability and toxic metabolic
wastes. Larval crowding imposes two opposing selection pressures. On one
hand, due to poor nutritional resources during developmental stages,
adults from the crowded larval environment have reduced investment in
reproductive tissues. On the other hand, a crowded larval environment acts
as a cue for future reproductive competition inducing increased investment
in reproductive tissues. Both these selection pressures are likely
affected by the level of crowding. The evolutionary consequence of
adaptation to larval crowding environment on adult reproductive investment
is bound to be a result of the interaction of these two opposing forces.
In this study, we used experimentally evolved populations of Drosophila
melanogaster adapted to larval crowding to investigate the effect of
adaptation to larval crowding on investment in reproductive organs (testes
and accessory glands) of males. Our results show that there is a strong
effect of larval developmental environment on absolute sizes of testes and
accessory glands. However, there was no effect of the developmental
environment when testis size was scaled by body size. We also found that
flies from crowded cultures had smaller accessory gland sizes relative to
body size. Moreover, the sizes of the reproductive organs were not
affected by the selection histories of the populations. This study
highlights that adaptation to two extremely different developmental
environments does not affect the patterns of reproductive investment. We
discuss the possibility that differential investment in reproductive
tissues could be influenced by the mating dynamics and/or investment in
larval survival traits, rather than just the developmental environment of
the populations.