Evolution of sex-specific heat stress tolerance and larval Hsp70 expression in populations of Drosophila melanogaster adapted to larval crowding

The ability to tolerate temperature stress is an important component of
adult fitness. In holometabolous insects like Drosophila
melanogaster, adult stress resistance can be affected by growth conditions
experienced during the larval stages. While evolution under crowded larval
conditions is known to lead to the correlated evolution of many adult
traits, its consequences on adult heat stress tolerance have not been
investigated. Therefore, in the present study, we assessed the adult heat
stress tolerance in populations of D. melanogaster adapted to a stressful
larval crowding environment. We used replicate populations of D.
melanogaster, selected for adaptation to larval crowding stress (MCUs),
for more than 230 generations, and their respective controls (MBs). Larvae
from selected and control populations were grown under crowded and
uncrowded conditions and their adult heat shock resistance at two
different temperatures was measured. Further, we compared Hsp70 expression
in crowded and uncrowded larvae of both populations and also measured the
Hsp70 expression after a mild-heat treatment in adults of selected and
control populations. Our results showed that adaptation to larval crowding
leads to the evolution of Hsp70 gene expression in larval stages and
improves adult heat-stress tolerance ability in males, but not in
females.