Experimental evolution reveals trade-offs between sexual selection and heat tolerance in Drosophila prolongata

Sexual selection promotes traits that enhance mating or fertilization
success, but these traits can be very costly under harsh environmental
conditions. The extent to which differential investment in costly traits
under varying intensities of sexual selection is related to their
susceptibility to environmental stress remains unclear. This study
explored how experimental evolution under different operational sex ratios
(OSRs) shapes traits and reproductive success of male Drosophila
prolongata, and how developmental and/or adult heat stress affect the
expression of these traits. We found males from even and slightly
male-biased OSRs to be larger and display greater reduction in body size
under developmental heat stress, suggesting pre-mating sexual selection on
body size and condition-dependent thermal sensitivity. These populations
also exhibited consistently high mating and fertilization success across
temperatures, potentially indicating selection for robust phenotypes with
‘good genes’ that perform well regardless of temperature. Conversely,
males from strongly male-biased OSR populations experienced more
pronounced decline in sperm competitiveness following exposure to
developmental or adult heat stress. These results highlight how
environmental stressors differentially impact populations, shaped by
varying strengths of pre- and post-mating sexual selection. These observed
patterns suggest potential interactions between past selection and the
ability to adapt to changing environments.