Recovery from heat-induced infertility: A study of reproductive tissue responses and fitness consequences in male Drosophila melanogaster

The predicted temperature increase caused by climate change is a threat to
biodiversity. Across animal taxa, male reproduction is often sensitive to
elevated temperatures leading to fertility loss and in more adverse
scenarios, this can result in sterility when males reach their upper
thermal fertility limit. Here we investigate temperature-induced changes
in reproductive tissues, fertility reduction, sterility, and the
associated fitness loss during the subsequent recovery phase in male
Drosophila melanogaster. We heat-stressed males during development and
either allowed them to recover or not in early adulthood, while measuring
several determinants of male reproductive success. We found significant
differences in recovery rate, organ sizes, sperm production, and other key
reproductive traits among males from our different temperature treatments.
Sperm maturation was impaired before reaching the upper thermal sterility
threshold. While some effects were reversible, this did not compensate for
the fitness loss due to damage imposed during development. Surprisingly,
developmental heat stress was damaging to accessory gland growth, and
female post-mating responses mediated by seminal fluid proteins were
impaired regardless of the possibility of recovery. We suggest that
sub-lethal thermal sterility and the subsequent fertility reduction is
caused by a combination of inefficient functionality of both the accessory
gland and testes.