Defects in mating behavior and tail morphology are the primary cause of sterility in C. elegans males at high temperature

Reproduction is a fundamental imperative of all forms of life. For all the advantages sexual reproduction confers, it has a deeply conserved flaw: it is temperature sensitive. As temperatures rise, fertility decreases. Across species, male fertility is particularly sensitive to elevated temperature. Previously, we have shown in the model nematode Caenorhabditis elegans hat all males are fertile at 20°C, but almost all males have lost fertility at 27°C. Male fertility is dependent on the production of functional sperm, successful mating and transfer of sperm, and successful fertilization post-mating. To determine how male fertility is impacted by elevated temperature, we analyzed these aspects of male reproduction at 27°C in three wild-type strains of C. elegans : JU1171, LKC34 and N2. We found no effect of elevated temperature on the number of immature non-motile spermatids formed. There was only a weak effect of elevated temperature on sperm activation. In stark contrast, there was a strong effect of elevated temperature on male mating behavior, male tail morphology and sperm transfer such that males very rarely completed mating successfully when exposed to 27°C. Therefore, we propose a model where elevated temperature reduces male fertility as a result of the negative impacts of temperature on the somatic tissues necessary for mating. Loss of successful mating at elevated temperature overrides any effects that temperature may have on the germline or sperm cells.