Subchronic elevation in ambient temperature drives alterations to the sperm epigenome and accelerates early embryonic development in mice

Forecasted increases in the prevalence and severity of extreme weather events accompanying changes in climatic behavior pose potential risk to the reproductive capacity of animals of ecological and agricultural significance. While several studies have revealed that heat stress induced by challenges such as testicular insulation can elicit a marked effect on the male reproductive system and in particular the production of spermatozoa, comparatively less is known about the immediate impact on male reproductive function following sub-chronic whole-body exposure to elevated ambient temperature. To address this knowledge gap, here we exposed unrestrained male mice to heat stress conditions that emulate a heatwave (daily cycle of 8?h at 35°C followed by 16 h at 25°C) for a period of seven days. Neither the testes or epididymides of heat exposed mice exhibited evidence of gross histological change and similarly, the spermatozoa of exposed males retained their functionality and ability to support embryonic development. Notably, however, the embryos sired by heat exposed spermatozoa experienced pronounced changes in gene expression linked to acceleration of early embryo development, aberrant blastocyst hatching and increased fetal weight. Such changes were causally associated with altered sperm small non-coding RNA (sncRNA) profiles, such that these developmental phenotypes were recapitulated by microinjection of wild-type embryos sired by control spermatozoa with RNAs extracted from heat exposed spermatozoa. Such data highlight that even a relatively modest excursion in ambient temperature can affect male reproductive function and identify the sperm sncRNA profile as a particular point of vulnerability to this imposed environmental stress. Overall design: To investigate the impact of paternal heat exposure on the sperm small RNA payload and subsequent transcriptome of the embryo we sequenced the small RNA fraction of sperm from control and heat-exposed mice (n=3 biological replicates per treatment) and performed RNA-seq on single 4-cell and morula stage embryos fertilized by control or heat-exposed sperm