Turning up the heat: how pupfish reproduction is affected by warming temperature

Authors: Sean Lema, Maddie Housh, Kseniya Krayeva

Year: 2023

Abstract

Reproduction in fishes is commonly sensitive to environmental temperature and increasing average temperatures or extreme ‘heat wave’ events associated with climate change have the potential to impact reproductive output. Even though pupfishes of the North American southwestern deserts have among the highest thermal limits for reproduction of any fishes, pupfish still show reduced reproductive performance at elevated temperatures. We have been examining the effects of warmer temperatures on reproduction in Amargosa Pupfish (Cyprinodon nevadensis amargosae) and other pupfish species under a variety of experimental temperature regimes. In fish as in other vertebrates, reproduction is regulated physiologically by the hypothalamic-pituitary-gonadal (HPG) hormone axis. Exposure of fish to atypically high or prolonged elevations in temperature can alter HPG hormone signaling with consequences for gametogenesis. Findings from several recent experimental studies with Cyprinodon pupfishes have demonstrated that egg development (oogenesis) in pupfish is considerably more sensitive to disruption by high temperatures than sperm production (spermatogenesis). Female pupfish experiencing elevated, but ecologically relevant, temperatures show reduced circulating levels of the estrogen hormone estradiol (E2). That lower E2 results in a downregulation of gene pathways essential to oogenesis, including significantly reduced gene expression of egg yolk (vitellogenin) and egg envelope (choriogenin) proteins in the liver. Ultimately, those effects of higher temperature result in diminished oogenesis and a reduced ovary mass. Females at elevated temperatures given supplemental E2 still respond by upregulating oogenesis gene pathways, implying that diminished oogenesis at higher temperatures is caused by the lower levels of E2. In contrast with females, the reproductive physiology of male pupfish is comparably insensitive to elevated temperature. Taken as a whole, these findings point to female pupfish being vulnerable to reproductive impairment when exposed to warmer temperatures and provide a foundation for understanding about how reproduction in wild pupfish populations will be affected by changing thermal conditions with climate change.