Effects of thermal environment on reproductive physiology in Amargosa pupfish, Cyprinodon nevadensis amargosae

Authors: Maddie Housh, Sean Lema

Year: 2021

Abstract

Desert pupfishes of southwestern North America present an ideal system for examining how thermal experience shapes physiology. As a consequence of being confined to isolated aquatic habitats, many of which show extreme environmental fluctuations, pupfishes have evolved some of the broadest and most extreme thermal tolerances among freshwater fishes. Despite this, pupfish reproduction is thermally sensitive, becoming impaired many degrees below the thermal maximum. Physiologically, reproduction is regulated through hormone production via the hypothalamic-pituitary-gonadal (HPG) axis, and high temperature exposure has been found to reduce circulating levels of estradiol, which is integral to processes such as egg development and mating behavior. Given current warming of inland aquatic ecosystems due to anthropogenic climate change, it is crucial to explore how thermal experience affects pupfish reproductive physiology, and whether thermal acclimation can impart physiological plasticity that benefits reproduction under future warming conditions. Here, we examined the effects of elevated temperatures on the reproductive physiology of Amargosa pupfish (Cyprinodon nevadensis amargosae) by maintaining adult fish for 44 days under three temperature conditions: stable 25?C, stable 35?C, or a regime of temperatures that fluctuated diurnally between ~25-35?C (mean: 30.5?C). Female pupfish at 35?C and under the fluctuating regime had lower gonadosomatic index (GSI) values compared to females at 25?C. Females at 35?C also had lower body condition (k) despite the absence of any difference in hepatosomatic index (HSI). Liver expression of genes involved in egg development, vitellogenin A (vtgA) and choriogenin L (cgL), were reduced in females at 35?C and fluctuating conditions compared to those at 25?C. Temperature did not, however, affect GSI or HSI in males. These findings suggest that oogenesis is more sensitive to elevated temperatures than spermatogenesis in pupfishes and demonstrate that oogenesis can be affected by elevated temperature even when increases occur in ecologically relevant fluctuating temperature regimes similar to those of stream and spring outflow channel pupfish habitats.