The influence of thermal signals during embryonic development on intrasexual and sexually dimorphic gene expression and circulating steroid hormones in American alligator hatchlings (Alligator mississippiensis)

Abstract

Incubation temperatures experienced by developing embryos exert powerful influences over gonadal sex determination and differentiation in many species. However, the molecular mechanisms controlling these impacts remain largely unknown. We utilize the American alligator to investigate the sensitivity of the reproductive system to thermal signals experienced during development and ask specifically whether individuals of the same sex, yet derived from different incubation temperatures display persistent variation in the expression patterns of sex biased transcripts and plasma sex hormones. Our analysis focuses on assessments of circulating sex steroids and transcript abundance in brain and gonad, two tissues that display sexually dimorphic gene expression and directly contribute to diverse sexually dimorphic phenotypes. Whereas our results identify sexually dimorphic patterns for several target gonadal genes in postnatal alligators, sex linked variation in circulating 17β-estradiol, testosterone, and expression of two brain transcripts (aromatase and gonadotropin releasing hormone) was not observed. Regarding intrasexual variation, we found that AMH transcript abundance in hatchling testes is positively correlated with temperatures experienced during sexual differentiation. We also describe highly variable patterns of gene expression and circulating hormones within each sex that are not explained by the intensity of embryonic incubation temperatures. The magnitude of sexually dimorphic gene expression, however, is directly associated with temperature for SOX9 and AMH, two transcripts with upstream roles in Sertoli cell differentiation. Collectively, our findings regarding temperature linked variation provide new insights regarding the connections between embryonic environment and persistent impacts on sexual differentiation in a reptile species that displays temperature dependent sex determination.

Keywords: Crocodilians; Intrasexual variation; Sexual dimorphism; Temperature dependent sex determination.