Wood Frog (Rana sylvatica) Testicular Microstructure Correlates with Plasma Hormone Concentrations During Late-Stage Spermatogenesis

The baseline relationships of hormones and reproductive structures in natural amphibian populations are important to understanding reproductive strategies in this declining group. We explored the relationships of hormones and testicular microstructures of a widespread amphibian, the Wood Frog (Rana sylvatica= Lithobates sylvaticus). We collected testes and plasma from 104 male R. sylvatica during the spring breeding season in southern Connecticut, USA. We quantified key testicular microstructure measurements as proxies for reproductive preparedness, including testes histosection area, germinal epithelium depth, maximum lobule area, and proportion of interstitial to lobule space, which are associated with hormone-producing cells and spermatogenesis. We compared these measurements with body mass and plasma testosterone (T) and estradiol (E2) concentrations. Contrary to our expectations, germinal epithelium depth was negatively correlated with T, histosection area was positively correlated with E2, and the proportion of interstitial space decreased with increasing body weight. These results may be explained by a biosynthesis shift from T to E2 that occurs towards the end of spermatogenesis in seasonal breeders. Understanding fundamental relationships of reproductive physiology and testicular microstructures improves our ability to assess strategies for amphibian conservation breeding programs and amphibian resilience in the face of environmental contaminants.