Abstract / Summary
Testicular descent is a characteristic feature of most mammals and is generally thought to support spermatogenesis through scrotal cooling. However, several mammalian lineages retain abdominal or inguinal testes while maintaining fertility, raising the question of how reproduction persists under elevated testicular temperatures. Here, we combined comparative evolutionary analyses, sperm morphological analyses, and functional assays to examine molecular and phenotypic changes associated with fertility maintenance in naturally cryptorchid mammals. Comparative evolutionary analyses of infertility-associated genes identified lineage-specific shifts in selective pressure in naturally cryptorchid mammals. Genes showing positive-selection signals or carrying ancestral-reconstruction-supported candidate recurrent amino acid substitutions showed significant enrichment for functions related to sperm flagellar organization and meiotic regulation when compared with a mouse testis-expressed gene background, although no significant enrichment was detected when the 65 candidate genes were used as the background. Across mammals, cryptorchid species showed shorter sperm, while representative observations in bottlenose dolphin further revealed prominent mitochondrial midpieces. Functional assays further showed that HORMAD1, a meiosis-related protein showing positive-selection signals in cryptorchid lineages, exhibited enhanced thermal stability, suggesting a possible role in maintaining meiotic function at elevated temperatures. Naturally cryptorchid mammals exhibited molecular and morphological changes associated with sperm architecture and meiotic regulation. Such changes, together with the enhanced thermal stability of HORMAD1, may contribute to fertility maintenance in the absence of scrotal cooling. Our study provided a comparative evolutionary framework for understanding reproductive persistence under chronic thermal constraint.