Fig. 6

A hypothesis for the testis-epididymis secreted signaling during evolution. Before Amniota, the epididymis and therefore testis-epididymis secreted signaling did not exist. In Amniota, in which the epididymis had developed from the mesonephros, testis-derived endocrine and lumicrine regulation of epididymal gene expression (represented by pink and blue arrows, respectively) emerged. Such contributions by endocrine or lumicrine action to the epididymal gene expression (represented by the size of arrows) can alter along evolution