Fig. 4

HoxA13 expression is sexually dimorphic in the claspers of males, and is not expressed in pelvic fins of females. At developmental stages 28 and 29, sex cannot be determined morphologically as the claspers have not yet begun to differentiate, however HoxA13 expression occurs in half of all embryos (Black arrows a, c), suggesting that HoxA13 is patterning the claspers before they form (a–d). At stages 30–32, clasper morphogenesis is apparent, and HoxA13 expression continues in males, with no expression in the pelvic fins of females (e–j)