Identification of novel genes involved in the development of the sword and gonopodium in swordtail fish.

Male swordtail fish of the genus Xiphophorus develop a sword, a colourful extension of the caudal fin, that evolved by sexual selection through female choice. Swords and gonopodia, an intromittent organ developing from the male anal fin, can be prematurely induced by exogenous testosterone, offering the opportunity to examine the identity and expression profiles of genes required during various stages of fin metamorphosis. Here, we employed suppression subtractive hybridisation to identify genes specifically up-regulated during two early stages of sword and gonopodium development. We identified 128 different sequences with significant similarity to known genes and characterized the rack1, dusp1, klf2, and tmsbeta-like genes as specifically up-regulated in developing as well as regenerating fin rays of the sword and gonopodium. We show that some of these genes follow distinct expression profiles in swords and gonopodia, suggesting differences in the genetic networks underlying the development of anal and caudal fin modifications.