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Background  One of Darwin's chosen examples for his idea of sexual selection through female choice was the "sword", a colourful extension of the caudal fin of male swordtails of the genus  Xiphophorus . Platyfish, also members of the genus  Xiphophorus , are thought to have arisen from within the swordtails, but have secondarily lost the ability to develop a sword. The sustained increase of testosterone during sexual maturation initiates sword development in male swordtails. Addition of testosterone also induces sword-like fin extensions in some platyfish species, suggesting that the genetic interactions required for sword development may be dormant, rather than lost, within platyfish. Despite considerable interest in the evolution of the sword from a behavioural or evolutionary point of view, little is known about the developmental changes that resulted in the gain and secondary loss of the sword. Up-regulation of  msxC  had been shown to characterize the development of both swords and the gonopodium, a modified anal fin that serves as an intromittent organ, and prompted investigations of the regulatory mechanisms that control  msxC  and sword growth.    Results  By comparing both development and regeneration of caudal fins in swordtails and platyfish, we show that  fgfr1  is strongly up-regulated in developing and regenerating sword and gonopodial rays. Characterization of the fin overgrowth mutant  brushtail  in a platyfish background confirmed that fin regeneration rates are correlated with the expression levels of  fgfr1  and  msxC . Moreover,  brushtail  re-awakens the dormant mechanisms of sword development in platyfish and activates  fgfr1/msxC -signalling. Although both genes are co-expressed in scleroblasts, expression of  msxC  in the distal blastema may be independent of  fgfr1 . Known regulators of Fgf-signalling in teleost fins,  fgf20a  and  fgf24 , are transiently expressed only during regeneration and thus not likely to be required in developing swords.    Conclusion  Our data suggest that Fgf-signalling is involved upstream of  msxC  in the development of the sword and gonopodium in male swordtails. Activation of a gene regulatory network that includes  fgfr1  and  msxC  is positively correlated with fin ray growth rates and can be re-activated in platyfish to form small sword-like fin extensions. These findings point towards a disruption between the  fgfr1/msxC  network and its regulation by testosterone as a likely developmental cause for sword-loss in platyfish.

Fgfr1 signalling in the development of a sexually selected trait in vertebrates, the sword of swordtail fish