For all you fish fin fans, there’s a good overview of Xiphophorus tails in this month’s BioEssays (Wilkins, 2004). Xiphophorus, or the swordtail, is a popular aquarium fish in which the males have a prominent ventral extension of the tail, and the females do not. It’s considered an exemplary product of sexual selection: a prominent and peculiar structure found only in males, and with no detectable selective advantage, other than that the ladies really seem to like it—they prefer to breed with males with longer swords. Basola (1990) also found that in related species in which the males do not have swords, females still have a predisposition to favor long-sworded males. She did this by fitting males with prosthetic swords, and found that it made them more attractive to female partners (I think the spammers who flood my emailbox may have also read this article).

One developmental question is how swords are made. Not all Xiphophorid species have them, suggesting that they are either easily lost or easily gained in evolution. Some of the swordless Xiphophorids (and some of the females!) can be induced to form swords by applying exogenous testosterone, which suggests that it may be a relatively simple matter of regulating a common pathway in development.

Zauner et al. (2003) have identified one of the molecular players in this process: members of the msx (muscle segment homeobox) family of genes. These genes are strongly expressed i ...

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