Charles Darwin regarded swordtails (Xiphophorus) as the perfect way of explaining his theory of sexual selection. Among these fish, it is the females and the length of the sword – a conspicuous extension of the caudal fin – that determine the mating partner. The team of researchers led by evolutionary biologist Dr. Gerrit Begemann at Constance University is using swordtails to investigate the development of traits that evolved by sexual selection. Their major focus is the molecular mechanism of growth control in the caudal fin. Or put more simply: why do these fish have swords?
The researchers also found that the system that regulates the growth of the sword in swordtails, also accounts for other differences. Swordtail males and females not only differ due to the presence of the male sword, but also through the shape of their anal fin. The male gonopodium is a testosterone-induced tubular modification of the anal fin that the males use to fertilise the eggs in the body of the females. Swordtails are viviparous fish. In evolutionary terms, the gonopodium is older than the sword. The researchers assume that the same genes code for gonopodium and sword, but are differentially regulated. They found that in juvenile males, as well as during testosterone-induced sword development, expression of the transcription factor msxC is markedly upregulated in the sword. In the case of gonopodium and swordtail caudal fin, evolution does not manifest itself through the alteration of proteins. “It is not only the genes, but also the switches in front of the genes that can undergo changes,” explained Gerrit Begemann. The ancestors of modern swordtails most likely had a developmental programme that led to the development of gonopodia when the animals reached sexual maturity, i.e. when they fell under the influence of testosterone. Evolution then led to the activation of this “growth module” in the caudal fin, which means that genes such as msxC and FGFR came under the regulatory control of the sex hormone testosterone. In order to show how the growth of the fins is controlled and what leads to their typical shape and colour, Gerrit Begemann’s colleagues transplanted individual sword rays from the lower to the upper side of the caudal fin and discovered that the sword also grew out from the upper part of the fin. It is assumed that a single fin ray triggers the growth of neighbouring rays, in which signalling molecules migrate from one cell to another. This is another of Begemann’s research goals. “We now know what to look for. We have to look for diffusing signalling molecules that are produced by the swords and are able to determine the growth and pigmentation of their direct environment.”