Xiphophorus is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. Xiphophorus species can be divided into three groups based on their evolutionary relationships: platyfish (or platies), northern swordtails, and southern swordtails. Platyfish formerly were classified in another genus, Platypoecilus, which is now obsolete. The type species is X. hellerii, the green swordtail. Like most other new world Poeciliids, platies and swordtails are live-bearers that use internal fertilization and give birth to live young instead of laying eggs like the bulk of the world's fishes. All are relatively small fishes, which reach a maximum length of depending on the exact species involved.

Three species and their hybrids are common in the aquarium trade: the green swordtail (X. hellerii), the southern platyfish (X. maculatus) and the variable platyfish (X. variatus). These three are the only species that have large native ranges.

All other species of Xiphophorus are highly localized and mostly endemic to Mexico. Only three of the localized species, the Chiapas swordtail (X. alvarezi), X. mayae and X. signum, range outside Mexico and the last two are the only Xiphophorus species not found in Mexico at all.

Human uses

Xiphophorus species are regularly used in genetic studies, and scientists have developed many interspecific hybrids,. Xiphophorus have proved a useful model to understand the consequences of hybridization, especially in the context of melanoma research since the 1920s. The Xiphophorus Genetic stock center, founded by Myron Gordon in 1939, is an important source of these fish for research.

Species and taxonomy

thumb|Males of the [[Montezuma swordtail have the proportionally longest tail "sword" among the swordtails, but as in all species the females lack it]]

thumb|One of the typical captive forms of the [[variable platyfish]]

There are currently 28 recognized species in this genus, according to FishBase. Two of these species, X. clemenciae and X. monticolus, are likely the result of natural hybrid speciation (ancestors of both are a platy species and a swordtail species). Contemporary hybridization in the wild was thought to be uncommon in this genus until recently.

Although traditionally divided into swordtails and platies, this separation is not supported by phylogenetic studies, which have shown that the swordtails are paraphyletic compared with the platies. These studies suggest that the genus can be divided into three monophyletic groups: the northern swordtails (of the Pánuco River basin, marked with a star* in the list), southern swordtails (southern Mexico to Honduras) and the platies. An interesting consequence of hybridization is the origin of spontaneous melanoma in hybrids. For example, in the well-studied Gordon-Kosswig cross between X. maculatus and X. hellerii, hybrids develop spontaneous melanoma due to an interaction between the oncogene xmrk and a repressor locus on a distinct chromosome.

Pigmentation: Xiphophorus pigmentation has been investigated since the 1920s using classical genetics techniques such as crosses. As a result, inheritance patterns of many traits have been described. More recently, researchers have identified specific genes and even mutations causing pigment pattern variation (for instance, a spot allowing males to mimic females) and investigated selective mechanisms on these traits.

Sexual selection: Researchers since Darwin have studied Xiphophorus species for their sexually selected ornaments (e.g. caudal fin "swords"). More recent studies have focused on male ornamentation, courtship strategies, and female mate choice.

Etymology

The genus name Xiphophorus derives from the Ancient Greek words ξίφος (xíphos), meaning 'sword', and φόρος (phóros), meaning 'bearing', referring to the gonopodium on the males.

References

  • xiphophorus genetic stock center
  • xiphophorus.org
  • xiphophorus.net(Chinese ver.)