New phylogeny reveals rainbowfish relationships

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A new phylogenetic study of rainbowfishes from Madagascar and Australasia has shown how the species are related and sheds new light on how they evolved.

The new study by Sparks and Smith of the Department of Ichthyology at the American Museum of Natural History, has just been published in the journal Molecular Ecology and Evolution.

The systematists examined a total of 4394 characters present on a small subset of the genes of the fishes in an attempt to infer their relationships to each other. This has allowed them to produce the most detailed analysis of the fishes ever undertaken.

It's the first time anyone has constructed a phylogeny (a sort of evolutionary family tree) for the Bedotia species of Madagascar, and the study covers practically all known representatives, including several that are as yet undescribed in a full taxonomic review of the genus.

The genetic study was also run in parallel with a morphological analysis of the fish and the phylogenetic trees produced are remarkably similar, which suggests that the study has been very effective in resolving the probable phylogeny of the group.

For the Malagasy species, the results show that Bedotia and Rheocles, as expected, are very closely related and are derived from a single ancestral species.

However, Sparks and Smith say that within these two genera, the patterns are somewhat different: "Rheocles is paraphyletic with the inclusion of two recently described species from northeastern Madagascar, R. vatosoa, and R. derhami. Rheocles vatosoa and R. derhami are sister taxa, and this clade is recovered as the sister group to Bedotia.

"The remaining species of Rheocles are not sexually dimorphic and comprise a clade that is recovered as the sister group to Bedotia (R. derhami+R. vatosoa), all of which are sexually dichromatic, and sexually dimorphic for pigmentation and fin development.

"Three geographically distinct clades are recovered within Bedotia, one comprising species with distributions ranging from mid- to southeastern Madagascar, another including species restricted to eastern drainages north of the Masoala Peninsula, and a third comprising taxa with distributions extending from the Masoala Peninsula south to the Ivoloina River."

As you would expect from a Gondwanaland split, the Australasian rainbows are distantly related to the Malagasy forms and form a sister group in the tree.

For more details see the paper: Sparks JS, Smith WL (2004) - Phylogeny and biogeography of the Malagasy and Australasian rainbowfishes (Teleostei: Melanotaenioidei): Gondwanan vicariance and evolution in freshwater. Mol Phylogenet Evol. 2004 Dec ; 33(3): 719-34