Scientists have undertaken a new molecular study on the members of the Discus genus Symphysodon and found three well-defined groups.
Hrbek and Farias of the Universidade Federal do Amazonas sampled 334 individuals from the Symphysodon genus, which were collected from 24 localities spanning the geographic range of the genus.
The specimens, which they believe to include all known phenotypic groups, species and subspecies, were analysed using a molecular technique to analyse their mitochondrial DNA, and the results show three distinct groups within the Symphysodon genus.
Hrbek and Farias, who are due to publish the results of their findings in a paper in the journal Molecular Phylogenetics and Evolution, believe that the groups revealed correspond to the 'green' and 'blue' groups, and to a previously morphologically unrecognised clade from the Rio Xingu drainage.
The authors said: "These three clades were nested within a paraphyletic assemblage consisting of the 'brown' group of S. aequifasciatus and of both described subspecies of S. discus, the 'Heckel' and the 'abacaxi' discus.
"Nuclear allele sharing was observed among groups, but there were significant differences in frequencies.
"We inferred several processes including past fragmentation among groups, and restricted gene flow with isolation by distance within the paraphyletic 'brown+Heckel+abacaxi' groups, and suggest that differences among the 'blue', 'Heckel' and 'brown' groups are potentially maintained by differences in water chemistry preferences.
"We further inferred colonization of the western Amazon basin by an ancestor of the 'green' clade. The 'green' group was the only group with a pattern of haplotype distribution consistent of a demographic expansion, and the divergence of this clade from other groups of discus was consistent with recent geologic evidence on the breach of the Purus Arch which separates western Amazon from eastern Amazon.
For more information see the paper: Farias IP, Hrbek T. (2008) - Patterns of diversification in the discus fishes (Symphysodon spp. Cichlidae) of the Amazon basin. Mol Phylogenet Evol. 2008 Jun 3.