American scientists have demonstrated that reticulate evolution involving three ancestral species is possible in fishes.
A study by published in a recent issue of the Proceedings of the National Academy of Sciences by David McDonald and coworkers has shown that some hybrid suckers (what the authors call muttsuckers) in a murky Wyoming stream appropriately named Muddy Creek have three parental species: the introduced white sucker (Catostomus commersoni) and two species native to the Colorado River Basin - the flannelmouth sucker (Catostomus latipinnis) and the bluehead sucker (Catostomus discobolus).
The authors based their study on an analysis of nuclear and mitochondrial DNA markers in all three species and their hybrids.
They found that the white sucker was hybridizing with the flannelmouth sucker and that the hybrids produced from these crosses were in turn breeding with bluehead suckers to produce muttsuckers with the genomes of all three species.
The flannelmouth sucker and the bluehead sucker were previously reproductively isolated from each other and incapable of producing hybrids.
This has changed with the introduction of the white sucker (the authors write the introduced white sucker and its hybrids appear to be acting as a bridge that could eventually lead to a hybrid swarm involving all three species).
The authors conclude that their findings have both conservation and evolutionary implications. From a conservation perspective, the introduced species now threatens the genetic integrity of not just one but two native speciesFrom an evolutionary perspective, the genetic bridging suggests that we cannot assume that vertebrate evolution proceeds in a strictly bifurcating manner.
For more information, see the paper: McDonald, DB, TL Parchman, MR Bower, WA Hubert and FJ Rahel (2008) An introduced and a native vertebrate hybridize to form a genetic bridge to a second native species. Proceedings of the National Academy of Sciences 105, pp. 10837"10842.