Molecular systematics, biogeography and population structure of Neotropical freshwater needlefishes of the genus Potamorrhaphis

Phylogenetic relationships of populations and species within Potamorrhaphis , a genus of freshwater South American needlefishes, were assessed using mi tochondrial cytochrome b sequences. Samples were obtained from eight widely distributed localities in the Amazon and Orinoco rivers, and represented a ll three currently recognized species of Potamorrhaphis. The phylogeny of h aplotypes corresponded imperfectly to current morphological species identit ies: haplotypes from P. guianensis, the most widespread species, did not ma ke up a monophyletic clade. Geography played a strong role in structuring g enetic variation: no haplotypes were shared between any localities, indicat ing restricted gene flow. Possible causes of this pattern include limited d ispersal and the effects of current and past geographical barriers. The hap lotype phylogeny also showed a complex relationship between fishes from dif ferent river basins. Based on the geographical distribution of clades, we h ypothesize a connection between the middle Orinoco and Amazon via rivers of the Guianas. More ancient divergence events may have resulted from Miocene alterations of river drainage patterns. We also present limited data for t wo other Neotropical freshwater needlefish genera: Belonion and Pseudotylos urus. Pseudotylosurus showed evidence of substantial gene flow between dist ant localities, indicating ecological differences from Potamorrhaphis.