GENE GENEALOGY AND DIFFERENTIATION AMONG ARBOREAL SPINY RATS (RODENTIA: ECHIMYIDAE) OF THE AMAZON BASIN: A TEST OF THE RIVERINE BARRIER HYPOTHESIS.

Sequence variation in the mitochondrial cytochrome b gene was examined in the arboreal spiny rat, Mesomys hispidus, collected at 15 sites along the Rio Juruá in western Amazonia, Brazil, to determine the importance of riverine barriers in the diversification of this taxon. Twenty individual haplotypes were uncovered, most of which were unique to single localities but some of which were shared among adjacent sites either along or across the river. Genealogical analyses suggest that gene flow is limited and, in combination with the unique distribution of most haplotypes, suggest that populations of this species are strongly substructured along the river. Thus, most sharing of haplotypes between adjacent localities is probably caused by historical association rather than to ongoing gene flow. Two haplotype clades were uncovered, but these correspond to headwaters versus mouth areas, not to opposite sides of the river, as would be expected by the Riverine Barrier Hypothesis. Moreover, haplotype sharing across the river was greater at its mouth than in the headwaters, a pattern opposite that expected if the river were a substantive barrier. Broader scale phylogeographic patterns of this species show that both clades have relationships to areas well outside the Rio Juruá basin. This suggests that the basin represents a relatively recent point of invasion between two more broadly distributed and differentiated geographic units of the species.