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 al ong the Rio Jurua in western Amazonia, Brazil, to determine the import ance of riverine barriers in the diversification of this taxon. Twenty individual haplotypes were uncovered, most of which were unique to si ngle localities but some of which were shared among adjacent sites eit her along or across the river. Genealogical analyses suggest that gene flow is limited and, in combination with the unique distribution of m ost haplotypes, suggest that populations of this species are strongly substructured along the river. Thus, most sharing of haplotypes betwee n adjacent localities is probably caused by historical association rat her than to ongoing gene flow. Two haplotype clades were uncovered, bu t these correspond to headwaters versus mouth areas, not to opposite s ides of the river, as would be expected by the Riverine Barrier Hypoth esis. 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 patter ns of this species show that both clades have relationships to areas w ell outside the Rio Jurua basin. This suggests that the basin represen ts a relatively recent point of invasion between two more broadly dist ributed and differentiated geographic units of the species.