RIVERINE BARRIERS AND GENE FLOW IN AMAZONIAN SADDLE-BACK TAMARINS

We describe patterns of genotypic and phenotypic variation in saddle-b ack tamarin (Saguinus fuscicollis) populations along the central and u pper Rio Jurua, western Brazilian Amazonia, The genetic data are seque nce haplotypes of the mitochondrial cytochrome b gene; phenotypic data are pelage colour variants that define sharply demarcated subspecies of this extremely variable tamarin species, We show that gene flow occ urs between adjacent subspecies, but that this phenomenon is restricte d to the headwater section of the river, which is consistent with expe ctations from the riverine barrier hypothesis. In this model, the majo r first-order tributaries of the Amazon form effective barriers to dis persal, with between-bank gene flow limited to the narrowed sections o f headwater streams and parallel divergence increasing along both bank s from the headwaters to the mouth of a given river. In meandering riv ers such as the Rio Jurua, we suggest passive transfer through river c hannel dynamics as the main mechanism permitting genetic contact betwe en populations on opposite banks of the river, Finally, we argue that in the case of plant and animal species that are largely restricted to unflooded (terra firme) forests, such as tamarins, seasonally flooded (varzea) forest can operate as a critical additional barrier to betwe en-bank gene flow.