How do rivers, geographic distance, and dispersal behavior influence genetic structure in two sympatric New World monkeys?

Dispersal, one of the major factors affecting the gene flow between populations, shapes the spatial distribution of genetic diversity within species. Alouatta macconnelli and Saguinus midas are two Neotropical monkey species that sympatrically inhabit the Guiana shield in northern Amazonia and are likely to differ in their dispersal behavior and vagility. We took advantage of their sympatry to investigate, over a fine geographical scale (similar to 50km long), the relationship between spatial genetic structure, on the one hand, and geographical features and the species' dispersal behavior on the other. A total of 84 A. macconnelli individuals from 25 social units and 76 S. midas individuals from 19 social units were genotyped for nine microsatellite markers. Both species displayed high genetic diversity and allelic richness. However, patterns of genetic structure differed between the two species. In A. macconnelli, no genetic substructuring was observed, while in S. midas we detected significant structuring, but this structuring was not correlated with geographical features, such as the location of individuals relative to the river and/or the distance between them. Instead, the geographical distribution of genetic variation observed for each species is predominantly explained by each species' dispersal pattern. We identified bisexual dispersal for both species, but with significant differences, either in the distance or in the rate of dispersal, between species and sexes. Genetic relatedness within social units was higher in S. midas than in A. macconnelli: gene flow between social units seems limited in S. midas, especially for females, while high dispersal characterizes A. macconnelli, where females seem to disperse at lower rate but at a longer distance than males.