RESUMEN
The genetic variability of New World primates is still poorly documented. We present the first genetic study on two threatened endemic titi monkey species in northern Bolivia (Plecturocebus modestus and Plecturocebus olallae) using six microsatellite markers to investigate genetic structure and variability of 54 individuals from two wild populations. A low level of genetic diversity was found (34 alleles in the total sampled population). Locus 1118 presented the greatest number of alleles. The mean number of alleles per locus in the total population was 5.6 and the average heterozygosity was 0.38 (range 0.12-0.88). The FIS value for the total population using all microsatellite loci shows a statistically significant heterozygote deficit. The inbreeding coefficients (FIS) were positive and significantly different from zero (0.064 for P. olallae and 0.213 for P. modestus). The genetic differentiation between populations (FST) was moderate with a pair-wise FST estimate of 0.14. Population structure analyses assigned the two populations to two differentiated clusters (K = 2). These results suggest that these two species with very close distributional ranges arose from a single population, and that they remain in a process of genetic differentiation and speciation. This study further underlines the urgent need for conservation actions for both endemic primate species.