Conservation genetics and genetic mating system of the yellow-shouldered blackbird (Agelaius xanthomus), an endangered island endemic

Abstract

The yellow-shouldered blackbird (Agelaius xanthomus) is a managed endemic species in Puerto Rico that experienced a bottleneck in the 1970s and currently faces high levels of brood parasitism and habitat loss. Little is known about its overall genetic profile and genetic mating system, which potentially shapes effective population size by contributing to variation in reproductive success. Here I used nine microsatellite loci to characterize the population genetics and mating system of a southwestern Puerto Rican population. Although the population showed no evidence of inbreeding, it had significantly lower allelic diversity, effective population size, and expected heterozygosity than both island and continental populations of the related red-winged blackbird (A. phoeniceus). In particular, the low effective population size of the yellow-shouldered blackbird (~71, 18 % of the 2012 census size) raises concerns about its long-term viability. Paternity tests on 30 nests provided the first evidence of genetic polyandry in this species. Of all young, 23 % were extra-pair (sired by a male other than the social father), and 37 % of nests contained extra-pair young, which does not differ from proportions observed in red-winged blackbirds. Extra-pair paternity was not found to contribute significantly to variation in individual reproductive output, suggesting the bottleneck is more likely to have contributed to the current population genetic profile than the species’ mating system. Together, these data suggest that while genetic diversity on the individual level may be intact, the population as a whole remains vulnerable to adverse stochastic events and loss of evolutionary potential.