Polygyny, census and effective population size in the threatened frog, Rana latastei

Effective population size (N-e) is a key determinant of genetic diversity of populations. In amphibians, the ratio effective population size/census size (N-e/N) is often very low, raising concerns for the long-term persistence of genetic diversity in isolated populations. It has been proposed that the phenomenon of 'genetic compensation' increases the ratio N-e/N in small populations, but the underlying mechanisms are poorly understood. Polygyny can decrease N-e/N because of the negative relationship between polygyny and N-e. We used genetic information (microsatellites) to evaluate the relationship between census size, polygyny and N-e in populations of the threatened Italian agile frog Rana latastei. We reconstructed parentage in tadpoles from nine populations with eight to 32 breeding females, using a likelihood-based method; we analysed simulated datasets with known properties to confirm the reliability of this approach in reconstructing polygyny. Furthermore, we estimated N-e using approximate Bayesian computation. The level of polygyny differed strongly among populations (average number of mates per breeding male: 2-6.4). Polygyny was greater in populations with larger census sizes. Moreover, variance in male mating success was larger in large populations. Effective population size increased with population size, but was negatively related to polygyny; as polygyny increased in large populations, this was associated with reduced N-e/N. In polygynous species, increasing levels of polygyny in large populations may explain the low N-e/N values, with important implications for the conservation of genetic diversity and for long-term population persistence.