Estimating the effective number of breeders from heterozygote excess in progeny

The heterozygote-excess method is a recently published method for estimating the effective population size (N(c)). It is based on the following principle: When the effective number of breeders (N(eb)) in a population is small, the allele frequencies will (by chance) be different in males and females, which causes an excess of heterozygotes in the progeny with respect to Hardy-Weinberg equilibrium expectations. We evaluate the accuracy and precision of the heterozygote-excess method using empirical and simulated data sets from polygamous, polygynous, and monogamous mating systems and by using realistic sample sizes of individuals (15-120) and loci (5-30) with varying levels of polymorphism. The method gave nearly unbiased estimates of N(eb) under all three mating systems. However, the confidence intervals on the point estimates of N(eb) were sufficiently small (and hence the heterozygote-excess method useful) only in polygamous and polygynous populations that were produced by <10 effective breeders, unless samples included > ~60 individuals and 20 multiallelic loci.