Estimating the Frequencies of Genetically Distinct Classes of Individuals in Hybridized Populations

Abstract

Hybridization and interspecific recombination result in the formation of novel morphologies and associations of molecular markers. Thus, in the past, the existence of hybridization and introgression has been inferred by identifying individuals exhibiting some defined hybrid or backcross phenotypes or genotypes. This approach can be unreliable because it may under- or overestimate the frequency of different classes of individuals in a hybridized population. Also, this method provides no formal framework for testing a null hypothesis that the frequency of a particular class of hybrids or introgressants is equal to zero. A model is presented which uses the method of maximum likelihood and codominant molecular markers segregating for parental species specific alleles to estimate the frequencies of six classes of individuals that may occur in a hybridized population. Estimates of the frequency of parental species, hybrid, and introgressed individuals are unbiased if model assumptions are met. If model assumptions are violated, estimates of the frequency of introgression are conservative. Monte Carlo computer simulations are used to examine the bias and variance of the model estimates and to evaluate the effects of different sampling strategies as the number of loci with unique alleles, the frequencies of these alleles, and the number of putative hybrid individuals sampled are varied. Simulations were also used to examine the accuracy of delta and bootstrap methods of obtaining true variances of the model estimates. This approach provides evolutionary geneticists with a simple method to quantify the occurrence and extent of hybridization and introgression in natural populations.