INTROGRESSION AMONG BRITISH BIRCH TREES

Hybridisation may lead to introgression of genes among species. Introgression may be bidirectional or unidirectional, depending on factors such as the demography of the hybridising species, or the nature of reproductive barriers between them. Previous microsatellite studies suggested bidirectional introgression between diploid Betula nana and tetraploid B. pubescens and also between B. pubescens and diploid B. pendula. Here we analyse introgression among these species using 76,587 variants in restriction-site associated (RAD) markers in 196 individuals. We found unidirectional introgression into B. pubescens from both of the diploid species, in clear clines with greater B. nana introgression in the north and greater B. pendula introgression in the south.  These patterns of introgression fit with the nature of the reproductive barrier between diploids and tetraploids, and historical range shifts of the three species since glaciation. To facilitate our analysis we developed a new script for genotyping polyploids based on read counts and average base quality, with an algorithm that calculates the maximum likelihood variant configuration at each locus. This is implemented in R and, besides population genetics, should be of use in cancer research and haplotype based analyses.