Data from: Unidirectional diploid–tetraploid introgression among British birch trees with shifting ranges shown by restriction site-associated markers

Hybridization may lead to introgression of genes among species.
Introgression may be bidirectional or unidirectional, depending on factors
such as the demography of the hybridizing species, or the nature of
reproductive barriers between them. Previous microsatellite studies
suggested bidirectional introgression between diploid Betula nana (dwarf
birch) and tetraploid B. pubescens (downy birch) and also between B.
pubescens and diploid B. pendula (silver birch) in Britain. Here, we
analyse introgression among these species using 51 237 variants in
restriction site-associated (RAD) markers in 194 individuals, called with
allele dosages in the tetraploids. In contrast to the microsatellite
study, we found unidirectional introgression into B. pubescens from both
of the diploid species. This pattern fits better with the expected nature
of the reproductive barrier between diploids and tetraploids. As in the
microsatellite study, introgression into B. pubescens showed clear clines
with increasing introgression from B. nana in the north and from B.
pendula in the south. Unlike B. pendula alleles, introgression of B. nana
alleles was found far from the current area of sympatry or allopatry
between B. nana and B. pubescens. This pattern fits a shifting zone of
hybridization due to Holocene reduction in the range of B. nana and
expansion in the range of B. pubescens.