Gene transfer across species boundaries in bryophytes: evidence from major life cycle stages in Homalothecium lutescens and H. sericeum.

BACKGROUND AND AIMS: The mosses Homalothecium lutescens and H. sericeum are genetically, morphologically and ecologically differentiated; mixed populations sometimes occur. In sympatric populations, intermediate character states among gametophytes and sporophytes have been observed, suggesting hybridization and introgression in such populations.
METHODS: We determined genotypes using bi-allelic co-dominant single nucleotide polymorphism (SNP) markers, specific to either H. lutescens or H. sericeum, to estimate the degree of genetic mixing in 449 moss samples collected from seven sympatric and five allopatric populations on the island of Öland, south Sweden. The samples represented three generations: haploid maternal gametophytes; diploid sporophytes; and haploid sporelings. KEY
RESULTS: Admixture analyses of SNP genotypes identified a majority as pure H. lutescens or H. sericeum, but 76 samples were identified as mildly admixed (17 %) and 17 samples (3.8 %) as strongly admixed. Admixed samples were represented in all three generations in several populations. Hybridization and introgression were bidirectional.
CONCLUSIONS: Our results demonstrate that admixed genomes are transferred between the generations, so that the populations behave as true hybrid zones. Earlier studies of sympatric bryophyte populations with admixed individuals have not been able to show that admixed alleles are transferred beyond the first generation. The presence of true hybrid zones has strong evolutionary implications because genetic material transferred across species boundaries can be directly exposed to selection in the long-lived haploid generation of the bryophyte life cycle, and contribute to local adaptation, long-term survival and speciation.