High-Throughput Sequencing of 5S-IGS rDNA in Fagus L. (Fagaceae) reveals
complex evolutionary patterns and hybrid origin of modern species
Standard models of speciation assume strictly dichotomous genealogies in
which a species, the ancestor, is replaced by two offspring species. The
reality is more complex: plant species can evolve from other species via
isolation when genetic drift exceeds gene flow; lineage mixing can give
rise to new species (hybrid taxa such as nothospecies and
allopolyploids). The multi-copy, potentially multi-locus 5S rDNA is one
of few gene regions conserving signal from dichotomous and reticulate
evolutionary processes down to the level of intra-genomic recombination.
Here, we provide the first high-throughput sequencing (HTS) 5S
intergenic spacer (5S-IGS) data for a lineage of wind-pollinated
subtropical to temperate trees, the Fagus crenata – F. sylvatica s.l.
lineage, and its distant relative F. japonica. The observed 4,963 unique
5S-IGS variants reflect a long history of repeated incomplete lineage
sorting and lineage mixing since the early Cenozoic of two or more
paralogous-homoeologous 5S rDNA lineages. Extant species of Fagus are
genetic mosaics and, at least to some part, of hybrid origin.