McAllister, H. A. | bioRxiv Science

bioRxiv2020

Molecular and morphological analyses clarify species delimitation and reveal a new Betula species in section Costatae

Background and AimsDelineating closely related and morphologically similar species with overlapping ranges can be difficult. Here, we use section Costatae (genus Betula) as a model to resolve species and subspecies boundaries in four morphologically similar trees: Betula ashburneri, Betula costata, Betula ermanii and Betula utilis (including ssp. utilis, and diploid and tetraploid races of ssp. albosinensis). MethodsWe genotyped 298 individuals (20-80 per species) from 38 populations at 15 microsatellite markers and a subset of 34 individuals from 21 populations using restriction-site associated DNA sequencing (RAD-seq). Morphometric analysis was conducted to characterise leaf variation for a subset of 89 individuals. Key ResultsMolecular analyses and leaf morphology found little differentiation between B. ashburneri, diploid B. utilis ssp. albosinensis and some samples of B. utilis ssp. utilis suggesting that these should be treated as a single species. By contrast, tetraploid Betula utilis ssp. albosinensis was divided into two groups with group I genetically similar to B. utilis ssp. utilis based on SNPs and group II, a very distinct cluster, which we propose as a new species, namely, Betula buggsii. Phylogenomic analysis based on 2,285,620 SNPs show a well-supported monophyletic clade of B. buggsii, forming a sister with a well-supported clade of B. ashburneri, diploid B. albosinensis and some samples of B. utilis ssp. utilis. Morphologically, Betula buggsii is characterised by elongated lenticels and a distinct pattern of bark peeling. Betula buggsii is geographically restricted to the Qinling-Daba Mountains. ConclusionsOur study reveals six genetically distinguishable species: B. ashburneri, B. buggsii, B. costata, B. utilis ssp. utilis, B. utilis ssp. albosinensis and B. ermanii. Our research demonstrates an integrative approach in delimitating species using morphological and genetic samples from their nearly entire distributions. Analyses based on subsets of species distributions may lead to erroneous species or subspecies delineation.

evolutionary biology↗

bioRxiv2020

Resolving phylogeny and polyploid parentage using genus-wide genome-wide sequence data from birch trees

Numerous plant genera have a history including frequent hybridisation and polyploidisation, which often means that their phylogenies are not yet fully resolved. The genus Betula, which contains many ecologically important allopolyploid tree species, is a case in point. We generated genome-wide sequence data for 27 diploid and 31 polyploid Betula species or subspecies using restriction site associated DNA (RAD) sequences assembled into contigs with a mean length of 675 bp. We reconstructed the evolutionary relationships among diploid Betula species using both supermatrix and species tree methods. We identified progenitors of the polyploids according to the relative rates at which their reads mapped to contigs from different diploid species. We sorted the polyploid reads into different putative sub-genomes and used the extracted contigs, along with the diploid sequences, to build new phylogenies that included the polyploid sub-genomes. This approach yielded a highly evidenced phylogenetic hypothesis for the genus Betula, including the complex reticulate origins of the majority of its polyploid taxa. The genus was split into two well supported clades, which differ in their seed-wing morphology. We propose a new taxonomy for Betula, splitting it into two subgenera. We have resolved the parentage of many widespread and economically important polyploid tree species, opening the way for their population genomic study.

evolutionary biology↗