Contrasting diversification patterns across wood rushes from Luzula sect. Luzula (Juncaceae) revealed by 3RAD genome-wide sequencing

Among the different mechanisms triggering diversification processes, chromosomal rearrangements that generate karyotypic changes are common in plants. Luzula (Juncaceae) is among the few angiosperm genera with holocentric chromosomes, which can undergo chromosome fission (agmatoploidy) or fusion (symploidy), resulting in karyotypes with different chromosome numbers and sizes. In this study, 3RAD genome-wide sequencing data and plastid sequences were used to explore evolutionary trends and patterns of genetic diversification among diploid taxa of Luzula sect. Luzula centred in the European Alpine System. In addition, we inferred its phylogenetic relationships to other closely related sections, of which several proved to be non-monophyletic. The species of Luzula sect. Luzula are segregated into three lineages, which show contrasting patterns regarding bifurcated branching, reticulation, and levels of coancestry as a result of different evolutionary histories. Agmatoploid species are found in two of these clades, displaying different karyotypes, while the third lineage comprises only L. campestris. Based on a molecular dating reconstruction, at least two putatively independent transitions towards agmatoploidy are estimated, which have occurred between the mid- and late Pleistocene in Luzula sect. Luzula. In addition, several trans-continental migrations, e.g. between Europe and Africa, were inferred. This study provides a new perspective on the complexity of diversification among wood rushes, which may serve as a basis for future exploration of the occurrence of agmatoploidy and its role in species diversification.