Genetic variation using ddRADseq within the arctic-alpine Calamagrostis stricta (Poaceae) species complex in Europe, with emphasis on the newly described C. lonana.

The Calamagrostis stricta (Poaceae) species complex is a circumpolar, boreo-arctic and montane taxon, which is described with numerous subdivisions into subspecies and varieties. The recent discovery of Calamagrostis lonana Eggenberg & Leibundgut in the Alps, called for relationships within C. stricta to be assessed. The main aim of the study was to elucidate the phylogenetic position, genetic structure, and ploidy level of C. lonana, as compared to the C. stricta species complex from Central Europe to the Arctic. Fifteen populations of the C. stricta species complex (including C. lonana) were sampled across Central and Northern Europe and their ploidy level was estimated using flow cytometry. Genetic variation was characterized using double digest RAD sequencing reads (ddRADseq) on a total of 115 individuals genotyped at 1157 single nucleotide polymorphisms (SNPs). Tetraploidy was observed in Arctic populations from Northern Europe and C. lonana in the Alps, contrasting with other populations exhibiting higher ploidy levels. C. lonana was genetically closely related to the arctic C. stricta subsp. groenlandica, while C. stricta subsp. stricta formed a second genetic cluster across Central Europe. A third, very distinct genetic cluster was observed in the northern Svalbard archipelago. Despite lacking evidence of sexual reproduction, substantially more genetic diversity than expected was detected within populations in C. lonana and other investigated taxa. The distribution and genetic structure of the C. stricta species complex has been shaped by major post-glacial environmental changes having affected cold regions, and specifically highlights C. lonana as a valuable relict taxon for the Alps.