Patterns of cytotype distribution and genome size variation in the genus SesleriaScop. (Poaceae)

Abstract

Polyploidization has played an important role in the diversification of the genus Sesleria (Poaceae), which comprises c. 48 species and subspecies mostly distributed in Europe. The genus' centre of diversity clearly is the Balkan Peninsula, harbouring about 80% of the species, half of which are endemic to this area. We employed chromosome counts, measurements of absolute genome size and determination of relative DNA-content for 460 populations belonging to 43 species of Sesleria. Our main aim was to provide essential baseline data for future molecular genetic reconstructions of the genus' evolutionary history. Relative genome size allowed for a mostly clear separation of four ploidy levels. The most frequent and widespread cytotypes are tetraploids followed by octoploids, while di- and dodecaploids were only found in a few species. We present first chromosome numbers for the tetraploid species S.doerfleri, S.phleoides, S.skipetarum and S.tuzsonii as well as for diploid S.ovata. Based on relative and partly also on absolute genome size measurements, ploidy level was determined in tetraploid S.rhodopaea and S.voronovii for the first time, and new cytotypes were identified in S.interrupta, S.kalnikensis and S.wettsteinii (tetraploids), S.caerulea, S.klasterskyi, S.latifolia, S.tenerrima, S.ujhelyii and S.vaginalis (octoploids), and S.albanica and S.vaginalis (dodecaploids). While most Sesleria species are ploidy-uniform, several comprise two or even, in the case of S.vaginalis, three ploidy levels. Genome downsizing after polyploidization was confirmed by significant negative correlation between ploidy level and monoploid genome size. Finally, we found a significant increase in monoploid relative genome size towards the margin of the genus' distribution area, which may be triggered by increased activity of transposable element in populations exposed to environmental or genomic stress.