Mosaic distribution of cytotypes in a mixed-ploidy plant species: nested environmental niches but low geographic overlap

Abstract

Polyploids often have divergent geographical ranges compared to their diploid progenitors, but the causes of such differentiation are poorly understood. The geographical ranges of cytotypes within polyploid complexes may be caused by multiple factors, including historical events, interactions among cytotypes and divergent environmental tolerances, but the fine-scale geographical arrangement of cytotypes is rarely known for most mixed-ploidy species. In this study, we assessed cytotype diversity and distribution patterns in the Jasione montana polyploid complex and examined whether environmental factors can explain the occurrence of tetraploids. Specifically, we reviewed all chromosome counts available in the literature, examined cytotype distributions in a large-scale population survey using flow cytometry (N = 278 populations, N = 3396 plants), and used niche modelling to compare cytotype environmental associations. Two cytotypes were detected: diploids, which are widespread across Europe, and tetraploids, restricted to the north-west quadrant of the Iberian Peninsula. The two cytotypes were distributed in a mosaic with areas dominated by diploids intermixed with those dominated by tetraploids, rarely forming mixed-ploidy populations (1.4%). Although having low geographical overlap, the tetraploid niche is fully nested within the diploid niche breadth and occupies only a subset of the environmental envelope of the diploid progenitor, suggesting that polyploidization has not caused niche expansion due to novel environmental preferences. The mosaic diploid-tetraploid contact zones and the lack of mixed-ploidy populations suggest that frequency-dependent selection may play a role in excluding minority cytotypes. Under this scenario, tetraploids would have to disperse to places unoccupied by diploids to successfully establish. The aggregation of tetraploid populations in areas suitable for diploids suggests that tetraploids may also outcompete diploids in certain areas. Collectively, our results indicate that environmental sorting has played a role, at least on a broader scale, in the successful establishment of polyploids in J. montana.