Contact zones as natural laboratories for cryptic speciation: genetic and ecological divergence in the autopolyploid...

Polyploidy is a major driver of plant evolution and speciation. Autopolyploidy, in particular, plays a key role in the emergence of cryptic lineages that are morphologically similar yet genetically and ecologically distinct. We investigated the extent of genetic exchange and climatic differentiation between diploid (2×) and hexaploid (6×) cytotypes of Aster amellus (Asteraceae) within a narrow contact zone in Central Europe to assess whether reproductive isolation and divergence are maintained where cytotypes occur in close proximity. We implemented a spatially explicit sampling of 735 individuals from 32 adjacent diploid and hexaploid populations located 1-10 km apart. Genetic variation was analysed using nuclear microsatellite markers, and ecological divergence was quantified through hypervolume-based niche analysis. We found strong genetic differentiation between cytotypes, with hexaploids exhibiting higher genetic diversity and occupying a broader climatic space than diploids. Genetic connectivity was high within cytotypes, but sharply reduced between them, and evidence of a recent polyploidization event in one population revealed hexaploid individuals genetically close to the diploid cluster, suggesting recurrent formation of hexaploids. Our findings indicate that reproductive isolation between cytotypes remains effective even at fine spatial scales, maintaining both genetic and ecological divergence despite geographical proximity. Fine-scale sampling reveals that narrow diploid-polyploid contact zones increase the likelihood of detecting early-stage polyploid formation before such signals are obscured by drift or selection, reinforcing their role as key natural laboratories for studying cryptic autopolyploid speciation.