Polytopic origin and scale-dependent spatial segregation of cytotypes in primary diploid-autopolyploid contact zones of Pilosella rhodopea (Asteraceae)
Abstrakt
Populations of sympatric diploid and autopolyploid cytotypes provide a unique opportunity to study early stages of polyploid evolution. Pilosella rhodopea is a diploid-autopolyploid complex forming frequent mixed-ploidy populations, most probably representing the largest primary contact zone documented so far in angiosperms.
Our aims were to elucidate: (1) the origin of autopolyploids (single vs. multiple); (2) cytotype distribution patterns at various spatial scales; and (3) potential ecological differentiation of the cytotypes by measuring several habitat variables along two elevational transects. In total, five cytotypes were found across the species range.
Triploids were the most frequent (50%), followed by diploids (29%) and tetraploids (15%), whereas pentaploids and hexaploids were rare. Most populations were mixed-ploidy.
Both amplified fragment length polymorphism and cytotype distribution patterns suggested multiple origins of autopolyploids and frequent intercytotype gene flow. Cytotype diversity and spatial aggregation of the cytotypes were scale dependent.
At a local scale (5 m x 5 m), cytotype co-occurrence was frequent, whereas micro-scale plots (1 m x 1 m) were cytotypically more homogeneous. We did not find any evidence for ecological differentiation of the cytotypes.
Our results show that P. rhodopea is a rare example of a diploid-autopolyploid complex with polytopic and ongoing polyploid formations in primary contact zones. Recurrent formation of polyploids owing to a frequent intercytotype gene flow among spatially close cytotypes might explain, at least in part, the sympatric coexistence of cytotypes.