Microsatellite evidence for low genetic diversity and reproductive isolation in tetraploid<i>Centaurea seridis</i>(Asteraceae) coexisting with diploid<i>Centaurea aspera</i>and triploid hybrids in contact zones

Abstract

Survival of polyploids in nature depends on several factors, including competition from diploid relatives and&#13;\nincreased genetic diversity. Unlike other reported Centaurea polyploid complexes, diploid Centaurea aspera and&#13;\ntetraploid Centaurea seridis coexist in hybrid zones with frequent triploid individuals. The polyploid origin of&#13;\nC. seridis, the genetic diversity and population structure of the three cytotypes, and the degree of genetic&#13;\ndifferentiation among them were analyzed in seven mixed-ploidy zones, involving different subspecies and&#13;\necological conditions. Ploidy was determined by flow cytometry. Microsatellite data suggested an allopolyploid&#13;\norigin of C. seridis. In the contact zones, diploids and tetraploids were genetically differentiated. When compared&#13;\nwith the related C. aspera, a low genetic diversity was observed in C. seridis, which is uncommon in tetraploids.&#13;\nFurthermore, although diploid individuals were grouped in a single widespread genetic cluster, tetraploids were&#13;\ngrouped in two highly differentiated clusters and showed significant isolation by distance. This genetic pattern in&#13;\nC. seridis may be related to a minimal gene flow with diploid relatives and/or other genetic factors, such as rare&#13;\npolyploidization events, founder effects or an increased selfing rate. Neither taxonomic assignment at subspecies&#13;\nlevel, nor ecological conditions could explain the genetic differentiation between tetraploid clusters.