The stability of Quaternary speciation: a case study in Primula sect. Auricula

Primula sect. Auricula, a group of 25 species distributed in the European Alpine System, has been hypothesised to have diversified in the Quaternary through speciation in geographically isolated glacial refugia. We here examine whether the integrity of species is endangered through hybridisation upon contact in the Holocene. To do this, we (1) critically screened the literature for reported hybrids and supplemented this with our own knowledge of the group, (2) performed an admixture analysis of AFLP variation of two partly sympatric species pairs, P. hirsuta/P. daonensis and P. latifolia/P. marginata, and (3) analysed long-known hybrid populations of P. lutea 9 P. hirsuta in Wipptal/Austria to identify possible mechanisms of reproductive isolation. The literature survey revealed that populations of the 32 hybrid combinations known have been observed at 63 localities. In the admixture analysis, two admixed individuals per species were found among 524 individuals of P. latifolia and P. marginata, and 21 admixed individuals were found among 234 individuals of P. hirsuta. The analysis of P. lutea 9 P. hirsuta hybrids revealed that they show reduced pollen and seed fertility, and are limited to soils with intermediate pH values. We conclude that although species of P. sect. Auricula can readily be hybridised experimentally, hybridisation is rare in nature and species are stable. Mechanisms of reproductive isolation include geographical and ecogeographical isolation, ecological hybrid inviability and reduced hybrid fertility.