A hybridisation barrier between two evolutionary lineages of Barbarea vulgaris (Brassicaceae) that differ in biotic resistances

Abstract

Hybridisation barriers are likely to evolve during allopatric separation of populations, in parallel with divergent adaptation to different conditions in the two ranges. If the populations secondarily come into contact, limited interbreeding between them may affect the subsequent spread of the two lineages and their adaptations in the region of sympatry. Barbarea vulgaris include two genetically divergent lineages that differ in secondary metabolites and resistances to insects and a pathogen. The two plant types grow in different Eurasian ranges but co-occur in Denmark and neighbouring countries, posing the question why they have not merged and the resistances spread from one type into the range of the other. Here, we tested whether a hybridisation barrier contribute to this. Different proportions of plants of the two types were placed in net tents and pollinated by flies, and paternity of the resulting seeds determined with genetic markers. Lower proportions of fruits and seeds developed successfully in mixtures with higher proportions of heterotypic plants (i.e. of the other plant type). When combined with results on offspring paternity in a statistical analysis, we found that heterotypic pollen was much less successful in fertilizing embryos and that heterotypic seeds survived less frequently than the contypic. Mature F1 hybrids in addition produced lower proportions of mature pollen. The two B. vulgaris plant types are thus separated by substantial prezygotic and postzygotic barriers, as strong as reported for crosses between closely related but taxonomically recognised plant species. This may explain why the two B. vulgaris types have not merged in sympatry and why genes for insect-resistance have not introgressed to any extent from one to the other.