Reproductive isolation and drift load in selfing populations of an otherwise obligately outcrossing species

• Applicant: Dr. Marc Stift
• Subject Area: Evolution and Systematics of Plants and Fungi
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388824194

Hermaphroditic plants can theoretically reproduce through self-fertilisation (selfing), but most are self-incompatible or have evolved other mechanisms to promote cross-fertilisation (outcrossing). Transitions from outcrossing to selfing are among the most frequent transitions in plant evolution. Nevertheless, only a relatively small proportion of plant species (10-15%) reproduce mainly by selfing. So why is this? As a potential explanation, it was proposed that selfing promotes speciation, but that this diversifying effect is countered by even higher extinction rates. Although there is some phylogenetic support for this hypothesis, the mechanisms that promote speciation of selfing lineages on the one hand, and their increased extinction rates on the other hand, are still largely a black box. Here, I propose three studies to address this using the North American species Arabidopsis lyrata. Most populations of this species are self-incompatible and thus obligately outcrossing, but six populations have been discovered that exclusively consist of self-compatible plants with high selfing rates (selfing populations). This provides an excellent model to study the ecological drivers of the evolution of selfing, and its negative consequences in terms of drift load. In the first study, we will perform a common garden experiment to test whether changes in phenology and insect visitation reproductively isolate selfing populations from other populations. In the second study, we will test whether mechanisms that act after pollination (but still before fertilisation) contribute to reproductive isolation of selfing populations. Specifically, we will test whether crosses between plants from selfing populations and plants from other populations yield fewer seeds than within-population reference crosses. The third study will test whether, in a common garden, plants from selfing populations have a reduced performance compared to those from conspecific outcrossing populations, based on multi-year seed production and taking into account seed viability. Together, the first two studies will improve our conceptual understanding of the ecological processes that play a role in allowing selfing populations to develop into reproductively isolated, divergent lineages. Additionally, the third study will allow to test whether this diversifying process is counteracted by fitness deterioration due to drift load.

• DFG Programme: Research Grants