Many factors contribute to reproductive isolation between self-pollinating and outcrossing morning glory
Authors: Kate Ostevik 1,2; Joanna Rifkin 2,3; Irene Liao 2,4; Mark Rausher 2
Affiliations: 1) University of California Riverside, Riverside, CA; 2) Duke University, Durham, NC; 3) University of Toronto, Toronto, ON; 4) University of California Los Angeles, Los Angeles, CA
Keywords: Complex traits
Highly selfing plant species have repeatedly diverged from outcrossing relatives. Diverse mechanisms, some specific to the selfer-outcrosser context, can lead to reproductive isolation between selfing and outcrossing species, and multiple mechanisms may act in any speciation event. Here, we dissect two reproductive barriers between the highly selfing morning glory Ipomoea lacunosa and its mixed-mating sister species I. cordatotriloba. We find that the crossing barrier (failure of hybrid seed set) is complex, with contributions from barrier components both before and after fertilization and extensive parental sex asymmetry. Fertilization failure varies by cross direction and is partially affected by pollen and style size. We find strong evidence that fertilized seeds in interspecific crosses fail to mature, and that cross direction affects seed provisioning. Genetic mapping of the crossing barrier also reveals multiple factors with variable strengths contribute to cross incompatibility between these species, parental sex asymmetries are common, and imprinting in hybrid seed provisioning is likely to be a major component. In contrast, hybrid pollen sterility is genetically simple and overwhelmingly caused by two epistatically interacting loci. This study highlights the importance of carefully dissecting reproductive barriers into their components, which in this case are diverse and could have independent evolutionary histories.