Uncovering genes involved in pollinator-driven mating system shifts and selfing syndrome evolution in Brassica rapa

BackgroundShifts in pollinator occurrence and their pollen transport effectiveness drive the evolution of mating systems in flowering plants. A decline in pollinator numbers can lead to the phenotypic evolution of floral traits favoring self-pollination (selfing syndrome). Understanding the genomic basis involved in such shifts of the mating system is crucial for predicting a species persistence or extinction under changing biotic and abiotic stressors in natural populations. We investigated loci showing high genetic differentiation before and after selection in fast-cycling Brassica rapa that were associated with rapid phenotypic evolution toward the selfing syndrome (reduction in petal width, stamen and pistil length, and herkogamy). Combining a genotyping-by-sequencing (GBS) approach with a genome-wide association study (GWAS), we shed light onto the genetic basis associated with the mating system shift over nine generations of pollination by Episyphrus balteatus (hoverflies), an abundant pollinator of B. rapa. Moreover, we functionally validated the involvement of candidate genes associated with changes in floral morphology by studying corresponding mutants in the model plant Arabidopsis thaliana. ResultsWe found that the mating system of B. rapa shifted from predominantly outcrossing to mixed mating with high rates of autonomous selfing, accompanied by a rapid reduction of floral morphological traits and herkogamy involving many loci. We found 31 candidate genes associated with the affected traits that were involved in a wide range of functions from DNA/RNA binding to transport. Our functional validation in A. thaliana confirmed that four of the identified genes are indeed involved in regulating the size of floral organs. Interestingly, two genes, both coding for the same protein complex responsible for active DNA methylation were successfully validated and closely linked to two correlated phenotypic traits, namely pistil and stamen length. ConclusionsAltogether, our study shows that hoverfly pollination leads to rapid evolutionary changes of the mating system through polygenic changes, highlighting the importance of using genomic approaches to understand pollinator-driven plant adaptation.