The abundance of pollen coat small signaling proteins shows limited convergence between independent selfing transitions in Arabidopsis and Capsella

In plants, a key example of convergence is the repeated evolution of floral traits associated with the transition from outcrossing to self-fertilization, often resulting in the 'selfing syndrome' (e.g. reduced flower size and loss of scent). However, potentially overlooked changes concern the pollen coat, which plays a role in different aspects of outcrossing strategy. To investigate this, we compared pollen coat thickness and pollen proteomes between selfing and outcrossing species in the Arabidopsis and Capsella genera, representing three independent selfing transitions. Pollen coat area diminished with the age of the selfing transition, with significant reductions in A. thaliana and Capsella rubella, but not in A. lyrata (recent transition, no selfing syndrome). Moreover, specific differentially abundant pollen coat proteins were shared across selfer-outcrosser contrasts more than expected by chance, even though the direction of changes was not consistent across the three comparisons. These specific proteins, while annotated as pathogen response due to their structure, are likely involved in pollen-pistil interactions instead. Overall, our findings reveal limited convergent evolution of pollen coat thickness and protein composition following selfing transitions. Our work identified a group of pollen coat proteins that are largely understudied, but whose abundance appears to be a hotspot for evolutionary changes.