Physical interactions between pollen and pistil tissues mediate cryptic female choice in Brassica rapa

Cryptic female choice - female-mediated bias in fertilization after mating - is well established in animals and can also occur in plants when multiple pollens compete on the same pistil. However, whether interactions between pollen and pistil tissues after pollen deposition contribute to this process remains unknown. Here, we experimentally test whether such interactions mediate cryptic female choice in the angiosperm Brassica rapa. We quantified fertilization success of pollen donors competing on the same pistil using paternity analyses, and in parallel, made semi-in vivo assays to measure pollen tubes trajectories emerging from the excised styles and growing toward unfertilized ovules for each donor-recipient pair. We show that pollen tube growth towards ovules predicts higher fertilization success under pollen competition. Thus, we document a previously unobserved mechanism of cryptic female choice based on physical interactions between male and female components of reproduction. In addition, different recipient plants favour different pollen donors, consistent with non-directional female choice. Plants with longer styles bias paternities more strongly towards the most successful pollen donor. Overall, our study demonstrates that interactions between pollen tubes and pistil tissues after pollen germination enable plants to bias paternity toward particular donors.