Recipient and donor characteristics govern the hierarchical structure of heterospecific pollen competition networks

Pollinator sharing can have negative consequences for plant fitness with the arrival of foreign (i.e. heterospecific) pollen, yet responses are often variable among species.  Plant traits and relatedness of donor and recipient species have been suggested to drive the variations in plant fitness, but how they shape the structure of pollen competition networks has been overlooked at the community level.
To understand the importance of reproductive traits and relatedness on the impacts of heterospecific pollen we conducted a controlled glasshouse experiment with an artificial co-flowering community. We performed 1800 reciprocal crosses by experimentally transferring 50% and 100% foreign pollen among 10 species belonging to three different plant families.
We found a significant reduction in seed set with 50% foreign pollen for 67% of the crosses driven largely by recipient traits and the interaction between recipient-donor traits under specific circumstances of trait-matching. In general, species with shorter styles, smaller stigmas and lower pollen:ovule ratios were more impacted by foreign pollen. These traits and their differences among species led to a hierarchical (or transitive) structure of pollen competition with clear winners and losers. However, phylogenetic distance among recipient and donor species did not explain the effects.

Synthesis: Our study shows that specific traits and trait combinations between donor and recipient species are important in determining fitness outcomes with heterospecific pollen deposition. Moreover, the differences in traits between species lead to a competitive structure with clear “winners” or “losers” species. The results of this study indicate the need to shift from pairwise to community level interactions to elucidate the mechanisms underlying foreign pollen impacts upon plant reproductive fitness.