What happens when pollinators bring the wrong pollen?

Explaining what plants are where is one of the goals of ecology. How do plants compete for space? In a Viewpoint article in Annals of Botany, Gerardo Arceo-Gómez argues that while there is a lot of study of floral visitation and pollinator choice, there have not been so many on heterospecific pollen (HP) transfer. This happens when a pollinator brings pollen from another species to a plant that it cannot use. Arceo-Gómez writes that this is a more direct form of plant to plant interaction that happens everywhere and can have strong fitness effects.

Which plants get heterospecific pollen? Arceo-Gómez lays out four predicted sources of within-species variation of receipt heterospecific pollen.

• Conspecific flower density, a measure of how many flowers around have the right pollen.
• Fine-scale spatial structure, how well-mixed in are the flowers with wrong pollen?
• Pollinator community composition, are there plenty of specialists or are there more generalists?
• Co-flowering community composition, how species-rich is the locality? This is also a factor for pollinators as well as the pollinated.

Arceo-Gómez goes on to discuss that once you have heterospecific pollen, there are factors that can change how it can change fitness. This might be due to environmental resource variability or whether a plant has a history of co-existing with another plant. Another issue might be its mating system. Does it rely on outcrossing, or can it self-pollinate?

“For instance, Fishman and Wyatt (1999) demonstrated that selection favoured selfing and selfing-related traits only in Arenaria uniflora populations that grew in sympatry with congeneric A. glabra,” writes Arceo-Gómez. “They further showed that HP transfer rather than pollinator competition was the main driver of selection (Fishman and Wyatt, 1999). Thus, HP transfer has the potential for generating divergence not only in floral traits among populations, but also in patterns of mating system evolution.”

“So far, the study of pollinator-mediated plant–plant interactions has been almost entirely dominated by studies of pre-pollination interactions even though their outcomes are influenced by plant–plant interactions that take place on the stigma after pollen has been deposited. Therefore, it is paramount that we fully evaluate the causes, consequences and context-dependency of HP transfer interactions in order to gain a more complete understanding of the role that plant–pollinator interactions play in generating and organizing plant biodiversity.”

RESEARCH ARTICLE

Arceo-Gómez G. 2021. Spatial variation in the intensity of interactions via heterospecific pollen transfer may contribute to local and global patterns of plant diversity. Annals of Botany 128: 383–394. https://doi.org/10.1093/aob/mcab082