Co-flowering community effects on the relative contribution of pollen quantity and quality limitation to the reproductive success of four Clarkia species

Background/Question/Methods

Most flowering plants rely on biotic pollination for seed production. However, pollination failure can still occur due to inadequate quantity or quality of pollen delivered (pollen limitation). In fact, 60% of plant populations have been shown to be pollen limited. Pollen limitation can occur due to low pollinator availability or when low quality pollen fails to fertilize the ovules. Co-flowering species share pollinators, and can influence each other's pollination by attracting a larger pollinator pool compared to when flowering alone (pollinator facilitation), resulting in greater pollen loads. On the contrary, a limited pollinator pool can lead to pollinator competition and decrease the amount of pollen received. Lastly, pollinator competition may increase self-pollination or heterospecific pollen transfer, hence increasing pollen quality limitation. Here, we evaluate the contribution of pollen quantity and quality limitation to plant reproductive success across a gradient of co-flowering community diversity.We used four co-flowering Clarkia species in the Sierra Nevada as our study system. Species bloom singly and in combination of 1-3 other Clarkia species. Styles from all species (~60/species) were collected across 25 sites varying in co-flowering diversity. 1600 styles were processed in the lab and the amount of pollen grains and pollen tubes were counted.

Results/Conclusions

Clarkia speciosa received the most pollen grains (477 grains) and Clarkia cylindrica the least (300 grains) on average across all populations. Preliminary data also shows that pollen quantity limitation decreases in communities where all four Clarkia species co-flower compared to when they flower singly or with only one-two other species. This pattern was observed for all Clarkia species. For instance, Clarkia unguiculata alone received 465.68 (± 30.63) pollen grains on average compared to 840.65 (±56.03) when co-flowering with all three other Clarkia species. Clarkia xantiana received 41.50 (± 48.82) pollen grains when flowering alone compared to 407.8 (± 62.19) when co-flowering with all other Clarkia species. We are currently collecting data on pollen tube number to evaluate differences in pollen quality limitation. Our preliminary data suggests pollinator facilitation can be a key mechanism reducing pollen quantity limitation, promoting species co-existence, and increasing plant reproductive success in this system. However, it is still not known if higher pollen loads lead to higher fertilization (work in progress). Clarkia species can self-pollinate (low quality pollen) or receive pollen from a different Clarkia which will not lead to fertilization and thus pollen quality could still affect reproductive success and mediate co-existence in these communities.