Day and night pollinators contribute separately to the reproduction of a sand verbena (Abronia fragrans, Nyctaginaceae)

Background/Question/Methods

Floral morphology is often shaped by evolution to effective pollinators; as suites of pollination-syndrome traits change across species or populations, it is tempting to conclude concurrent shifts in functional pollinator groups. But while floral morphology variation is often solely attributed to primary pollinator variation, less is known about the roles of less-effective, secondary pollinators in driving shifts in pollination-syndrome traits. To explore the evolutionary drivers and ecological consequences of floral trait changes within a species, we investigated a white-pink floral transition in a widespread sand verbena, Abronia fragrans. Across most of its range, A. fragrans has day-closing, white flowers and is nocturnally pollinated, compatible with a moth pollination syndrome; yet, in a small region intersecting NW Texas and SW Oklahoma, A. fragrans has pink flowers that stay open later into the day. To test whether these floral morphs are adapted to local pollinator communities, we excluded floral visitors from inflorescences during the day and/or night, in natural white- and pink-flowered populations to isolate nocturnal and diurnal pollinator contributions. We applied exclusion treatments for the entirety of individual inflorescence flowering periods on 2,115 white flowers (in KS) and 18,259 pink flowers (in TX).

Results/Conclusions

Consistent with morphological predictions, we found no daytime pollination and no difference between night-only and overall seed set in a white-flowered A. fragrans population — suggesting that nocturnal pollinators are the sole pollinators of the white-floral morph. In contrast, in a pink-flowered population, more than 10% of pollination occurred during the day (probably entirely from butterflies), and daytime pollination was nearly additive with nighttime (90% contribution from nocturnal moths). This distinction between respective day and night pollination suggests that diurnal and nocturnal pollinators provide independent fitness benefits for pink-flowered A. fragrans. Since all flowers close through at least most of the day and nocturnal pollination accounts for most fitness, these pink A. fragrans populations still rely primarily on nocturnal moths; however, diurnal pollinators contribute significantly to their overall reproductive output. In a night-pollinated system, the reproductive gain from day pollination suggests that the pink-floral morph is adapted to secondary, diurnal pollinators. This A. fragrans pollination-syndrome trait shift, seemingly driven by a secondary pollinator, is fascinating because secondary lepidopteran pollinators typically contribute no fitness advantage. Future work will include pollinator-observations and -exclusion experiments with a reciprocal transplant to explore the secondary-pollinator local-adaptation hypothesis as a mechanism driving this floral trait transition.