Distance Dependent Contribution of Ants to Pollination but Not Defense in a Dioecious, Ambophilous Gymnosperm

ARANDA-RICKERT, ADRIANA; TORRÉNS, JAVIER; YELA, NATALIA I.; BRIZUELA, MARÍA MAGDALENA; DI STILIO, VERÓNICA S.

Frontiers in Plant Science

Andre Kessler, Cornell University, United States

Año: 2021 vol. 12

Dioecious plants are obligate outcrossers with separate male and female individuals,which can result in decreased seed set with increasing distance between the sexes.Wind pollination is a common correlate of dioecy, yet combined wind and insectpollination (ambophily) could be advantageous in compensating for decreased pollenflow to isolated females. Dioecious, ambophilous gymnosperms Ephedra (Gnetales)secrete pollination drops (PDs) in female cones that capture airborne pollen and attractants that feed on them. Plant sugary secretions commonly reward ants in exchange forindirect plant defense against herbivores, and more rarely for pollination. We conductedfield experiments to investigate whether ants are pollinators and/or plant defenders ofSouth American Ephedra triandra, and whether their contribution to seed set and seedcone protection varies with distance between female and male plants. We quantifiedpollen flow in the wind and assessed the effectiveness of ants as pollinators byinvestigating their relative contribution to seed set, and their visitation rate in femaleplants at increasing distance from the nearest male. Ants accounted for most insectvisits to female cones of E. triandra, where they consumed PDs, and pollen load waslarger on bigger ants without reduction in pollen viability. While wind pollination wasthe main contributor to seed set overall, the relative contribution of ants was distancedependent. Ant contribution to seed set was not significant at shorter distances, yetat the farthest distance from the nearest male (23 m), where 20 times less pollenreached females, ants enhanced seed set by 30% compared to plants dependingsolely on wind pollination. We found no evidence that ants contribute to plant defenseby preventing seed cone damage. Our results suggest that, despite their short-rangemovements, ants can offset pollen limitation in isolated females of wind-pollinated plantswith separate sexes. We propose that ants enhance plant reproductive success viatargeted delivery of airborne pollen, through frequent contact with ovule tips whileconsuming PDs. Our study constitutes the first experimental quantification of distancedependentcontribution of ants to pollination and provides a working hypothesis forambophily in other dioecious plants lacking pollinator reward in male plants.