Pollinator-mediated selfing erodes the flexibility of the best-of-both-worlds mating strategy in 'Bulbine vagans'

Abstract

1. The best-of-both-worlds hypothesis proposes that delayed autonomous selfing assures reproduction when pollinators are absent, but allows outcrossing when pollinators are present without incurring seed and pollen discounting. However, because self-compatibility is necessary for delayed selfing, discounting can be incurred if pollinators mediate selfing concurrently with outcrossing. The extent that pollinator-mediated selfing erodes the best-of-both-worlds mating strategy is largely unknown. 2. In this study, we assess how delayed selfing and pollinator-mediated selfing contribute to reproductive output under contrasting pollination environments in 'Bulbine vagans' (Asphodelaceae). 3. We manipulated floral phenotype under contrasting weather conditions, plant densities and floral display. We emasculated flowers at early-flowering (allowing only outcrossing) or late-flowering (eliminating delayed selfing) stages and compared reproductive output with intact flowers (outcrossing + pollinator-mediated selfing + delayed selfing). 4. During inclement weather, few or no pollinators visited flowers. Reproductive output was severely limited by cross pollen, and delayed selfing provided reproductive assurance. During fine weather, cross pollen was also limiting, but pollinator-mediated selfing boosted reproductive output before delayed selfing could occur. 5. On isolated plants, flowers received fewer pollinator visits and reproductive output was limited more by cross pollen compared with grouped plants. Despite these differences, reproductive output of both isolated and grouped plants was boosted to similar levels by pollinator-mediated selfing before delayed selfing could occur. 6. One- and three-flowered plants were similarly limited by cross pollen. On three-flowered plants, facilitated and geitonogamous selfing boosted reproductive output before delayed selfing could occur. By contrast, on one-flowered plants geitonogamy was eliminated, and pollinator-mediated selfing was reduced. This provided mating flexibility by retaining opportunities for subsequent outcrossing, although delayed selfing boosted reproductive output. 7. Reproductive output was consistently limited by cross-pollination. Yet, delayed selfing provided reproductive assurance only during inclement weather and on one-flowered plants. Under other conditions, pollinators mediated selfing that could render ovules unavailable for outcrossing as selfing occurred and for outcrossing that might occur subsequently. We conclude that best-of-both-worlds flexibility will often be eroded by pollinator-mediated selfing that inevitability occurs with outcrossing.