A geographic cline in the ability to self‐fertilize is unrelated to the pollination environment

Abstract

The reproductive assurance (RA) hypothesis predicts that the ability to autonomously self-fertilize should be favored in environments where a lack of mates or pollinators limits outcross reproduction. Because such limits to outcrossing are predicted to be most severe at range edges, elevated autonomy in peripheral populations is often attributed to RA. We test this hypothesis in 24 populations spanning the range of Campanula americana, including sampling at the range interior and three geographic range edges. We scored autonomous fruit set in a pollinator-free environment and detected clinal variation-autonomy increased linearly from the southern to the northern edge, and from the eastern to the western edge. We then address whether the cline reflects the contemporary pollination environment. We measured population size, plant density, pollinator visitation, outcross pollen limitation and RA in natural populations over two years. Most populations were pollen limited, and those that experienced higher visitation rates by bumblebees had reduced pollen limitation. Reproductive assurance, however, was generally low across populations and was unrelated to pollen limitation or autonomy. Neither pollen limitation nor RA displayed geographic clines. Finally, autonomy was not associated with pollinator visitation rates or mate availability. Thus, the data do not support the RA hypothesis; clinal variation in autonomy is unrelated to the current pollination environment. Therefore, geographic patterns of autonomy are likely the result of historical processes rather than contemporary natural selection for RA.

Selfing ability and drift load evolve with range expansion

“…There is also little apparent geographic structure to mate or pollinator availability across the species' geographic range (Koski et al. ). Viewed together, these results indicate that variation in a key reproductive trait is not explained by the current environments in which that trait is expressed.…”

“…The capacity to autonomously self is higher in populations in the northwestern part of the range (e.g., Minnesota and Iowa) (Koski et al. ). Here, we work with 24 previously described populations west of the Appalachian Mountains (Koski et al.…”

“…However, most studies of variation in mating systems explore its association with reproductive assurance in contemporary environments (Eckert ; Herlihy & Eckert ; Koski et al. ; Opedal et al. ).…”

“…Importantly, there is clinal variation among populations in the capacity to autonomously self‐fertilize—populations in the northwestern part of the range self‐fertilize more readily than those in the southeast (Koski et al. ). Koski et al.…”

“…Koski et al. () found variation among natural populations in mate and pollen limitation across two years. However this variation was not associated with the geographic cline in autonomy, suggesting that contemporary ecological selective conditions are unlikely to underlie variation in this important mating system trait.…”