Maintenance of self-incompatibility in peripheral populations of a circumboreal woodland subshrub

Compared with self-incompatible (SI) species, species that shift to self-compatibility (SC) are more likely to colonize a new habitat. Self-incompatibility and fruit-set failure have been widely reported in European populations of Linnaea borealis (twinflower), whereas at the eastern margin of its North American distribution it showed potential SC. We investigated the breeding system of L. borealis in northwestern China, the eastern margin of the species' distribution in Eurasia. Pollinators, breeding system and pollen limitation were examined in a nature reserve with thousands of L. borealis individuals. To investigate whether fruit set was limited by mating opportunity, we compared fruit set in high-, medium- and low-density patches of L. borealis. To examine whether clonal reproduction resulted in higher fruit-set failure, we compared fruit set among different sizes of clonal ramets. Flies contributed most pollinator visits in the studied population. It was strictly SI and natural fruit set depended on insect visits. Patch density comparisons showed that L. borealis was not pollen limited in low-density patches that had significantly fewer flowers. However, it produced significantly fewer fruits per flower when clonal ramet size increased, suggesting that the high failure of fruit set in larger clones with more flowers may be caused by geitonogamy. Generalist pollinators and clonal reproduction may help L. borealis to colonize in marginal areas without the transition of the breeding system from SI to SC, but experiencing fruit-set failure resulting from geitonogamy within clones.