Explaining outcrossing rate in Campanulastrum americanum (Campanulaceae):: Geitonogamy and cryptic self-incompatibility

A flexible mating system may enable self-compatible plants to prevent self-fertilization under certain circumstances. Campanulastrum americanum is a self-compatible, protandrous herb. Although within-plant pollen transfer is likely and self pollen can produce a full set of seeds, selfing in natural populations is rare. We investigated explanations for the high outcrossing rate. One possibility was that self pollen contacts the stigma rarely. We surveyed floral displays in nature to gauge the potential for within-plant pollen transfer. Floral displays in nature are large enough to enable frequent geitonogamous pollinator visits and subsequent self-pollination. Alternatively, the high outcrossing rate could result from cryptic self-incompatibility, a mechanism that favors outcross pollen over self pollen. To determine whether outcross pollen has a seed-siring advantage over self pollen, we pollinated maternal plants with equal mixtures of self and outcross pollen. Genotyping the offspring revealed that outcross pollen sired significantly more seeds than self pollen. We explored whether differential growth rates of self and outcross pollen tubes produce cryptic self-incompatibility. Growth rates did not differ, indicating that cryptic self-incompatibility occurs by some other mechanism. Cryptic self-incompatibility enables C. americanum to reduce inbreeding when outcross pollen is present yet ensure reproduction when only self pollen is available.