In self-compatible plants, small populations may experience reduced outcrossing owing to decreased pollinator visitation and mate availability. We examined the relation between outcrossing and population size in eastern Ontario populations of Aquilegia canadensis. Experimental pollinations showed that the species is highly self-compatible, and can achieve full seed-set in the absence of pollinators via automatic self-pollination. We estimated levels of outcrossing (t) and parental inbreeding coefficients (F) from allozyme variation in naturally pollinated seed families for 10 populations ranging in size from 32 to 750 reproductive individuals. The proportion of seeds produced through outcrossing was generally low (mean = 0.29 +/- 0.02 SE) and varied widely among populations (range = 0.00-0.83). Accordingly, estimates of F were large (mean = 0.26 +/- 0.05) and significantly greater than zero in seven populations. As expected, four small populations (N < 40) outcrossed less (0.17 +/- 0.03) than six large populations (N > 90; 0.38 +/- 0. 03). However, parental plants were not significantly more inbred in small than large populations (P = 0.18). There was no difference in the germination of seeds from hand self- and cross-pollinations. However, population genetic estimates of inbreeding depression for survival expressed from seed to reproductive maturity were very high (mean delta = 1 - relative fitness of selfed seed = 0.88 +/- 0.14). The combination of self-compatibility and automatic self-pollination makes the mating system of A. canadensis sensitive to variation in ecological factors that affect the likelihood of cross-pollination.
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