Effects of Density on Mixed Mating Systems and Reproduction in Natural Populations ofImpatiens capensis

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Abstract
Understanding the role of plant density in natural populations necessitates evaluation of density effects on both mating system and reproduction. I estimated parameters of the mixed mating system of Impatiens capensis, including plant outcrossing rate (t) and level of biparental inbreeding (mb), along with reproductive components at three densities typically found within three natural populations. As a result of coexisting cleistogamous (CL) and chasmogamous (CH) flowers on individual I. capensis, an estimate of t at each density was obtained by multiplying the percentage of CH seeds and the multilocus estimate of outcrossing rate among CH seeds (tCH). Estimation of mb was based on the relationships between fixation indices of selfed and outcrossed progeny. These estimates led to evaluation of average inbreeding coefficients in seeds (f) and at equilibrium (fe). When plant density decreased from high (>100 plants/m2) to low (ca. 20–30 plants/m2), the estimated tCH increased significantly from 0.44 to 0.62. The estimated mb and tCH were not correlated, but lower mb and higher tCH were significantly associated with higher fruit set and more seeds per CH fruit. I extended Ritland's estimation of inbreeding depression to the case of biparental inbreeding and found that mb affects inbreeding depression positively; the effect of t on inbreeding depression can be negative or positive, depending on the level of mb; magnitudes of both effects rely on the level of f. Plant mixed mating system not only responds to densities but also assists in maintaining population density through its association with reproduction and inbreeding depression.