Pollen dispersal and optimal outcrossing in Delphinium nelsoni [11]

Abstract

NATURAL SELECTION, in sexually reproducing plants, should often favour matings between individuals of intermediate genetic similarity. Matings between very similar individuals may lead to inbreeding depression because segregational load is revealed1,2, while matings between very dissimilar individuals may disrupt favourable gene combinations and lead to outbreeding depression3-5. Outbreeding depression in plants has been documented in crosses between species, varieties and isolated populations6-9, and reports of inbreeding depression date back at least a century10 We suggest that outbreeding depression will often occur on a much finer scale than previously recognised, especially in plants subject to restricted pollen and seed dispersal. Such plants are likely to show pronounced microgeographic genetic differentiation resulting from drift in subpopulations isolated by distance or from adaptation to local edaphic and biotic conditions 11,12 Under these circumstances, a short outcrossing distance may be optimal not only because of intragenotypic effects, but also because it produces offspring sufficiently similar to the female parent to grow successfully near her, yet sufficiently genotypically diverse to maximise success of the total progeny in the face of coarse-grained temporal environmental variation 13-15, frequency-dependent sibling competition16-18 or predation19-20. Here we present evidence that a short outcrossing distance is optimal for Delphinium nelsoni Greene and discuss the relationship between the optimal outcrossing distance for D. nelsoni and actual pollen dispersal by its main pollinators. © 1979 Nature Publishing Group.