Effects of Geitonogamy on the Seed Set of Magnolia obovata Thunb. (Magnoliaceae)

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Abstract
The effects of geitonogamy on the seed set of a protogynous hermaphroditic tree, Magnolia obovata Thunb., were investigated by electrophoretic allozyme analysis and pollination experiments. The multilocus outcrossing rates (tm) estimated in 1995 and 1997 by using four polymorphic loci were 0.39 and 0.42, respectively, indicating that more than half of the seeds are produced by geitonogamous selfing. Manual pollination demonstrated that the follicle set per fruit (frequency of ripe carpels) and seed set per fruit from self‐pollination were 2.6 and 3.8 times less than those from cross‐pollination, respectively, and the frequency of aborted seeds from self‐pollination was significantly higher than that from cross‐pollination. There was no significant difference between self‐ and cross‐pollination in the fruit set and the frequency of remnant ovules (ovules unfertilized or aborted before endosperm development). These results indicate that inbreeding depression is a primary factor reducing the seed set of self‐pollinated fruits. The fruit set, follicle set per tree, and seed set per tree from natural pollination were significantly lower than those from the manual pollinations. From these results, we estimated that 91% of ovules in M. obovata were not fertilized owing to pollen shortage, and 65% of embryos failed to develop because of geitonogamous selfing. The high embryo mortality rate from geitonogamy in M. obovata can be regarded as a cost of this tree’s pollination system, which is based on automimicry via asynchronous flowering.