Neighborhood Structure in Plants Under Diverse Reproductive Methods

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Abstract

Neighborhood size and area, and equations for their calculation, have been treated in relation to reproduction under diverse reproductive methods. With allogamy via homomorphic incompatibility as the standard system, consideration has been given to the effect of autogamy, allogamy via heteromorphic incompatibility and dioecy, and apomixis upon the neighborhood parameters of colonies. It has been shown that neighborhood size contracts under autogamy, as it does under dioecy or heteromorphic incompatibility if the sex or morph ratios are disproportionate. Neighborhood size may expand under heteromorphic incompatibility or dioecy through enhanced pollen carry-over if the sex ratio is 1:1 and if the plants are insect-pollinated. Because of the formation of clones which are incompatible inter se, it also expands if apomixis is an adjunct to allogamy. As an expansion of neighborhood size may be equated with enhanced outbreeding and a contraction with reduced outbreeding, it is evident that heteromorphic incompatibility and dioecy have no inherent properties which promote outbreeding, although they do promote outcrossing or cross-pollination. Of the mechanisms considered, only apomixis enhances outbreeding, pollination system notwithstanding. This is achieved through an "effective" increase in generation time. As a gene migration gauge, neighborhood area is useful in assessing the potential of a colony to differentiate in the face of spatially disparate selective forces. The larger the area, the smaller is this potential, and vice versa. The potential for local differentiation thus is enhanced by autogamy and retarded by heteromorphic incompatibility, dioecy, and apomixis, the latter being the case in spite of the fact that apomixis is the best method for perpetuating superior genotypes.

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