Evolution towards dioecy in heterostylous populations

The effective gender of a plant is defined as the proportion of the plant's genes that are transmitted through pollen (its maleness) or through ovules (its femaleness). Formulae are derived that enable the average effective gender of the morphs of a heterostylous population and the gender of individual plants to be estimated. Estimates require a knowledge of the morph ratio and the seed set that results in each morph from self-fertilization and from legitimate and illegitimate cross-fertilizations. If no illegitimate fertilizations occur, the average gender of the morphs can be estimated from their seed production alone; in this situation the average femaleness of one morph is exactly equal to the average maleness of the other morph. The average gender of long- and short-styled morphs is calculated for populations of fiveCordia species (Boraginaceae) from published data ofOpler & al. (1975). In two species, both morphs transmit their genes equally through pollen and ovules. In the other species, the long-styled morph acts predominantly (or in one species exclusively) as an ovule parent and the short-styled morph succeeds predominantly (or exclusively) as a pollen parent.The features of the evolutionary pathway from heterostyly to dioecy and the selective forces that may be responsible are outlined.