Five Evolutionary Pathways to Subdioecy

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Abstract
Apparently, subdioecy in the seed plants has evolved repeatedly, by at least 5 evolutionary pathways, from hermaphroditism via gynodioecy; from monoecy; from distyly; from hermaphroditism via andromonoecy and androdioecy; and directly from hermaphroditism. The evidence for the 5 pathways is summarized and a genetic model which represents hermaphrodite or monoecious populations, and their direct differentiation toward dioecy is presented. There is a morphological, physiological and evolutionary continuum between hermaphroditism and dioecy, so that some populations cannot easily be placed into discrete classes. Populations of Vaccinium angustifolium may have small proportions of males or females, together with many hermaphrodites, which may vary in fertility. The fertility genes are apparently not organized into an XX/XY chromosome system. The selective forces apparently responsible for the evolution of dioecy are reviewed, and several selection models, involving fertility variation, resource allocation, sexual selection, and overdominance, are proven equivalent. Evidently, outbreeding advantage is sometimes but not always an important selective force.