Some Functions of Dioecy in Seed Plants

• Published in: The American naturalist 1984-08, Vol.124 (2), p.149-158
• Main Author: Baker, Herbert G.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Biological taxonomies; Dioecy; Ecological genetics; Evolution; Fundamental and applied biological sciences. Psychology; Genera; Outbreeding; Plant physiology and development; Plants; Pollen; Pollinators; Taxa; Vegetative and sexual reproduction, floral biology, fructification
• ISSN: 0003-0147; 1537-5323
• DOI: 10.1086/284260

Comments are made on recent publications that emphasize that dioecy in seed plants can be more than just an outcrossing mechanism. It is accepted that "sexual selection" may play an important role in the evolution of dioecy. Circumstantial evidence is presented that when habitat conditions favor outcrossing easily evolved dioecy may be selected. Nevertheless, the disputed suggestion made by me a quarter of a century ago that dioecy appears to have arisen more frequently in hermaphrodite or monoecious taxa that do not have the genetical resources to produce a functional self-incompatibility system is defended, with some indication that this relationship should be examined at the genus level rather than at the level of the family as originally proposed. Disadvantages (to the plants) of dioecy compared to a self-incompatibility system, particularly a monomorphic multiple-allele one, are considered in this light. Distyly is the only incompatibility breeding system that may be expected to give rise sometimes to dioecy (for the flowers are already morphologically halfway to dioecious) although the classic example of distyly, the large genus Primula, shows no sign of dioecy, and some postulated cases of this transition (Bauhinia and Morinda) need to be reexamined. Examples of the apparent evolution of gynodioecy or dioecy after the loss of self-incompatibility are also considered (Armeria, Fragaria, Rubus, and possibly Solanum). It is suggested that all of the "outcrossing" and "sexual selection" hypotheses that have been put forward may have a basis in fact and may even be operative simultaneously.