Androdioecy and the evolution of dioecy

The likelihood that dioecy could evolve via androdioecy is examined. It is concluded that female-sterility mutations are unlikely to be able to invade populations of self-compatible hermaphrodite species, even if the resources that an hermaphrodite devotes to seed production can be diverted to yield increased survival and also to increase male fertility. These findings are in agreement with the great rarity of androdioecy. Claimed cases of androdioecy are reviewed. All of the species in question appear to be functionally dioecious, with females retaining substantial anther vestiges. It is argued that this morphological androdioecy is in no way indicative of a previous functionally androdioecious state. The details of the reproductive biology of many of these species seem rather to be consistent with their having evolved dioecy via gynodioecy.
The rarity of androdioecy, as a route to the evolution of dioecy, suggests that re-allocation of reproductive resources is unlikely to be the sole factor of importance, and supports an important role for inbreeding avoidance. The fact that females in some dioecious species retain anthers of substantial size, containing considerable quantities of pollen, gives further support to the view that male-sterility mutations can sometimes be favoured even when little or no resources are re-allocated to male functions. This is impossible without substantial selfing and inbreeding. It is therefore concluded that inbreeding avoidance is generally important in the evolution of dioecy, though reallocation of reproductive resources is also necessary.