Androdioecy

Androdioecy is a reproductive system characterized by the coexistence of males and hermaphrodites. Androdioecy is rare in comparison to the other major reproductive systems dioecy and hermaphroditism. In animals, androdioecy has been considered an important stepping stone in the transition from dioecy to hermaphroditism, and vice versa.

The fitness requirements for androdioecy to arise and sustain itself are theoretically so improbable that it was long considered that such systems do not exist. Particularly, males and hermaphrodites have to have the same fitness, in other words the same number of offspring, in order to be maintained. However, males only have offspring by fertilizing eggs or ovules of hermaphrodites, while hermaphrodites have offspring both through fertilizing eggs or ovules of other hermaphrodites and their own ovules. This means that all else being equal, males have to fertilize twice as many eggs or ovules as hermaphrodites to make up for the lack of female reproduction.

Androdioecy can evolve either from dioecious ancestors through the invasion of hermaphrodites or from hermaphroditic ancestors through the invasion of males. The ancestral state is important because conditions under which androdioecy can evolve differ significantly.

In roundworms, clam shrimp, tadpole shrimp and cancrid shrimps, androdioecy has evolved from dioecy. In these systems, hermaphrodites can only fertilize their own eggs (self-fertilize) and do not mate with other hermaphrodites. Males are the only means of outcrossing. Hermaphrodites may be beneficial in colonizing new habitats, because a single hermaphrodite can generate many other individuals. In the well-studied roundworm Caenorhabditis elegans, males are very rare and only occur in populations that are in bad condition or stressed.

In plants, corals and barnacles, androdioecy has evolved from hermaphroditism. Many plants self-fertilize, and males may be sustained in a population when inbreeding depression is severe because males guarantee outcrossing.

Despite their unlikely evolution, 115 androdioecious animal and about 50 androdioecious plant species are known. These species include

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