Androdioecy is a reproductive system characterized by the coexistence of males and hermaphrodites. Androdioecy is rare in comparison with the other major reproductive systems: dioecy, gynodioecy and hermaphroditism. In animals, androdioecy has been considered a stepping stone in the transition from dioecy to hermaphroditism, and vice versa. Androdioecy, trioecy and gynodioecy are sometimes referred to as a mixed mating systems. Androdioecy is a dimorphic sexual system in plants comparable with gynodioecy and dioecy.

Evolution of androdioecy

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

Androdioecy with dioecious ancestry

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 Caenorhabditis elegans androdioecy is thought to have evolved from dioecy, through a trioecous intermediate.

Androdioecy with hermaphroditic ancestry

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

Types of androdioecy

The most common form of androdioecy in animals involves hermaphrodites that can reproduce by autogamy or allogamy through ovum with males. However, this type does not involve outcrossing with sperm. This type of androdioecy generally occurs in predominantly gonochoric taxonomy groups. One type of androdioecy contains outcrossing hermaphrodites which is present in some angiosperms. Another type of androdioecy has males and simultaneous hermaphrodites in a population due to developmental or conditional sex allocation. Like in some fish species small individuals are hermaphrodites and under circumstances of high density, large individuals become male.

Androdioecious species

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

Anthozoa (Corals)

Nematoda (Roundworms)

Rhabditidae (Order Rhabditida) Diplogastridae (Order Rhabditida) Steinernematidae (Order Rhabditida) Allanotnematidae (Order Rhabditida) Dorylaimida

Nemertea (Ribbon worms)

Arthropoda

Clam shrimp Tadpole shrimp Barnacles Lysmata Insects

Annelida (Ringed worms)

Chordata

Angiosperms (Flowering plants)