When males and hermaphrodites coexist: a review of androdioecy in animals

Abstract

Androdioecy (populations consisting of males and hermaphrodites) is a rare mating system in plants and animals: up to 50 plants and only 36 animals have been described as being androdioecious, with most of the latter being crustaceans. To date, a thorough comparative analysis of androdioecy in animals has not been undertaken. Herein we present such an analysis. Androdioecy has only been extensively surveyed in 2 animal taxa: the nematode Caenorhabditis and the clam shrimp Eulimnadia. The other major taxon having androdioecious species is the Cirripedia (barnacles), but there are only limited studies on androdioecy in this group. In animals, androdioecy is found either in species that have morphologically and ecologically distinct sexes (that is, hermaphrodites and small, "complemental" males) that are derived from hermaphroditic ancestors (that is, the barnacles) or in species that have similarly-sized males and hermaphrodites that have been derived from dioecious ancestors (the remaining androdioecious species). We suggest that the barnacles have evolved a sexual specialization in the form of these complemental males that can more efficiently use the constrained habitats that these barnacles often experience. For the remaining species, we suggest that androdioecy has evolved as a response to reproductive assurance in species that experience episodic low densities. Additionally, we hypothesize that the development of mechanisms allowing reproductive assurance in species with a number of sexually differentiated traits is most likely to result in androdioecy rather than gynodioecy (mixtures of females and hermaphrodites), and that these species may be developmentally constrained to stay androdioecious rather than being capable of evolving into populations solely consisting of efficient, self-compatible hermaphrodites. We conclude by suggesting several areas in need of further study to understand more completely the evolution and distribution of this interesting mating system in animals.

Adaptive evolution of sexual systems in pedunculate barnacles

“…Previous theories predict that androdioecy is an extremely rare sexual system compared with hermaphroditism, dioecy or gynodioecy [6,7,50]. In fact, in the best-studied taxon, angiosperms, gynodioecy is much more common than androdioecy [6,50].…”

“…Diverse sexual systems in organisms and the evolutionary forces driving them have been a major topic of inquiry in evolutionary biology [1][2][3][4][5][6][7]. However, few empirical studies have addressed these problems, especially in animals.…”

Androgenesis: a review through the study of the selfish shellfish Corbicula spp.

“…Within the genus Corbicula, biflagellate sperm are unique to androgenetic lineages (including males coexisting with androgens), as far as is known. Moreover, the situation of androdioecy, found recently in C. leana (Houki et al, 2011), is very rare in the animal kingdom (Weeks et al, 2006).…”

Phenotypic differences between the sexes in the sexually plastic mangrove rivulus fish (Kryptolebias marmoratus)

“…The mangrove rivulus is a simultaneous hermaphrodite, possessing both ovarian and testicular tissue in a single ovotestis (Harrington, 1974), and is one of two known self-fertilizing hermaphroditic vertebrates, the other being its sister species K. hermaphroditus (Costa et al, 2010;Harrington and Kallman, 1968;Weeks et al, 2006). Mangrove rivulus are androdiecious, with populations consisting only of males and hermaphrodites Harrington, 1974;Tatarenkov et al, 2012;Weeks et al, 2006).…”