Androdioecy Inferred in the Clam Shrimp Eulimnadia Agassizii (Spinicaudata: Limnadiidae)

Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both the plant and animal kingdoms. Androdioecy has been described in three branchiopod species, and is best known from the clam shrimp Eulimnadia texana Packard. Herein we describe sex ratio, genetic and histological evidence from the clam shrimp Eulimnadia agassizii Packard that suggest androdioecy is also found in this species. The E. agassizii population sampled had all-females, and when these females were isolated and allowed to produce eggs, those eggs yielded 100% female offspring in 15 out of 15 cases. Additionally, the originally isolated females proved to be completely homozygous at each of the six allozyme loci scored. The offspring from these isolated females also proved to be homozygous for the same alleles as their parent. Tissue sectioning of the gonad found that the ''females'' actually produced testicular tissue in the posterior portion of the gonad. Taken together, these data are entirely consistent with those of the androdioecious E. texana, and thus indicate that E. agassizii is also an androdioecious species, bringing the total number of branchiopod species with this form of reproduction to four. Branchiopod crustaceans exhibit a broad array of reproduc- tive mechanisms, ranging from complete apomixis in several cladoceran species (Hebert and Finston, 2001) to obligate outcrossing in most of the anostracan species (Clark and Bowen, 1976). The clam shrimp, originally classified as ''conchostracans'' but now split into the Laevicaudata, Spinicaudata, and Cyclestherida (Martin and Davis, 2001), encapsulate most of the overall reproductive variation found in the branchiopods as a whole (Sassaman, 1995). Caenthestheriella gynecia Mattox is described as strictly asexual (Mattox, 1950), Cyclestheria hislopi (Baird) exhibits cyclic parthenogenesis (Nair, 1968), Limnadia lenticularis (L.) is a self compatible hermaphrodite (Scanabissi and Mondini, 2002), Eulimnadia texana Packard is androdioe- cious (Sassaman and Weeks, 1993), and members of the genus Lynceus are obligately amphimictic (Sassaman, 1995). Of these various mating systems, androdioecy is the least common (Pannell, 2002). Androdioecy is defined as populations comprised of males and hermaphrodites, but lacking true females. This mating system has been described in only a handful of animals, but has been found in three branchiopods: Triops newberri (Packard) (Sassaman, 1989b, 1991); Eulimnadia antlei Mackin (Sassaman, 1988); and Eulimnadia texana (Sassaman and Weeks, 1993). Of these three species, the specific mechanism of androdioecy has been most extensively examined in the latter species (Zucker et al., 1997; Weeks et al., 1999; Weeks and Zucker, 1999; Weeks et al., 2000; 2001a; 2001b). In E. texana, maleness is determined by a recessive allele at a single sex-determining locus (Sassaman and Weeks, 1993). Hermaphrodites are comprised of two morphologically indistinguishable forms: the dominant homozygote, termed ''monogenic,'' and the heterozygote, termed ''amphigenic.'' Monogenic her- maphrodites can only produce hermaphrodites when self- fertilizing or mating with a male, while the amphigenic hermaphrodites produce 25% males when self-fertilizing and 50% males when mating with a male. Hermaphrodites lack the clasping appendages to allow the pairing necessary for mating, and thus hermaphrodites cannot fertilize one another (Sassaman and Weeks, 1993). Although androdioecy has been found in two Eulimnadia species, most other species in this genus have not been examined for reproductive mode. The mating system of the uncommon E. agassizii has been only briefly mentioned in two publications. Asexuality has been inferred because of a lack of males in a population from Massachusetts (Zinn and Dexter, 1962), and Sassaman (1995) mentioned ''unisexu- ality'' in the rearing of offspring from eggs produced by isolated ''females.'' However, neither author explored the mating system further. Herein we report the results of examinations of the mating system of E. agassizii, which is a rare clam shrimp found in the northeastern United States (Smith, 1992). Evidence from offspring sex ratios, allozyme data, and histological examination suggest that the mating system of E. agassizii is the same as that described for E. texana (Sassaman and Weeks, 1993; Zucker et al., 1997), with the ''all-female'' populations described for E. agassizii actually being mono- genic hermaphrodites that have lost all males and amphi- genic hermaphrodites.