A general purpose genotype in an ancient asexual.

Many parthenogenetic species are geographically more widely distributed than their sexual relatives. Their success has been partly attributed to the existence of general purpose genotypes (GPGs). Darwinula stevensoni is an ostracod species, which has persisted for >25 million years without sex, and is both ubiquitous and cosmopolitan. Here, we test the hypothesis that this ancient asexual species may possess a highly generalised (or general purpose) genotype. The ecological tolerance of D. stevensoni was compared with asexual populations of Heterocypris incongruens, a common cypridinid species with mixed reproduction, as well as with that of another ancient asexual darwinulid species with a limited geographic and ecological distribution, Vestalenula molopoensis. The unusually wide tolerance range for both salinity (0-30 g/l) and temperature (10°C, 20°C and 30°C) of the freshwater species D. stevensoni, supports the hypothesis that this ancient asexual has indeed developed a GPG. This coincides with its wide geographic and ecological distribution and might explain its persistence as an obligate asexual in its long-term evolution. The more restricted salinity tolerance of V. molopoensis (maximum at 12 g/l) shows that not all species of the ancient asexual family Darwinulidae have a GPG. D. stevensoni has a much broader tolerance than the asexuals of H. incongruens. We argue why a GPG is most likely to develop in long-term asexuals.