A general purpose genotype in an ancient asexual

Abstract

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.

How ancient are ancient asexuals?

“…Moreover the species was rather common, indicating a high ecological tolerance. The Recent D. stevensoni is both cosmopolitan and ubiquitous and it has recently been demonstrated (Van Doninck et al 2002) that the general purpose genotype (GPG) in this species is most probably closely linked to its long-term asexuality. A. leguminella could be the Mesozoic ecological equivalent of D. stevensoni, its long stratigraphical range representing ample time for the development of a GPG.…”

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

“…If a newly colonized niche is suitable, asexual lineages will not suffer an ecological disadvantage and are expected to thrive and spread: only one individual is sufficient to establish a new population, and each individual can have as many as 90 000 offspring in one breeding season (McMahon, 1999). If there is sufficient clonal diversity either due to colonization by diverse genotypes or due to new mutations, a successful clone with a broad tolerance could be selected for and in the long term a general-purpose genotype may evolve (Van Doninck et al, 2002).…”

Two families of non-LTR retrotransposons, Syrinx and Daphne, from the Darwinulid ostracod, Darwinula stevensoni

“…This population has also been subjected to ecological (Van Doninck et al, 2002) and genetic (Schön et al, 1998, Schön and) studies and appears to be monoclonal (Van Doninck et al, 2004). DNA was extracted from 50 pooled adult females with the DNA Minikit (Qiagen) following the manufacturer's protocol, yielding about 0.5 µg of DNA.…”

“…It can further be characterized by an exceptionally low mutation rate in both the hsp82 nuclear gene and the ribosomal ITS1 region Schön et al, 1998), an exceptionally long generation time of 1 to 4 years depending on the latitude (McGregor, 1969;Ranta, 1979;Van Doninck et al, 2003) and low fecundity (Ranta, 1979;Van Doninck et al, 2003). Furthermore, ecological investigations have shown that D. stevensoni has an extremely wide tolerance for temperature, pH (Van Doninck et al, 2002) and oxygen (Rossi et al, 2002).…”