Life history traits in Darwinula stevensoni (Crustacea: Ostracoda) from Southern European populations under controlled conditions and their relationship with genetic features

We describe life history traits of the ostracod Darwinula stevensoni through laboratory experiments. This small (about 0.7 mm), ancient, obligate parthenogenetic species appeared to be particulary difficult to handle as its long life cycle (up to 3-4 years) has made lab experiments over several generations very difficult. D. stevensoni is an eurythermal and euryhaline species with low variability in size and shape (both of the carapace and the soft parts). Its genetic variability has also been found to be very low. Survival, clutch size, deposition timing and hatching were evaluated in acclimated and non-acclimated females from seven populations: six from Northern Italy and one from Spain. The samples were collected from three different habitats: four lakes, two streams and one spring. A genetic survey using starch gel electrophoresis had previously revealed that only Glucose phosphate isomerase (Gpi) locus was polymorphic. A clone, homozygous at Gpi locus, is the most common in lacustrine and spring habitats in the whole biogeographic range. Surprisingly, two heterozygous clones dominate in Northern Italian lotic environments. Enzymatic activity of heterozygous and homozygous genotypes at Gpi locus was assayed in order to evaluate the relationship between Gpi activity and fitness. Survival and developmental time were not affected by acclimation, while reproductive potential decreased in acclimated females. In females from running waters, reproduction started later and lasted for a shorter time, clutch size was smaller, and hatching percentage lower than females from lakes. These differences are not directly correlated with differences in enzymatic activities at Gpi locus. This does not rule out an overall genetic control of these characteristics since, in a parthenogenetic species, the whole genome is transmitted without recombination. We stress the intriguing case of D. stevensoni in relation to the concept of the "general purpose genotype"

Spatial segregation of Darwinula stevensoni (Crustacea: Ostracoda) genotypes in lentic and lotic habitats of Northern Italy

The clonal structure of apomictic populations of Darwinula stevensoni from Northern Italy lacustrine and riverine habitats has been analysed by allozyme electrophoresis. Thirty two sites were sampled and a total of 748 individuals were analysed. Only one polymorphic locus, Gpi, turned out to have enough variation to be informative. Two thirds of all individuals were homozygous for the most frequent allele but the frequency of this genotype was significantly lower in rivers (16%) than in lakes (92%). As a consequence homozygous clonal females are dominant in lacustrine habitats while heterozygous clonal females are dominant in riverine ones. Differences in genetic structure between habitats were observed even at distances of a few meters. Allele and genotype frequencies are significantly different between Lake Montorfano and its outflow (River Seveso) and between Mantova lakes and their two tributaries, the River Mincio and Corniano Channel. The former is also the outflow of Lake Garda. By laboratory experiments, we previously reported difference between genotypes: homozygous females from Mantova lakes show significantly lower starvation tolerance than heterozygous females from River Mincio. We discuss the hypothesis that habitat segregation is related to this difference. Riverine females should be better adapted to low productivity ecosystems than lacustrine ones