Low genetic variability in the ancient asexual ostracodDarwinula stevensoni

Abstract

The genetic variability and genotypic (clonal) structure of 34 populations of the ancient asexual Darwinula stevensoni was assayed using allozymes. In this study of 1964 individuals from nine countries in Europe, Israel and South Africa, little genetic variability was found. Out of nine enzyme systems only two revealed sufficient enzyme activity to produce consistent zymograms. Only one locus (Gpi 1) was polymorphic, and only in 14 populations: 83% of all screened individuals from all over Europe shared the same genotype. There was no significant deviation of Hardy--Weinberg equilibrium in 10 out of 14 polymorphic populations, a result unexpected in an asexual species. Among possible causes we discuss null alleles whose presence would make genotype frequencies discordant with Hardy-Weinberg equilibrium. Allele and genotype frequencies were different in lakes and rivers (heterozygotes were dominant in rivers, homozygotes in lakes). Habitat choice might be driven by a different starvation tolerance by different genotypes.

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

“…Clonal (dilocus genotypes) variability in local populations of D. stevensoni was similar to what reported for non marine parthenogenetic ostracod species and in aquatic asexual taxa where, in general, a single population is made up of up to five clones (Rossi et al 1998;Cywinska & Hebert 2002). In populations from Northern Italy we found the highest clonal diversity in Europe (Rossi et al 2004).…”

“…In spite of the general low genetic variability observed in European D. stevensoni populations Gandolfi et al 2001a;Rossi et al 2004), at a local scale the species shows a relatively high number of genotypes per population. In fact, on the basis of only one polymorphic marker, 5 different alleles and up to 8 different dilocus genotypes were found in the Mincio population.…”

“…Only two loci, Gpi (E.C.5.3.1.9) and Pgm (E.C.2.7.5.1), that were previously shown to be consistently scorable, were used (Rossi et al 2004). The number of alleles per locus and the number of multilocus genotypes per sample were calculated from the scores obtained from individual organisms.…”

“…On a total of 9 loci scored, only Gpi locus was polymorphic (Rossi et al 2004). All 33 populations analysed, from Finland to Israel, share the most common allele carried by 91% of all individuals while the most common genotype (homozygous for the most common allele) is carried by 83% of the almost 2000 individuals that were analysed.…”

Ostracod Assemblages in the Frasassi Caves and Adjacent Sulfidic Spring and Sentino River in the Northeastern Apennines of Italy

“…F. fabaeformis), and an unidentified species of Paralimnocythere, all taxa listed in Table 1 have been previously reported from various habitats elsewhere in Europe (Meisch, 2000). They have been also recorded from various present-day water bodies and late Pliocene to Holocene deposits in the Italian Peninsula, Sardinia, and Sicily (e.g., Ambrosetti et al, 1995;Barberi et al, 1995;Calderini et al, 1998;Gliozzi and Mazzini, 1998b;Gliozzi, 2000;Meisch, 2000;Karanovic and Pesce, 2001;Rossi et al, 2003Rossi et al, , 2004Rossetti et al, 2004Rossetti et al, , 2005Rossetti et al, , 2006Pieri et al, 2006Pieri et al, , 2007Pieri et al, , 2009Rossi et al, 2010).…”