SLOW MOLECULAR EVOLUTION IN AN ANCIENT ASEXUAL OSTRACOD

Genetic variability of the non-marine ostracod species Darwinula steve nsoni was estimated by sequencing part of the nuclear and the mitochon drial genome. As Darwinulidae are believed to be ancient asexuals, acc umulation of mutations should have occurred, both between alleles with in lineages and between lineages, during the millions of years of part henogenetic reproduction. However, our sequence data show the opposite : no variability in the nuclear ITS1 region was observed within or amo ng individuals of D. stevensoni, despite sampling a geographical range from Finland to South Africa. Lack of allelic divergence might be exp lained by concerted evolution of rDNA repeats. Homogeneity among indiv iduals may be caused either by slow molecular evolution in ITS1 or by a recent selective sweep. Variability of mitochondrial cytochrome oxid ase (COI) was similar to intraspecific levels in other invertebrates, thus weakening the latter hypothesis. Calibrating interspecific, genet ic divergences among D. stevensoni and other Darwinulidae using their fossil record enabled us to estimate rates of molecular evolution. Bot h COI and ITS1 evolve half as fast, at most, in darwinulids as in othe r invertebrates, and molecular evolution has significantly slowed down in ITS1 of D. stevensoni relative to other darwinulids. A reduced ITS 1 mutation rate might explain this inconsistency between nuclear and m itochondrial evolution in D. stevensoni.