Intercontinental allozyme differentiation among four holarctic Daphnia species

The species overlap between European and North American cladoceran faunas has been questioned from the very beginning of faunistic studies on Nearctic zooplankton and still remains unsettled. However, more recently, genetic techniques seem to overcome the limits of morphology as the basis for taxonomy. We analyzed 8-11 allozyme loci to assess the genetic differentiation between intercontinental counterparts of four daphniids: Daphnia curvirostris, Daphnia obtusa, Daphnia pulex, and Daphnia pulicaria. Despite their morphological likeness, all equivalents showed a large amount of genetic divergence between continents due to complete allelic substitutions and major gene frequency shifts. From an evolutionary perspective, this study shows that as in other organisms, geographic isolation seems to be sufficient to minimize gene flow to levels at which the origin of new daphniid species can be considered. From a practical point of view, a revision of Holarctic daphniid taxonomy should be considered.