We report the genetic structure of 47 all-female populations of Heterocypris incongruens from Italy. Most populations (34) were collected from ricefields in the Po Plain (Northern Italy). Using three polymorphic allozyme markers (MPI, PGM and GPI) we found 125 different multilocus genotypes (clones) in a total of 3235 studied females. One clone was present almost everywhere (66 % localities) and in about a third (35 %) of all analysed females. Only a few clones (9 or 7 %) were found in more than ten localities while the majority (77 or 62 %) were found in only one population (private clones). Almost half (48 %) of all private clones were from the eight ricefields with the highest clonal richness while 16 % were from isolated temporary pools sampled for comparison. Clonal diversity seems to be the result of the presence of at least two different clonal lineages. One of the two, made up of the largest number of clones, probably had an autochthonous origin, a condition that probably favoured adaptation to local conditions. The other lineage was possibly introduced with imported rice seeds and its descendants carry adaptations to warm ricefield conditions. The weak relationship between genetic and geographic distance and the lack of spatial patterns in genetic variability might be due to the species' long-distance dispersal ability and to the persistence of founder effects sustained by large resting egg banks, conditions that blur isolation by distance. The lack of phylogeography in genetic variation may also be due to the spatially stochastic process of initiation and cessation of local ricefield cultivation known to have happened since the 15(th) Century in this area of Northern Italy.
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