Two asexual lineages, JPN1 and JPN2, of panarctic Daphnia pulex expanded the distribution range to Japan from North America, independent of each other. According to the mutation rates within these lineages, JPN1 lineage colonized Japan earlier than JPN2 lineage. Moreover, the ratio of nonsynonymous to synonymous mutation rates (dN/dS) was lower in JPN1 than in JPN2 lineage.

Accordingly, it is hypothesized that variations of phenotypic traits differ between these two lineages. In addition, since they are obligate parthenogenetic animals, the lineage occupying a larger distribution range should have a larger phenotypic plasticity. To test these hypotheses, we experimentally examined the phenotypic variations of fitness-related traits, including digestive, life history and morphological traits, among several genotypes of these lineages.

We found that within-lineage variations of most traits were smaller in JPN1 than in JPN2. In addition, the overall phenotypic variations were also smaller within the JPN1 lineage than within the JPN2 lineage. These results support the idea that the JPN1 lineage has been more efficiently subjected to negative selection, as expected from the lower dN/dS ratio.

However, the magnitude of the phenotypic plasticity to changing food levels was at the same level for both the JPN1 and JPN2 lineages, while variations found in the phenotypic plasticity were smaller in the JPN1 lineage. The difference in the variations of the phenotypic traits and plasticity between the two lineages suggests that these two lineages have evolved under somewhat different environmental conditions and that genotypes of the JPN2 lineage may have exploited niches that differed somewhat from that of the JPN1 genotypes.

Detailed methods are described in the paper.

Data set used for all figures and tables in the paper.