Abstract
BackgroundSeveral genetic lineages of obligate parthenogenetic Daphnia pulex, a common zooplankton species, have invaded Japan from North America. Among these, a lineage named JPN1 is thought to have started colonization as a single genotype several hundred to thousand years ago and subsequently produced many genotypes in Japan. To examine the phenotypic variations due to ecological drivers diverging the genotypes in new habitats, we measured heritability and variation in 17 traits, including life history, morphology and digestive traits, and the genetic distance among the D. pulex JPN1 genotypes in Japan.ResultsWe found that most of the traits measured varied significantly among the genotypes and that heritability was highest in the morphological traits, followed by the digestive and life history traits. In addition, 93% of the variation in these traits was explained by the first three components in the principal component analysis, implying that variation of these heritable traits is not random but rather converged into a few directions. These relations among traits revealed the potential importance of predation pressures and food conditions as factors for diverging and selecting different genotypes. However, the magnitude of the difference in any single trait group did not correlate with the genetic distance.ConclusionsOur findings show that the divergent traits evolved within D. pulex JPN1 lineage without genetic recombination, since their ancestral clone invaded Japan. Large variations and covariations of the phenotypic traits, irrespective of the genetic distance among the genotypes, support the view that the invasive success of D. pulex JPN1 was promoted by a genetic architecture that allowed for large phenotypic variations with a limited number of functionally important mutations without recombination.
Highlights
Several genetic lineages of obligate parthenogenetic Daphnia pulex, a common zooplankton species, have invaded Japan from North America
The growth coefficient k and the body length at maturation and at day 5 only differed among genotypes, the intermoult duration before maturation differed between food levels, and the lipase activity and intermoult duration after maturation did not differ among genotypes or food levels (Table 1)
This study showed that D. pulex JPN1 evolved divergent traits without genetic recombination for several hundred to thousand years since their ancestral clone originally invaded Japan
Summary
Several genetic lineages of obligate parthenogenetic Daphnia pulex, a common zooplankton species, have invaded Japan from North America. Individuals of D. pulex JPN1 are obligate parthenogenetic animals, several genotypes (clones) were found within this lineage, suggesting that they genetically evolved without recombination after the initial invasion and likely adapted to new habitats in Japan [7]. Since they reproduce asexually, their populations are established by and composed of single genotypes [8, 9]. If phenotypic traits vary among these genotypes (clones) and are heritable, the genotypes of D. pulex that invaded Japan are a good model to explore whether the magnitude of the difference in the phenotypic traits is related to genetic distance among the genotypes and what environmental factors promoted evolution after Daphnia invasions into new habitats
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