Environmental gradients structure Daphnia pulex × pulicaria clonal distribution.

The rarity of eukaryotic asexual reproduction is frequently attributed to the disadvantage of reduced genetic variation relative to sexual reproduction. However, parthenogenetic lineages that evolved repeatedly from sexual ancestors can generate regional pools of phenotypically diverse clones. Various theories to explain the maintenance of this genetic diversity as a result of environmental and spatial heterogeneity [frozen niche variation (FNV), general-purpose genotype] are conceptually similar to community ecological explanations for the maintenance of regional species diversity. We employed multivariate statistics common in community ecological research to study population genetic structure in the freshwater crustacean, Daphnia pulex × pulicaria. This parthenogenetic hybrid arose repeatedly from sexual ancestors. Daphnia pulex × pulicaria populations harboured substantial genetic variation among populations and the clonal composition at each pond corresponded to nutrient levels and invertebrate predator densities. The interclonal selection process described by the FNV hypothesis likely structured our D. pulex × pulicaria populations.