FLAT REACTION NORMS AND FROZEN PHENOTYPIC VARIATION IN CLONAL SNAILS (POTAMOPYRGUS-ANTIPODARUM)

The Frozen Niche-Variation hypothesis (FNV) suggests that clones rando mly sample and ''freeze'' the genotypes of their ancestral sexual popu lations. Hence, each clone expresses only a fraction of the total nich e-use variation observed in the sexual population, which may lead to s election for ecological specialization and coexistence of clones. A ge neralized form of the FNV model suggests that the same is true for lif e-history (as well as other) traits that have important fitness conseq uences, but do not relate directly to niche use. We refer to the gener al form of the model as the Frozen Phenotypic Variation (FPV) model. A mixed population of sexual and parthenogenetic snails (Potamopyrgus a ntipodarum) in a New Zealand lake allowed us to examine the phenotypic variation expressed by coexisting clones in two benthic habitats, and to compare that variation to the sexual population. Three clones were found primarily in an aquatic macrophyte zone composed of Isoetes kir kii (1.5-3.0 m deep), and three additional clones were found in a deep er macrophyte: zone composed of Elodea canadensis (4.0-6.0 m deep). Th ese clones showed significant variation between habitats, which mirror ed that observed in the sexual population. Specifically, clones and se xuals from the deeper habitat matured at a larger size and had larger broods. There was also significant among-clone variation within habita ts; and as expected under the FPV model, the within-clone coefficients of variation for size at maturity were low in both habitats when comp ared to the sexual population. In addition, we found four clones that were common in both macrophyte zones. The reaction norms of these clon es were fiat across habitats, suggesting little phenotypic plasticity for morphology or life-history traits. Flat reaction norms, high among -clone variation, and low coefficients of variation (relative to the s exual population) are in accordance with the FPV model for the origin of clonal lineages. We also measured the prevalence of infection by tr ematode larvae to determine whether clones are inherently more or less infectable, or whether they are freezing phenotypic variation for res istance from the sexual population. We did this in the deep habitats o f the lake where recycling of the parasite by the vertebrate host is u nlikely, thereby reducing the complications raised by frequency-depend ent responses of parasites to host genotypes. We found no indication t hat clones are either more or less infectable than the resident sexual population. Taken together, our results suggest that phenotypic varia tion for both life-history traits and resistance to parasites is froze n by clones from the local sexual population.