Life-history variation, phenotypic plasticity, and subpopulation structurein a freshwater snail

Earlier studies of habitat-specific subpopulations of mixed clonal and sexu al freshwater snails of the species Potamopyrgus antipodarum have revealed clinal variation by depth in several life-history traits, risk of parasite infection, mixed population structure, and the genetic structure of the clo nal population. Clinal variation is pronounced in life-history traits: snai ls are larger and start reproduction later in the deeper habitats. The prop ortion of clonal individuals increases with depth, and many clones are habi tat-specific. While these patterns are well documented, it is not known whi ch processes have led to the observed genetic divergence in the clonal popu lation. In this study, we experimentally investigated the contribution of p henotypic plasticity to habitat-specific life-history trait variation using reciprocal transplant experiments with adult and juvenile snails. Assessme nt of phenotypic plasticity is important because canalized habitat-specific life-history trait variation is one of the alternative explanations for ha bitat-specific genetic divergence seen in the clonal population. However, i f life-history trait variation is largely due to adaptive phenotypic plasti city, canalized optimization of life-history traits is unlikely to explain the divergence observed in the clonal structure. We found significant habit at-induced variation fur growth rate, proportion of brooding females, brood size, number of surviving offspring, and juvenile survival, indicating tha t much of the life-history variation must be considered the result of pheno typic plasticity. Based on these results, it seems that life-history trait divergence is unlikely to explain habitat-specific clonal structure. In con trast, we found genetically based differences in resistance to parasite inf ections, snails of the deeper Isoetes habitat were more susceptible to infe ction than snails of the shallow shorebank habitat. To our surprise, we fou nd only a few habitat-by-origin interactions that could directly contribute to the maintenance of the observed habitat-specific clonal structure. One potentially important interaction, however, was that in the deeper Isoetes habitat, reproductive output of snails transplanted from the shallow habita t was lower than that of resident snails. In addition, we also found that s urvival of clonal snails may be poorer in unfamiliar habitat's than that of sexual P. antipodarum, potentially promoting the maintenance of habitat-sp ecific clonal assemblages. Thus, higher parasite resistance of shallow-wate r snails in both habitats and lower reproductive output of shallow-water sn ails in the deep habitat are likely to contribute to the maintenance of the habitat-specific clonal structure in P. antipodarum populations, whereas m ost of the observed variation in life-history traits seems to be due to phe notypic plasticity, which is likely to be adaptive.