Life-History Variation, Phenotypic Plasticity, and Subpopulation Structure in a Freshwater Snail

Abstract

Earlier studies of habitat-specific subpopulations of mixed clonal and sexual 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 clonal population. Clinal variation is pronounced in lifehistory traits: snails are larger and start reproduction later in the deeper habitats. The proportion of clonal individuals increases with depth, and many clones are habitat-specific. While these patterns are well documented, it is not known which processes have led to the observed genetic divergence in the clonal population. In this study, we experimentally investigated the contribution of phenotypic plasticity to habitat-specific life-history trait variation using reciprocal transplant experiments with adult and juvenile snails. Assessment of phenotypic plasticity is important because canalized habitat-specific life-history trait variation is one of the alternative explanations for habitat-specific genetic divergence seen in the clonal population. However, if life-history trait variation is largely due to adaptive phenotypic plasticity, canalized optimization of life-history traits is unlikely to explain the divergence observed in the clonal structure. We found significant habitat-induced variation for growth rate, proportion of brooding females, brood size, number of surviving offspring, and juvenile survival, indicating that much of the life-history variation must be considered the result of phenotypic plasticity. Based on these results, it seems that life-history trait divergence is unlikely to explain habitat-specific clonal structure. In contrast, we found genetically based differences in resistance to parasite infections; snails of the deeper Isoetes habitat were more susceptible to infection than snails of the shallow shorebank habitat. To our surprise, we found 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 habitat was lower than that of resident snails. In addition, we also found that survival of clonal snails may be poorer in unfamiliar habitats than that of sexual P. antipodarum, potentially promoting the maintenance of habitatspecific clonal assemblages. Thus, higher parasite resistance of shallow-water snails in both habitats and lower reproductive output of shallow-water snails in the deep habitat are likely to contribute to the maintenance of the habitat-specific clonal structure in P. antipodarum populations, whereas most of the observed variation in life-history traits seems to be due to phenotypic plasticity, which is likely to be adaptive.

Genetic variation in sexual and clonal lineages of a freshwater snail

“…We would agree that the possibility exists that there are microhabitats within each of these areas, upon which the many different clones might specialize, but the existence of these microhabitats is not obvious to us at the present time. Soft selection might, however, play some role in the distribution of clones among habitats in both Lakes Alexandrina and Tennyson, but recent reciprocal transplant experiments have suggested that clones may be quite plastic with respect to habitat use (Negovetic & Jokela, 2001).…”

Adaptive responses and invasion: the role of plasticity and evolution in snail shell morphology

“…; Dybdahl and Drown ). In the native range of P. antipodarum , variation in shell morphology reflects adaptive responses to abiotic and biotic factors (Negovetic and Jokela ; Haase ; Holomuzki and Biggs ). However, little is known about how variation in shell morphology affects the success of invasive populations across broad environmental gradients.…”

“…A single clonal genotype, US 1, has spread rapidly in the western United States since 1987, and sometimes reaches great abundance (Hall et al 2003(Hall et al , 2006Kerans et al 2005;Dybdahl and Drown 2011). In the native range of P. antipodarum, variation in shell morphology reflects adaptive responses to abiotic and biotic factors (Negovetic and Jokela 2001;Haase 2003;Holomuzki and Biggs 2006). However, little is known about how variation in shell morphology affects the success of invasive populations across broad environmental gradients.…”

Adaptive phenotypic plasticity in a clonal invader

“…Native P. antipodarum are intermediate hosts for at least 14 species of endoparasites including castrating trematodes (Hechinger, ; Winterbourn, ). Infection of juvenile P. antipodarum by parasites, and in particular castrating trematodes, may affect shell morphology and brood size through changes in resource allocation of the growing snail (Levri, Dillard, & Martin, ; Negovetic & Jokela, ). For example, infected P. antipodarum are wider and less spiny than uninfected counterparts (Lagrue, McEwan, Poulin, & Keeney, ; Levri et al., ).…”