Amy R. Wethington

Inheritance at five loci in the freshwater snail,Physa heterostropha

Interest in the genetics of pulmonate snails has arisen in at least three contexts. Population geneticists have often been attracted to snails as convenient organisms for basic research, from the classic studies of the banded land snail Cepaea (Jones et al., 1977) to more recent isozyme studies of other land snails (Selander and Kaufman, 1975), slugs (McCracken and Selander, 1980), and a variety of freshwater species (Jarne and Delay, 1991). Interests of a more medical and parasitological nature have driven investigations into the genetics of Biomphalaria and other freshwater pulm0nate populations serving as intermediate hosts of human schistosomes (Richards, 1970; Mulvey and Woodruff, 1985). Most recently, evolutionary biologists have been drawn to pulmonate snails as model organisms for the study of reproductive allocation (Rudolph and Bailey, 1985; Rollinson et al., 1989; Schrag et al., 1992; Jarne et al., 1993). Physa heterostropha pomilia (Conrad) is a freshwater pulmonate snail common in ponds and backwaters throughout North America. Genetic variability, reproductive plasticity, ease of culture, and rapid generation time combine to make it an ideal laboratory animal for general studies of sex allocation (Wethington and Dillon, 1993; in review). Our isofemale lines of Physa have their origins in 1989 collections from a pond at Charles Towne Landing State Park, within the city limits of Charleston, SC (site described by Dillon and Dutra-Clarke, 1992). We isolated 35 wild-caught snails and collected all offspring produced by each over a period of 60 days, in an effort to assess sperm storage capabilities (Wethington and Dillon, 1991). Small

The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa

BackgroundThe cosmopolitan freshwater snail Physa acuta has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that Physa carolinae, recently described from the American southeast, is a sister species of P. acuta. The divergence of the acuta/carolinae ancestor from the more widespread P. pomilia appears to be somewhat older, and the split between a hypothetical acuta/carolinae/pomilia ancestor and P. gyrina appears older still.ResultsHere we report the results of no-choice mating experiments yielding no evidence of hybridization between gyrina and any of four other populations (pomilia, carolinae, Philadelphia acuta, or Charleston acuta), nor between pomilia and carolinae. Crosses between pomilia and both acuta populations yielded sterile F1 progeny with reduced viability, while crosses between carolinae and both acuta populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between gyrina and all four of our other Physa populations, between pomilia and carolinae, and between pomilia and Charleston acuta, but not between pomilia and the acuta population from Philadelphia, nor between carolinae and either acuta population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between pomilia and acuta may have been reinforced by selection for prezygotic reproductive isolation in sympatry.ConclusionsWe propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five Physa populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, Physa.

EMPIRICAL ESTIMATES OF REPRODUCTIVE ISOLATION AMONG THE FRESHWATER PULMONATE SNAILS PHYSA ACUTA, P. POMILIA, AND P. HENDERSONI

Physa hendersoni collected from its type locality near Yemassee, South Carolina, and Physa pomilia from its type locality near Claiborne, Alabama, both display the penial morphology that has been characterized as “type-bc” by Te (1978, 1980). Mate choice tests returned no evidence of premating reproductive isolation between these two populations, and no-choice breeding experiments confirmed outcross fecundity, F1 viability and F1 fertility comparable to incross controls. Significant premating reproductive isolation was documented, however, between the P. hendersoni population and a population of Physa acuta from Charleston, South Carolina, bearing the “type-c” penial morphology. No-choice breeding experiments involving P. acuta and P. hendersoni yielded a mixture of hybrid and selfed progeny, the hybrids apparently sterile. Thus the nomen Physa hendersoni is a junior synonym of P. pomilia, whereas P. pomilia and P. acuta are distinct biological species.

NO REPRODUCTIVE ISOLATION BETWEEN FRESHWATER PULMONATE SNAILS PHYSA VIRGATA AND P. ACUTA

Abstract Mate choice tests provided no evidence of prezygotic reproductive isolation between a population of Physa virgata (Gould, 1855) collected from its type locality in the Gila River of Arizona and Physa acuta (Draparnaud, 1805) from a control site in Charleston, South Carolina. Reared in a no-choice experimental design, 10 outcross Arizona × South Carolina pairs initiated reproduction at approximately the same age as Arizona × Arizona controls, and earlier than South Carolina × South Carolina controls. Parents in the outcross experiment did not differ significantly from either control in their median weekly fecundity across 10 weeks of observation, yielding an F1 generation with significantly improved viability. We detected no evidence of reduction in F1 fertility. Thus, P. virgata, the most widespread freshwater gastropod of the American Southwest, should be considered a junior synonym of the cosmopolitan P. acuta.

Effects of host outcrossing on the interaction between an aquatic snail and its locally adapted parasite

In this study, we investigated the interaction between host outcrossing and infection in the Biomphalaria glabrata–Schistosoma mansoni system. Snails collected from three susceptible isofemale lines were mated with either siblings or snails recently derived from a field site in Brazil. Resulting inbred and outcrossed progeny were then exposed to S. mansoni larvae and monitored for a 10-week period. Interestingly, all snails exhibited equal susceptibility whether they were the result of inbreeding or outcrossing. However, further examination of both host and parasite life-history traits uncovered significant differences between the groups. In uninfected snails, outcrossed progeny tended to exhibit greater fitness relative to inbred progeny. When snails were parasitized, these differences were magnified in certain life-history traits, particularly host reproduction and survival. As an extension of the work, we also investigated virulence within this host–parasite system. Estimates of parasite reproduction and host size were combined to generate a novel “exploitation index,” and these indices were regressed with host survivorship. As predicted, there was a significant and negative correlation between the variables, but this was restricted to a single snail line. Results from this study demonstrate that infection outcomes (as measured by prevalence) may not differ between inbred and outcrossed hosts. However, outcrossing may enhance snail fitness through life-history trait expression.

Sex Bias in Interspecific Copulation between the Hermaphroditic Freshwater Snails, Physa acuta and P. pomilia (Physidae)

Abstract. Previously-published mate-choice experiments have uncovered significant prezygotic reproductive isolation between South Carolina populations of Physa acuta (Draparnaud, 1805) and P. pomilia (Conrad, 1834). Here we report the results of similar tests returning no evidence of such barriers between an Alabama population of P. pomilia and three separate populations of P. acuta, two from Alabama and one from South Carolina. However, heterogametic copulations did demonstrate both a significant sex bias and a significant size bias. The smaller P. acuta was more likely to copulate in the male role and hence less exposed to any potential fitness decrements due to the reduced viability and sterility of the hybrids produced. Our observations on defeated mating attempts suggested that P. acuta may be more aggressive than Alabama P. pomilia in mating contests (its smaller size perhaps contributing to greater agility) and hence more likely to prevail in the male role. The absence of a sex bias in our previously-published mate-choice observations involving South Carolina P. pomilia and P. acuta may be attributable to our use of size-matched snails, which equalized the agility of the prospective copulants, thus perhaps prolonging the mating contests, and ultimately lessening the likelihood that any mating took place at all.

Assessing predator risk: how leeches affect life history and behaviour of the freshwater snail Physa acuta

Physa acuta, an important consumer in freshwater ecosystems, is known to have morphological, life history and behavioural responses to fish and crayfish as shell-breaking predators. However, less is known about physid responses to their leech predators, which consume snails without damaging the shell and are commonly found in close proximity to P. acuta and their egg masses. Because juvenile physids are more vulnerable to leech predators than adults, we studied the effects that leech predation may have on a physid’s life history and reproductive behaviour. Snails that were reared with predatory-leech cue (fed conspecific snails) experienced a delay in reproduction. Snails also delayed their reproduction in the presence of crushed-snail cue alone. Snails did not exhibit any size or shell-shape differences over time when exposed to leech cue, although snail growth was depressed over time in the crushed-snail treatment. Both the cue from crushed snails and that from leeches caused a depression in the number of noticeable behaviours that snails displayed during mating-behaviour trials. In mating trials, control snails mated more frequently than snails reared in the crushed-snail treatment. This is the first study to show that chemical cues from predatory leeches affect life history strategies in physids via delayed production of viable egg masses.