Amy C. Krist

Experimental exposure of juvenile snails (Potamopyrgus antipodarum ) to infection by trematode larvae (Microphallus sp.): infectivity, fecundity compensation and growth

Abstract Host-parasite interactions that result in host castration are evolutionarily similar to predator-prey interactions because both interactions terminate reproduction for the host or prey. Yet, host-parasite interactions differ from predator-prey interactions in that infected hosts remain alive and potentially can make adjustments to their life-history strategy before castration is complete. Here we exposed juvenile snails (Potamopyrgus antipodarum) to infection by a digenetic trematode (Microphallus sp.) in order to determine whether: (1) pre-reproductive individuals could be infected, (2) individuals that were exposed to infection shifted resources to early reproduction (fecundity compensation), and (3) infected individuals exhibit altered growth rates relative to uninfected individuals. We found that juveniles are susceptible to infection; hence P. antipodarum could be selected for earlier maturation in populations where the risk of infection is high. We also found that fecundity compensation does not occur in this snail. Finally, we found that Microphallus-infected snails exhibit altered growth rates; individuals infected as juveniles have lower growth rates and are smaller than uninfected snails. These results suggest that growth is altered by infection of a trematode parasite but reproduction in uninfected snails is not induced by exposure to trematode eggs.

Phosphorus-mediated changes in life history traits of the invasive New Zealand mudsnail (Potamopyrgus antipodarum)

Understanding the mechanisms that species use to succeed in new environments is vital to predicting the extent of invasive species impacts. Food quality is potentially important because it can affect population dynamics by affecting life history traits. The New Zealand mudsnail, Potamopyrgus antipodarum, is a worldwide invader. We examined how mudsnail growth rate and fecundity responded to the C:P ratio of algal food in laboratory conditions. Mudsnails fed low-P algae (C:P 1,119) grew more slowly, matured later, produced smaller offspring, and grew to a smaller adult size than snails reared on algae with high levels of P. A relatively small increase in algal C:P (203–270) significantly increased mudsnail age at maturity. We suggest that the relatively high body P requirements of mudsnails make them susceptible to allocation trade-offs between growth and reproduction under P-limited conditions. The elemental composition of algae varies greatly in nature, and over half of the rock biofilms in streams surveyed within the introduced range of mudsnails in the Greater Yellowstone Area had C:P ratios above which could potentially pose P limitation of life history traits. High growth rate and fecundity are common traits of many species that become invasive and are also associated with high-P demands. Therefore, fast-growing consumers with high P demands, such as mudsnails, are potentially more sensitive to P limitation suggesting that limitation of growth and reproduction by food quality is an important factor in understanding the resource demands of invasive species.

Spatial variation in susceptibility to infection in a snail-trematode interaction

Parasites should be better at infecting hosts from sympatric populations than allopatric populations most of the time (parasite local adaptation). In a previous study of a population of snail parasites (Microphallus sp.) from Lake Alexandrina, New Zealand, we found that Microphallus was more infective to snails (Potamopyrgus antipodarum) in shallow water but not in deep water. Here, we repeated the original study and also monitored the development of the parasite. We found that parasites from shallow water were more infective to hosts from shallow water and developed more rapidly in these hosts. In contrast, parasites from deep water were not more infective to hosts from deep water and did not develop more rapidly in them. These results suggest clinal variation in the susceptibility of these snails, with shallow-water snails more susceptible than deep-water snails. We offer 2 possible explanations for these results. First, gene flow in the Microphallus population is primarily from shallow to deep water, leading to an asymmetric pattern of local adaptation. Alternatively, snails from shallow water may be more susceptible for reasons independent of gene flow, perhaps due to differences in host condition between habitats.

SENSITIVITY TO PHOSPHORUS LIMITATION INCREASES WITH PLOIDY LEVEL IN A NEW ZEALAND SNAIL

Evolutionary and ecological factors that explain natural variation in ploidy level remain poorly understood. One intriguing possibility is that nutrient costs associated with higher per‐cell nucleic acid content could differentially influence the fitness of different ploidy levels. Here, we test this hypothesis by determining whether access to phosphorus (P), a main component of nucleic acids, differentially affects growth rate in asexual freshwater snails (Potamopyrgus antipodarum) that differ in ploidy. As expected if larger genomes generate higher dietary P requirements, tetraploid P. antipodarum experienced a more than twofold greater reduction in growth rate in low‐P versus high‐P conditions relative to triploids. Mirroring these results, tetraploid P. antipodarum also had a significant reduction in body P content under low P relative to high P, whereas triploid body P content was unaffected. Taken together, these results set the stage for the possibility that P availability could influence the distribution and relative frequency of P. antipodarum of different ploidy levels. These findings could be applicable to many other animal taxa featuring ploidy‐level variation, which includes many mixed sexual/asexual taxa.

Can resource costs of polyploidy provide an advantage to sex

The predominance of sexual reproduction despite its costs indicates that sex provides substantial benefits, which are usually thought to derive from the direct genetic consequences of recombination and syngamy. While genetic benefits of sex are certainly important, sexual and asexual individuals, lineages, or populations may also differ in physiological and life history traits that could influence outcomes of competition between sexuals and asexuals across environmental gradients. Here, we address possible phenotypic costs of a very common correlate of asexuality, polyploidy. We suggest that polyploidy could confer resource costs related to the dietary phosphorus demands of nucleic acid production; such costs could facilitate the persistence of sex in situations where asexual taxa are of higher ploidy level and phosphorus availability limits important traits like growth and reproduction. We outline predictions regarding the distribution of diploid sexual and polyploid asexual taxa across biogeochemical gradients and provide suggestions for study systems and empirical approaches for testing elements of our hypothesis.

Response to Phosphorus Limitation Varies among Lake Populations of the Freshwater Snail Potamopyrgus antipodarum

Local adaptation – typically recognized as higher values of fitness-related traits for native vs. non-native individuals when measured in the native environment - is common in natural populations because of pervasive spatial variation in the intensity and type of natural selection. Although local adaptation has been primarily studied in the context of biotic interactions, widespread variation in abiotic characteristics of environments suggests that local adaptation in response to abiotic factors should also be common. Potamopyrgus antipodarum, a freshwater New Zealand snail that is an important model system for invasion biology and the maintenance of sexual reproduction, exhibits local adaptation to parasites and rate of water flow. As an initial step to determining whether P. antipodarum are also locally adapted to phosphorus availability, we examined whether populations differ in their responses to phosphorus limitation. We found that field-collected juvenile P. antipodarum grew at a lower rate and reached an important size threshold more slowly when fed a relatively low vs. a relatively high- phosphorus diet. We also detected significant across-population variation in individual growth rate. A marginally significant population-by-dietary phosphorus interaction along with a two-fold difference across populations in the extent of suppression of growth by low phosphorus suggests that populations of P. antipodarum may differ in their response to phosphorus limitation. Local adaptation may explain this variation, with the implication that snails from lakes with relatively low phosphorus availability should be less severely affected by phosphorus limitation than snails from lakes with higher phosphorus availability.

Experimental Evolution of Antibiotic Resistance in Bacteria

Bacteria are ideal organisms for studying evolution because they reproduce quickly and asexually, they permit the study of large sample sizes, they are easy to propagate, and it is easy to manipulate their environment (Elena & Lenski, 2003). In this simple experiment, the common soil bacterium, Bacillus thuringiensis, evolves resistance to the antibiotic streptomycin. Evolution of antibiotic resistance can be observed over a few weeks due to the short generation time of B. thuringiensis, which can divide every 45 minutes; the natural occurrence of random spontaneous mutations that occur during cell division; and experimental conditions that allow natural selection to occur. Thus, mutation and natural selection, the higher rate of reproduction or survival of organisms possessing favorable heritable traits, are the mechanisms of evolutionary change in this experiment.

Phenotypic plasticity of the introduced New Zealand mud snail, Potamopyrgus antipodarum, compared to sympatric native snails.

Phenotypic plasticity is likely to be important in determining the invasive potential of a species, especially if invasive species show greater plasticity or tolerance compared to sympatric native species. Here in two separate experiments we compare reaction norms in response to two environmental variables of two clones of the New Zealand mud snail, Potamopyrgus antipodarum, isolated from the United States, (one invasive and one not yet invasive) with those of two species of native snails that are sympatric with the invader, Fossaria bulimoides group and Physella gyrina group. We placed juvenile snails in environments with high and low conductivity (300 and 800 mS) in one experiment, and raised them at two different temperatures (16°C and 22°C) in a second experiment. Growth rate and mortality were measured over the course of 8 weeks. Mortality rates were higher in the native snails compared to P. antipodarum across all treatments, and variation in conductivity influenced mortality. In both experiments, reaction norms did not vary significantly between species. There was little evidence that the success of the introduced species is a result of greater phenotypic plasticity to these variables compared to the sympatric native species.

Host diet alters trematode replication and elemental composition

Highly productive ecosystems often exhibit high rates of parasitism. Theory and experimental evidence suggest that consumption of high-quality diets by hosts living in these ecosystems facilitates parasite replication, presumably by enhancing the parasite’s access to nutrients. However, a direct link between parasite replication and the ability to acquire nutrients from host resources has not been shown. We manipulated diet quality and quantity of field-collected infected and uninfected snails (Physella sp.). After 16 d on experimental diets, we compared parasite replication and virulence in infected snails. To estimate the parasite’s ability to acquire nutrients, we compared %C, N, and P of parasitized and unparasitized host tissues. Parasites in snail hosts fed diets enriched or depleted in P maintained their replication rates, but those in hosts fed a diet with intermediate P exhibited decreased replication (assayed by release of trematode transmission stages). Regardless of the quality of the host’s diet, tissue occupied by parasites was P rich relative to the same tissue in unparasitized snails. However, compared to the same tissues in unparasitized snails, %C and N of tissues occupied by parasites were elevated only in snails fed the low-P diet. Neither parasitism nor diet treatment affected snail mortality. Our results indicate that the quality of a host’s diet can alter trematode replication, but not in the direction we predicted. However, we found no evidence that high-quality diets enhance the parasite’s ability to acquire C, N, or P. Instead, this parasite may be capable of enriching the nutrient content of its host, regardless of the quality of the host’s diet.

Foraging differences between the native snail, Fossaria sp. and the invasive New Zealand mudsnail (Potamopyrgus antipodarum) in response to phosphorus limitation

Animals require food adequate in quantity and quality to grow and reproduce. Therefore, environments limited in key nutrients may promote competitive interactions, with the organisms best equipped to exploit these resources gaining a competitive advantage. We compared the foraging behavior of the New Zealand mudsnail (Potamopyrgus antipodarum), a worldwide invader that can dominate secondary productivity to Fossaria (Bakerlymnaea) bulimoides which is native to western North America to determine whether either of these animals modify their behavior to compensate for nutrient limitation. We used the amount of dietary phosphorus (P) as a measure of food quality, because P is often a limiting resource in freshwater ecosystems. We compared the feeding rate and food preference of each snail species to food containing two levels of P. Because potential competitors can also affect foraging decisions, we conducted additional experiments comparing the effects of conspecifics and heterospecifics under low and high biomass. Consistent with predictions, both species increased feeding rate of low P food, and P. antipodarum consumed more food per unit mass than Fossaria. However, only Fossaria displayed a preference for high P food. The two snails also differed in their response to interacting snails. Fossaria significantly reduced foraging activity in the presence of both conspecifics and heterospecifics, while P. antipodarum spent more time feeding under high conspecific density. Although the presence of high densities of P. antipodarum may negatively impact Fossaria in its native habitat, Fossaria’s ability to locate and preferentially feed on higher quality resources may mitigate some of the impact, potentially facilitating coexistence between these two species.