Plants buffer some of the effects of a pair of cadmium-exposed zebrafish on the un-exposed majority.

Certain individuals have a disproportionate effect on group responses. Characteristics may include susceptibility to pollutants, such as cadmium (Cd), a potent trace metal. Here, we show how a pair of Cd-exposed individuals can impact the behavior of unexposed groups. We used behavioral assessments to characterize the extent of the effects of the Cd-exposed individuals on group boldness, cohesion, foraging, activity, and responses to plants. We found that groups with a pair of Cd-exposed fish remained closer to novel stimuli and plants than did groups with untreated (control) fish. The presence of plants reduced Cd-induced differences in shoal cohesion and delays feeding in male shoals. Shoals with Cd- and water-treated fish were equally active. The results suggest that fish acutely exposed to environmentally relevant Cd concentrations can have profound effects on the un-exposed majority. However, the presence of plants may mitigate the effects of contaminants on some aspects of social behavior.

A Pair of Cadmium-exposed Zebrafish Affect Boldness and Landmark use in the Un-exposed Majority.

Some individuals have a disproportionate effect on group responses. These individuals may possess distinct attributes that differentiate them from others. These characteristics may include susceptibility to contaminant exposure such as cadmium, a potent trace metal present in water and food. Here, we tested whether a pair of cadmium-exposed individuals could exert an impact on the behavior of the unexposed majority. We used behavioral assessments to characterize the extent of the effects of the cadmium-exposed pair on group boldness, cohesion, activity and responses to landmarks. We found that groups with a pair of cadmium-exposed fish approached and remained closer to novel stimuli and landmarks than did groups with pairs of fish treated with uncontaminated water (control). Shoals with cadmium and water treated fish exhibited similar levels of cohesion and activity. The results suggest that fish acutely exposed to environmentally-relevant cadmium concentrations can have profound effects on the un-exposed majority.

A pair of cadmium-exposed zebrafish affect social behavior of the un-exposed majority.

To account for global contamination events, we must identify direct and indirect pollutant effects. Although pollutants can have direct effects on individuals, it is unknown how a few contaminated individuals affect groups, a widespread social organization. We show environmentally relevant levels of cadmium (Cd) can have indirect social effects revealed in the social context of a larger group. Cd-contaminated individuals had poor vision and more aggressive responses, but no other behavioral effects. The presence of experienced Cd-exposed pairs in the groups had an indirect effect on the un-exposed individual's social interactions leading to the shoal becoming bolder and moving closer to a novel object than control groups. Because a few directly affected individuals could indirectly affect social behavior of the un-exposed majority, we believe that such acute but potentially important heavy metal toxicity could inform reliable predictions about the consequences of their use in a changing world.

Post-association barrier to host switching maintained despite strong selection in a novel mutualism.

Following a host shift, repeated co-passaging of a mutualistic pair is expected to increase fitness over time in one or both species. Without adaptation, a novel association may be evolutionarily short-lived as it is likely to be outcompeted by native pairings. Here, we test whether experimental evolution can rescue a low-fitness novel pairing between two sympatric species of Steinernema nematodes and their symbiotic Xenorhabdus bacteria. Despite low mean fitness in the novel association, considerable variation in nematode reproduction was observed across replicate populations. We selected the most productive infections, co-passaging this novel mutualism nine times to determine whether selection could improve the fitness of either or both partners. We found that neither partner showed increased fitness over time. Our results suggest that the variation in association success was not heritable and that mutational input was insufficient to allow evolution to facilitate this host shift. Thus, post-association costs of host switching may represent a formidable barrier to novel partnerships among sympatric mutualists.

Asymmetric density-dependent competition does not contribute to the maintenance of sex in a mixed population of sexual and asexual Potamopyrgus antipodarum.

Asexual reproduction is expected to have a twofold reproductive advantage over sexual reproduction, owing to the cost of producing males in sexual subpopulations. The persistence of sexual females, thus, requires an advantage to sexual reproduction, at least periodically. Here, we tested the hypothesis that asexual females are more sensitive to limited resources. Under this idea, fluctuations in the availability of resources (per capita) could periodically favour sexual females when resources become limited. We combined sexual and asexual freshwater snails (Potamopyrgus antipodarum) together in nylon mesh enclosures at three different densities in an outdoor mesocosm. After 1 month, we counted the brood size of fertile female snails. We found that fecundity declined significantly with increasing density. However, sexual females did not produce more offspring than asexual females at any of the experimental densities. Our results, thus, suggest that the cost of sexual reproduction in P. antipodarum is not ameliorated by periods of intense resource competition.

Pre- and post-association barriers to host switching in sympatric mutualists.

Coevolution between mutualists can lead to reciprocal specialization, potentially causing barriers to host switching. Here, we conducted assays to identify pre- and post-association barriers to host switching by endosymbiotic bacteria, both within and between two sympatric nematode clades. In nature, Steinernema nematodes and Xenorhabdus bacteria form an obligate mutualism. Free-living juvenile nematodes carry Xenorhabdus in a specialized intestinal receptacle. When nematodes enter an insect, they release the bacteria into the insect hemocoel. The bacteria aid in killing the insect and facilitate nematode reproduction. Prior to dispersing from the insect, juvenile nematodes must form an association with their symbionts; the bacteria must adhere to the intestinal receptacle. We tested for pre-association barriers by comparing the effects of bacterial strains on native versus non-native nematodes via their virulence towards, nutritional support of, and ability to associate with different nematode species. We then assessed post-association barriers by measuring the relative fitness of nematodes carrying each strain of bacteria. We found evidence for both pre- and post-association barriers between nematode clades. Specifically, some bacteria were highly virulent to non-native hosts, and some nematode hosts carried fewer cells of non-native bacteria, creating pre-association barriers. In addition, reduced infection success and lower nematode reproduction were identified as post-association barriers. No barriers to symbiont switching were detected between nematode species within the same clade. Overall, our study suggests a framework that could be used to generate predictions for the evolution of barriers to host switching in this and other systems.

DNA Content Variation and SNP Diversity Within a Single Population of Asexual Snails.

A growing body of research suggests that many clonal populations maintain genetic diversity even without occasional sexual reproduction. The purpose of our study was to document variation in single-nucleotide polymorphism (SNP) diversity, DNA content, and pathogen susceptibility in clonal lineages of the New Zealand freshwater snail, Potamopyrgus antipodarum. We studied snails that were collected from multiple field sites around a single lake (Lake Alexandrina), as well as isofemale clonal lineages that had been isolated and maintained in the laboratory. We used the kompetitive allele specific PCR (KASP) method to genotype our samples at 46 nuclear SNP sites, and we used flow cytometry to estimate DNA content. We found high levels of SNP diversity, both in our field samples and in our clonal laboratory lines. We also found evidence of high variation in DNA content among clones, even among clones with identical genotypes across all SNP sites. Controlled pathogen exposures of the laboratory populations revealed variation in susceptibility among distinct clonal genotypes, which was independent of DNA content. Taken together, these results show high levels of diversity among asexual snails, especially for DNA content, and they suggest rapid genome evolution in asexuals.