Biology subjects

Schaal, K. A.

Publications and source records attributed to Schaal, K. A..

6 recordsLinked to original sources

bioRxiv2025

Species interactions determine plasmid persistence in a 3-member bacterial community

Microbial communities are shaped by multiple forces, including interspecies interactions and the effects of mobile genetic elements such as plasmids. How these forces interact to affect community responses to environmental perturbations remains unclear, particularly considering the qualitatively different natures and the complexity of competitive and plasmid transfer interactions. We investigated the role of bacterial and plasmid interaction networks in community responses to single and multiple environmental perturbations, using a model community of three bacteria and two plasmids grown in rich media over five 2-day transfers. Bacterial interactions were the primary driver of community response, to the extent that plasmids were not always retained in the community even when they carried relevant resistance genes. Overall, our results indicate that while bacterial and plasmid interactions both shape community responses to environmental perturbations, bacteria-bacteria interactions may be the primary driver of community dynamics and their response to perturbations.

ecology↗

bioRxiv2025

Predation, evo-devo, and historical contingency: A nematode predator drives evolution of aggregative multicellularity

Regulated aggregation and differentiation of cells is an intriguing form of multicellularity that produces diverse morphological forms, resulting from an interplay of complex evolutionary, regulatory, behavioral, and social mechanisms. We investigate the potential for predation to shape aggregative multicellularity evolution across different combinations of three species comprising a multi-trophic food web. The bacterium Myxococcus xanthus, which can develop into spore-filled fruiting bodies, is a mesopredator; the bacterivorous nematode Pristionchus pacificus is an apex predator, and the bacterium Escherichia coli is a shared basal prey for both predators. The number and morphology of M. xanthus fruiting bodies responds evolutionarily to nematodes, regardless of whether E. coli is present. E. coli alone with M. xanthus reduces both fruiting body formation and spore production, but the presence of nematodes eliminates those effects. M. xanthus lineages with an ancestral antibiotic resistance mutation evolved less overall, revealing strong historical contingency and suggesting potential tradeoffs between antibiotic resistance and responsiveness to biotic selection. Our results suggest that predation both of and by mesopredators has played important roles in the evolution of aggregative multicellularity and reveal complex inter-trophic evolutionary interactions in a relatively simple three-species food web.

evolutionary biology↗

bioRxiv2023

Ecological histories determine the success of social exploitation

Ecological context often modifies biotic interactions, yet effects of ecological history are poorly understood. In experiments with the bacterium Myxococcus xanthus, resource-level histories of genotypes interacting during cooperative multicellular development were found to strongly regulate social fitness. Yet how developmental spore production responded to variation in resource-level histories between interactants differed greatly between cooperators and cheaters; relative-fitness advantages gained by cheating after high-resource growth were generally reduced or absent if one or both parties experienced low-resource growth. Low-resource growth also eliminated facultative exploitation in some pairwise mixes of cooperation-proficient natural isolates that occurs when both strains have grown under resource abundance. Our results contrast with previous studies in which cooperator fitness correlated positively with resource level and suggest that resource-level variation may be important in regulating whether exploitation of cooperators occurs in a natural context.

ecology↗

bioRxiv2022

Antagonistic, synergistic, and social pleiotropy in microbial cheaters

Cooperation is widespread among microbes. One mechanism proposed to constrain cheating is antagonistic pleiotropy, wherein mutations that cause defection from cooperation, while potentially under positive selection for this effect, reduce fitness at other traits. The bacterium Myxococcus xanthus engages in pleiotropically connected complex multicellular behaviors, including motility, predation, and starvation-induced fruiting-body development. Sporulation during development is susceptible to cheating. Here we investigate pleiotropic impacts on how cheater spores respond to stressful environmental changes after development, relative to cooperator spores. A cheater with a mutation in the developmental-signaling gene asgB shows antagonistic pleiotropy under both heat and basic-pH stress, while a cheater mutated at csgA shows synergistic pleiotropy under basic-pH stress. Further, in a social form of pleiotropy, cooperator spores formed in mixture with the asgB cheater are less resilient under basic pH than those from pure groups; interaction of cooperators with cheaters reduces the cooperators physical robustness. Our results indicate that, depending on the mutation, pleiotropy can promote as well as limit cheating alleles. They additionally demonstrate that alleles can pleiotropically alter traits in organisms not carrying those alleles. Synergistic and social pleiotropy may contribute to shaping the evolutionary dynamics of cooperation and cheating in many social systems.

evolutionary biology↗

bioRxiv2021

Behavioral interactions between bacterivorous nematodes and predatory bacteria in a synthetic community

Theory and empirical studies in metazoans predict that apex predators should shape the behavior and ecology of mesopredators and prey at lower trophic levels. Despite the ecological importance of microbial communities, few studies of predatory microbes examine such behavioral responses and the multiplicity of trophic interactions. Here, we sought to assemble a three-level microbial food chain and to test for behavioral interactions between the predatory nematode Caenorhabditis elegans and the predatory social bacterium Myxococcus xanthus when cultured together with two basal prey bacteria that both predators can eat - Escherichia coli and Flavobacterium johnsoniae. We find that >90% of C. elegans worms failed to interact with M. xanthus even when it was the only potential prey species available, whereas most worms were attracted to pure patches of E. coli and F. johnsoniae. In addition, M. xanthus altered nematode predatory behavior on basal prey, repelling C. elegans from two-species patches that would be attractive without M. xanthus, an effect similar to that of C. elegans pathogens. The nematode also influenced the behavior of the bacterial predator: M. xanthus increased its predatory swarming rate in response to C. elegans in a manner dependent both on basal-prey identity and on worm density. Our results suggest that M. xanthus is an unattractive prey for some soil nematodes and is actively avoided when more lucrative prey are available. Most broadly, we find that nematode and bacterial predators mutually influence one anothers predatory behavior, with likely consequences for coevolution within complex microbial food webs.

ecology↗

bioRxiv2021

Allopatric divergence limits cheating range and alters genetic requirements for a cooperative trait

Social defectors may meet diverse cooperators. Genotype-by-genotype interactions may constrain the ranges of cooperators upon which particular defectors can cheat, limiting cheater spread. The bacterium Myxococcus xanthus undergoes cooperative multicellular development, but some developmental defectors cheat on cooperators during this process. In this study, interactions between a cheater disrupted at the signaling gene csgA and allopatrically diversified cooperators reveal a very small cheating range. Expectedly, the cheater failed to cheat on all natural-isolate cooperators owing to non-cheater-specific antagonisms. Surprisingly, lab-evolved cooperators that diverged from their cheating-susceptible ancestor by fewer than 20 mutations and without experiencing cheating had already exited the csgA mutants cheating range. Cooperators might also diversify in the potential for a mutation to reduce expression of cooperative trait or generate a cheating phenotype. A new csgA mutation constructed in several highly diverged cooperators generated diverse sporulation phenotypes, ranging from a complete defect to no defect, indicating that genetic backgrounds can limit the set of genomes in which a mutation creates a defector. Our results suggest that natural populations feature geographic mosaics of cooperators that have diversified in their susceptibility to particular cheaters and in the phenotypes generated by any given cooperation-gene mutation. Significance statementSelection on cooperators exploited by obligate cheaters can induce evolution of resistance to cheating. Here we show that cooperators can also rapidly evolve immunity to cheating simply as a byproduct of evolutionary divergence in environments in which cooperation and cheating at the focal trait do not occur because the trait is not expressed. We also find that differences in the genomic context in which a cooperation-gene mutation arises can profoundly alter its phenotypic effect and determine whether the mutation generates a social defect at all - a pre-requisite for obligate cheating. These findings suggest that general divergence of social populations under a broad range of environmental conditions can restrict both the set of mutations that might generate social defectors in the first place and the host range of such defectors once they arise.

evolutionary biology↗