Kaitlin A. Schaal

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Schaal

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Kaitlin A.

ORCID

0000-0001-9033-9646

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Publications 1 - 7 of 7

Behavioral Interactions between Bacterivorous Nematodes and Predatory Bacteria in a Synthetic Community
Mayrhofer, Nicola; Velicer, Gregory J.; Schaal, Kaitlin A.; et al. (2021)
Microorganisms
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 res-ponses 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 found 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 other prey are available. Most broadly, we found that nematode and bacterial predators mutually influence one another’s predatory behavior, with likely consequences for coevolution within complex microbial food webs.
Ecological histories govern social exploitation by microorganisms
Schaal, Kaitlin A.; Manhes, Pauline; Velicer, Gregory J. (2025)
The ISME Journal
Exploitation is a common feature of social interactions, which can be modified by ecological context. Here, we investigate effects of ecological history on exploitation phenotypes in bacteria. In experiments with the bacterium Myxococcus xanthus, prior resource levels of different genotypes interacting during cooperative multicellular development were found to regulate social fitness, including whether cheating occurs. Responses of developmental spore production to manipulation of resource-level histories differed between interacting cooperators and cheaters, and 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 exploitation in some pairwise mixes of cooperative natural isolates that occurs when both strains have grown under resource abundance. Our results contrast with previous experiments in which cooperator fitness correlated positively with concurrent resource level and suggest that resource-level variation may be important in regulating whether exploitation of cooperators occurs in a natural context.
Ecology and evolution in microbial food webs
Schaal, Kaitlin A. (2022)
All natural evolution occurs within an ecological context. Studying evolution in its simplest terms, for example via the LTEE, distills patterns and basic mechanisms. Yet biology is as much about complexity as it is about simplicity, and considering the factors that disrupt the patterns and complexify the mechanisms can reveal emergent truths. Microbial interactions are often studied as pairwise modules, yet they are influenced by the complexity of interaction networks. The prey in predator-prey interactions have competitors and cross-feeders, and the predators have their own predators and competitors to avoid or fight. In this thesis, I explore how behavior and evolution respond in multi-species microbial communities that are structured by predator-prey interactions. I focus on the bacterium Myxococcus xanthus, a social soil bacterium that swarms cooperatively through soil habitats, preying on other bacteria and fungi it encounters and, when nutrients are depleted, aggregating and developing into spore-filled fruiting bodies. The mechanisms, evolution, and origin of M. xanthus’s charismatic social traits offer a fascinating and fruitful target for this lens, and I investigate how M. xanthus responds over ecological and evolutionary time to interactions with lower trophic levels (prey) and higher trophic levels (predators). First, I construct a four-member community with two prey bacteria and M. xanthus and the bacterivorous nematode Caenorhabditis elegans as competing predators, and I measure the behavioral responses of the predators to each other. C. elegans generally avoids M. xanthus, leaving agar plates where it is the only food source (although worms who remain preferentially interact with M. xanthus over sterile buffer) and choosing patches of the lower-quality prey when M. xanthus is present in the patch of higher-quality prey. M. xanthus also shows a behavioral response to C. elegans, although this depends on third-party interactions – when the prey is of high quality, M. xanthus swarms faster in the presence of C. elegans. Next, I construct a three-member community comprising a food chain: Escherichia coli as a prey, M. xanthus as a mesopredator, and another bacterivorous nematode, Pristionchus pacificus, as an apex predator. I allow M. xanthus to evolve for 20 weeks in different community compositions: alone, with just E. coli, with just P. pacificus, or with both E. coli and P. pacificus. I first consider specific adaptation of evolved M. xanthus lineages to the different evolution treatments. Almost all lineages show evidence of adaptation to their own evolution environment, and it seems that exposure to at least one other organism during evolution provides some kind of benefit when encountering the other. However, exposure to both organisms is needed to do well in the three-member community. Swarming rates decreased relative to the ancestor, indicating that motility evolved across environments. I next consider trait-based evolution of M. xanthus in each of the communities in order to identify their contribution to the different long-term forces that shape M. xanthus as an organism. E. coli seems to simply offer a nutrient source, relaxing selection on starvation-related traits and allowing lack of maintenance to introduce phenotypic changes – in many cases, lineages that evolved in the presence of E. coli show lower levels of sporulation and no longer make fruiting bodies under starvation conditions. P. pacificus, on the other hand, selects for M. xanthus populations to form more and smaller fruiting bodies (although with unchanged levels of spore production), which may reflect an increased need for sheltering benefits. Strikingly, evolved lineages which descend from a variant of the wild type which contains a single point mutation in rpoB, and is therefore resistant to rifampicin, show a much smaller degree of trait-based evolution than lineages which descend from the wild type itself. This suggest that trade-offs between antibiotic resistance and anti-predator defense may shape evolutionary responses to inter-species antagonism under natural conditions. Finally, I argue for the development of microbial systems, such as the one I present here, both as a way to enrich the ecological, evolutionary, and mechanistic understanding of microbial communities and the specific trophic relationships which structure them and as a tool for addressing open questions about the evolution of predator behaviors and prey defenses more broadly. As we study interactions between simple ecological and evolutionary rules, surprising and beautiful complexity emerges.
Allopatric divergence of cooperators confers cheating resistance and limits effects of a defector mutation
Schaal, Kaitlin A.; Yu, Yuen-Tsu Nicco; Vasse, Marie; et al. (2022)
BMC Ecology and Evolution
BACKGROUND: 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. Upon starvation, the soil bacterium Myxococcus xanthus cooperatively develops into spore-bearing fruiting bodies, using a complex regulatory network and several intercellular signals. Some strains (cheaters) are unable to sporulate effectively in pure culture due to mutations that reduce signal production but can exploit and outcompete cooperators within mixed groups. RESULTS: 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, some lab-evolved cooperators had already exited the csgA mutant's cheating range after accumulating fewer than 20 mutations and without experiencing cheating during evolution. Cooperators might also diversify in the potential for a mutation to reduce expression of a 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. CONCLUSIONS: Our results demonstrate that natural populations may feature geographic mosaics of cooperators that have diversified in their susceptibility to particular cheaters, limiting defectors' cheating ranges and preventing them from spreading. This diversification may also lead to variation in the phenotypes generated by any given cooperation-gene mutation, further decreasing the chance of a cheater emerging which threatens the persistence of cooperation in the system.
Simulation of Pre-planetesimal Collisions with Smoothed Particle Hydrodynamics II
Geretshauser, Ralf J.; Meru, Farzana; Schaal, Kaitlin A.; et al. (2013)
High Performance Computing in Science and Engineering ‘12
In the frame of planet formation by coagulation the growth step from millimetre-sized highly porous dust aggregates to massive kilometre-sized planetesimals is not well constrained. In this regime of pre-planetesimals, collisional growth is endangered by disruptive collisions, disintegration by rotation as well as mutual rebound and compaction. Since laboratory studies of pre-planetesimal collisions are infeasible beyond centimetre-size, we perform numerical simulations. For this purpose, utilise the parallel smoothed particle hydrodynamics (SPH) code parasph. This program has been developed to simulate macroscopic highly porous dust aggregates consisting of protoplanetary material. We briefly introduce our porosity model and use it to perform simulations on the growth criteria of pre-planetesimals. With the aid of parameter studies we investigate fragmentation criteria in dust collisions depending on aggregate size and aggregate porosity. We extend a previous study on bouncing criteria of equally sized aggregates depending on their porosity and the presence of compacted shells of various porosities. Regarding the rotational stability of highly porous dust aggregates we theoretically derive fragmentation criteria for dust cylinders depending on angular velocity as well as porosity and perform suitable simulations.
The evolution and ecology of multiple antipredator defences
Kikuchi, David W.; Allen, William L.; Arbuckle, Kevin; et al. (2023)
Journal of Evolutionary Biology
Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such “defence portfolios” that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies.
Group Formation: On the Evolution of Aggregative Multicellularity
La Fortezza, Marco; Schaal, Kaitlin A.; Velicer, Gregory J. (2022)
Evolutionary Cell Biology ~ The Evolution of Multicellularity

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Kaitlin A. Schaal

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