Biology subjects

Strassmann, J.

Publications and source records attributed to Strassmann, J..

3 recordsLinked to original sources

bioRxiv2023

Experimental evolution of symbiotic microbes without their partners can imply the presence of cooperative or antagonistic adaptations

Microbes adapt to the presence of other species, but the fitness consequences of specific interactions are difficult to study in their natural context. We experimentally evolved symbiotic microbes in an artificial environment without access to the partners with whom they interact in nature. As organisms will tend to lose adaptations that they do not need due to drift or pleiotropic tradeoffs, we expect normally symbiotic microbes evolved in isolation to lose adaptations to help or harm their natural partners. The direction and magnitude of such changes can suggest whether the microbes had historically been selected to help or harm one another. We apply this method to the symbiosis between the social amoeba Dictyostelium discoideum and three intracellular bacterial endosymbionts, Paraburkholderia agricolaris, P. hayleyella, and P. bonniea. A minority of strains of Paraburkholderia and D. discoideum evolved differences in their effects on one anothers fitnesses, implying the existence of adaptations to one another that were lost when no longer relevant. Our results suggest that the degree to which D. discoideum and Paraburkholderia have adapted to help or harm one another can differ substantially between strains within each species, with some strains appearing to have a historically adversarial relationship, some strains a more benign relationship, and many strains no clear adaptations to one another at all. Our results underscore the complexity of microbial interactions in nature and suggest experimental evolution under relaxed selection is a potentially useful approach for studying adaptation in microbes.

evolutionary biology↗

bioRxiv2023

Reduced social function in experimentally evolved Dictyostelium discoideum implies selection for social conflict in nature

Many microbes interact with one another, but the difficulty of directly observing these interactions in nature makes interpreting their adaptive value complicated. The social amoeba Dictyostelium discoideum forms aggregates wherein some cells are sacrificed for the benefit of others. Within chimeric aggregates containing multiple unrelated lineages, cheaters can gain an advantage by undercontributing, but the extent to which wild D. discoideum has adapted to cheat is not fully clear. In this study, we experimentally evolved D. discoideum in an environment where there were no selective pressures to cheat or resist cheating in chimeras. D. discoideum lines grown in this environment evolved reduced competitiveness within chimeric aggregates and reduced ability to migrate during the slug stage. By contrast, we did not observe a reduction in cell number, a trait for which selection was not relaxed. The observed loss of traits that our laboratory conditions had made irrelevant suggests that these traits were adaptations driven and maintained by selective pressures D. discoideum faces in its natural environment. Our results suggest that D. discoideum faces social conflict in nature, and illustrate a general approach that could be applied to searching for social or non-social adaptations in other microbes. SIGNIFICANCE STATEMENTMicrobes interact in diverse and important ways, but the difficulty of directly observing microbes in nature can make it challenging to understand the adaptive significance of these interactions. In this study, we present an experimental evolution approach to infer the selective pressures behind an apparently social trait in the microbe Dictyostelium discoideum. We take advantage of the observation that organisms use it or lose it - when selective pressures are relaxed, adaptations that evolved in response to those pressures tend to be lost. Our work helps resolve debate over the importance of cheating in D. discoideum, and demonstrates a general approach that could be applied to the study of other microbial traits that are difficult to observe in nature.

evolutionary biology↗

bioRxiv2023

In the social amoeba D. discoideum, shortened stalks limit obligate cheater success even when exploitable partners are available.

Cooperation is widespread across life, but its existence can be threatened by exploitation. The rise of obligate social cheaters that are incapable of contributing to a necessary cooperative function can lead to the loss of that function and to the extinction of populations. In the social amoeba Dictyostelium discoideum, obligate social cheaters cannot form dead stalk cells and in chimeras instead form living spore cells. This gives them a competitive advantage within chimeras. However, obligate cheaters of this kind are not found in nature, probably because they are often enough in clonal populations that they need to retain the ability to produce stalks. In this study we discovered an additional cost to obligate cheaters. Even when there are wild-type cells to parasitize, the chimeric fruiting bodies that result have shorter stalks and these are disadvantaged in spore dispersal. The inability of obligate cheaters to form fruiting bodies when they are on their own combined with the lower functionality of fruiting bodies when they are not represent limits on obligate social cheating as a strategy.

evolutionary biology↗