Biology subjects

Doulcier, G.

Publications and source records attributed to Doulcier, G..

8 recordsLinked to original sources

bioRxiv2024

Evolutionary dynamics of nascent multicellular lineages

The evolution of multicellular organisms involves the emergence of cellular collectives that eventually become units of selection in their own right. The process can be facilitated by ecological conditions that impose heritable variance in fitness on nascent collectives with long-term persistence depending on capacity of competing lineages to transition reliably between soma- and germ-like stages of proto-life cycles. Prior work with experimental bacterial populations showed rapid increases in collective-level fitness with capacity to switch between life cycle phases being a particular focus of selection. Here we report experiments in which the most successful lineage from the earlier study was further propagated for 10 life cycle generations under regimes that required different investment in the somalike phase. To explore the adaptive significance of switching, a control was included in which reliable transitioning between life cycle phases was abolished. The switch proved central to maintenance of fitness. Moreover, in a non-switch treatment, where solutions to producing a robust and enduring soma-phase were required, evolution of mutL-dependent switching emerged de novo. A newly developed computational pipeline (colgen) was used to display the moment-by-moment evolutionary dynamics of lineages providing rare visual evidence of the roles of chance, history and selection. Colgen, underpinned by a Bayesian model, was further used to propagate hundreds of mutations back through temporal genealogical series, predict lineages and time points corresponding to changes of likely adaptive significance, and in one instance, via a combination of targeted sequencing, genetics and analyses of fitness consequences, adaptive significance of a single mutation was demonstrated. Overall, our results shed light on the mechanisms by which collectives adapt to new selective challenges and demonstrate the value of genealogy-centered approaches for investigating the dynamics of lineage-level selection.

evolutionary biology↗

bioRxiv2023

Stability of Ecologically Scaffolded Traits During Evolutionary Transitions in Individuality

Evolutionary transitions in individuality (ETIs), such as the emergence of multicellularity, are events in the history of life during which entities at one level of organisation (particles) form collective-level entities that subsequently become individuals in their own right. Recent empirical and theoretical studies advocate the importance of an externally imposed meta-population structure or "ecological scaffold" for the emergence of new levels of individuality. Such a scaffold enables survival and reproduction at the collective level and thus the possibility of selection for beneficial traits on that level. However, a long-standing difficulty for the ecological scaffolding approach has been its inability to adequately explain how collective-level trait values that evolved under scaffolding conditions can be retained once these conditions are lifted. We call this difficulty "the problem of endogenisation." Here, we derive general conditions for the possibility of endogenisation. Key to endogenisation is the existence of a fitness valley that can be circumvented when scaffolding occurs. Using a stochastic meta-population model, we implement two versions of ecological scaffolding (one temporal and one spatial) and study subsequent evolutionary trajectories using the modelling techniques of adaptive dynamics. Our analysis yields several important results. The temporal model reveals that only collective traits based on particle-particle interactions can be endogenised when a temporary scaffold is applied to the entire population. The spatial model shows that, given the presence of an environmental gradient of externally imposed meta-population structure, ecological scaffolding can only occur in a limited "Goldilocks" zone of the environment. Further, if endogenisation conditions are also fulfilled, scaffolded collectives can colonise non-scaffolding areas of the environment. We conjecture that Goldilocks zones could act as initiators of ETIs and help explain the near ubiquity of collective-level individuality even if the conditions that promote it prove to be rare.

evolutionary biology↗

bioRxiv2023

Rapid dissemination of host-metabolism-manipulating transposon-like entities via integrative and conjugative elements

Integrative and conjugative elements (ICEs) are self-transmissible mobile elements that transfer functional genetic units across broad phylogenetic distances. Accessory genes shuttled by ICEs can make significant contributions to bacterial fitness, yet ICEs that carry accessory genes encoding functions other than antimicrobial resistance remain poorly characterized. Recent observation of the rapid acquisition of ICEs in a pandemic lineage of Pseudomonas syringae pv. actinidae led to investigation of the structural and functional diversity of these elements among a diverse array of P. syringae. Fifty-three unique ICE types were identified across multiple phylogroups. These ICEs display distinct evolutionary histories compared to their bacterial hosts, are highly recombinogenic, exhibit a conserved structure and are punctuated by hotspots of accessory gene integration. Many carry a 16 kb transposon-like entity (Tn6212) that shows little polymorphism indicating recent dissemination. Deletion of Tn6212 did not alter pathogen growth in planta, but mutants displayed significant fitness defects when grown on TCA cycle intermediates. These were largely attributable to a single LysR regulator. RNA-seq analysis of a set of nested Tn6212 deletions confirmed a central role of LysR in enhanced expression of more than 300 genes and down-regulation of genes controlling expression of energetically demanding loci. Together the transcriptional data indicate a major role for Tn6212 in manipulation of bacterial metabolism with primary effects on RNA degradation, protein synthesis and potential diversion of ATP to growth.

microbiology↗

bioRxiv2022

Exit from lag phase in populations of Pseudomonas fluorescens is determined by strong interactions among cells

The relationship between the number of cells colonizing a new environment and time for resumption of growth is a subject of long-standing interest. In microbiology this is known as the "inoculum effect". Its mechanistic basis is unclear with possible explanations ranging from the independent actions of individual cells, to collective actions of populations of cells. Progress requires precise measurement of lag time distributions while at the same time, experimentally controlling inoculum size. Here we use a millifluidic droplet device in which the growth dynamics of hundreds of populations founded by different numbers of Pseudomonas fluorescens cells, ranging from a single cell, to one thousand cells, were followed in real time. Our data show that lag phase decreases with inoculum size. The average decrease, variance across droplets, and distribution shapes, follow predictions of extreme value theory, where the inoculum lag time is determined by the minimum value sampled from the single-cell distribution. Our experimental results show that exit from lag phase depends on strong interactions among cells, consistent with a "leader-cell" triggering end of lag phase for the entire population.

microbiology↗

bioRxiv2021

Beyond Fitness Decoupling: Tradeoff-breaking during Evolutionary Transitions in Individuality

Evolutionary transitions in individuality (ETIs) involve the formation of Darwinian collectives from Darwinian particles. The transition from cells to multicellular life is a prime example. During an ETI, collectives become units of selection in their own right. However, the underlying processes are poorly understood. One observation used to identify the completion of an ETI is an increase in collective-level performance accompanied by a decrease in particle-level performance, for example measured by growth rate. This seemingly counterintuitive dynamic has been referred to as "fitness decoupling" and has been used to interpret both models and experimental data. Using a mathematical approach, we show this concept to be problematic in that the fitness of particles and collectives can never decouple--calculations of particle and collective fitness performed over appropriate and equivalent time intervals are necessarily the same. By way of solution, we draw attention to the value of mechanistic approaches that emphasise traits, and tradeoffs among traits, as opposed to fitness. This trait-based approach is sufficient to capture dynamics that underpin evolutionary transitions. In addition, drawing upon both experimental and theoretical studies, we show that while early stages of transitions might often involve tradeoffs among particle traits, later--and critical--stages are likely to involve the rupture of such tradeoffs. Thus, tradeoff-breaking stands as a useful marker for ETIs.

evolutionary biology↗

bioRxiv2020

Division of labor and fitness decoupling in evolutionary transitions in individuality

Reproductive division of labor has been proposed to play a key role for evolutionary transitions in individuality (ETIs). This chapter provides a guide to a theoretical model that addresses the role of a tradeoff between life-history traits in selecting for a reproductive division of labor during the transition from unicellular to multicellular organisms. In particular, it focuses on the five key assumptions of the model, namely (1) fitness is viability times fecundity; (2) collective traits are linear functions of their cellular counterparts; (3) there is a tradeoff between cell viability and fecundity; (4) cell contribution to the collective is optimal; and (5) there is an initial reproductive cost in large collectives. Thereafter the chapter contrasts two interpretations of the model in the context of ETIs. Originally, the model was interpreted as showing that during the transition to multicellularity the fitness of the lower-level (the cells) is "transferred" to the higher level (the collective). Despite its apparent intuitiveness, fitness transfer may obscure actual mechanisms in metaphorical language. Thus, an alternative and more conservative interpretation of the model that focuses on cell traits and the evolutionary constraints that link them is advocated. In addition, it allows for pursuing subsequent questions, such as the evolution of development.

evolutionary biology↗

bioRxiv2019

Eco-evolutionary dynamics of nested Darwinian populations and the emergence of community-level heredity

Interactions among microbial cells can generate new chemistries and functions, but exploitation requires establishment of communities that reliably recapitulate community-level phenotypes. Using mechanistic mathematical models, we show how simple manipulations to population structure can exogenously impose Darwinian-like properties on communities. Such scaffolding causes communities to participate directly in the process of evolution by natural selection and drives the evolution of cell-level interactions to the point where, despite underlying stochasticity, derived communities give rise to offspring communities that faithfully re-establish parental phenotype. The mechanism is akin to a developmental process (developmental correction) that arises from density dependent interactions among cells. Knowledge of ecological factors affecting evolution of developmental correction has implications for understanding the evolutionary origin of major egalitarian transitions, symbioses, and for top-down engineering of microbial communities.

evolutionary biology↗

bioRxiv2019

Experimental manipulation of selfish genetic elements links genes to microbial community function

Microbial communities underpin earths biological and geochemical processes, but their complexity hampers understanding. Motivated by the challenge of diversity and the need to forge ways of capturing dynamical behaviour connecting genes to function, biologically independent experimental communities comprising hundreds of microbial genera were established from garden compost and propagated on nitrogen-limited minimal medium with cellulose (paper) as sole carbon source. After one year of bi-weekly transfer, communities retained hundreds of genera. To connect genes to function we used a simple experimental manipulation that involved periodic collection of selfish genetic elements (SGEs) from separate communities, followed by pooling and redistribution across communities. The treatment was predicted to promote amplification and dissemination of SGEs and thus horizontal gene transfer (HGT). Confirmation came from comparative metagenomics, which showed substantive movement of ecologically significant genes whose dynamic across space and time could be followed. Enrichment of genes implicated in nitrogen metabolism, and particularly ammonification, prompted biochemical assays that revealed a measurable impact on community function. Our simple experimental strategy offers a conceptually new approach for unravelling dynamical processes affecting microbial community function.

microbiology↗