From populations of multicellular organisms to selfish genetic elements, conflicts between levels of biological organization are central to evolution. Plasmids are extrachromosomal, self-replicating g...

Dihydrogen Monoxide Research Division provides an international clearinghouse for information on Dihydrogen Monoxide including current research results, governmental regulations, and exposure and cont...

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Profiling host RNA shed in feces reveals disease states in the gut.

Bacterial quorum sensing is often assumed to follow a strict hierarchy, limiting understanding of how multiple signals interact. This study finds that Pseudomonas aeruginosa uses a reciprocal, coopera...

Although metagenomic sequencing has revealed a rich microbial biodiversity in the mammalian gut, methods to genetically alter specific species in the microbiome are highly limited. Here, we introduce ...

It is commonly thought that bacterial species borders tend to be fuzzy, due to frequent exchange of DNA. Here, Diop et al. quantify the patterns of gene flow between core genomes across 50 major bacte...

Symposium Schedule All presentation times are shown in Pacific Daylight Time (GMT -07:00 / UTC -07:00) Time Talk / Session 9:00 Welcoming remarks by Sid Venkatesh Session One: Neural Effects

The nature of standing genetic variation remains a central debate in population genetics, with differing perspectives on whether common variants are almost always neutral as suggested by neutral and n...

In simple microbial populations, the fitness effects of most selected mutations are generally taken to be constant, independent of genotype frequency. This assumption underpins predictions about evolutionary dynamics, epistatic interactions, and the maintenance of genetic diversity in populations. Here, we systematically test this assumption using beneficial mutations from early generations of the Escherichia coli Long-Term Evolution Experiment (LTEE). Using flow cytometry-based competition assays, we find that frequency-dependent fitness effects are the norm rather than the exception, occurring in approximately 80\% of strain pairs tested. Most competitions exhibit negative frequency-dependence, where fitness advantages decline as mutant frequency increases. Furthermore, we demonstrate that the strength of frequency-dependence is predictable from invasion fitness measurements, with invasion fitness explaining approximately half of the biological variation in frequency-dependent slopes. Additionally, we observe violations of fitness transitivity in several strain combinations, indicating that competitive relationships cannot always be predicted from fitness relative to a single reference strain alone. Through high-resolution measurements of within-growth cycle dynamics, we show that simple resource competition explains a substantial portion of the frequency-dependence: when faster-growing genotypes dominate populations, they deplete shared resources more rapidly, reducing the time available for fitness differences to accumulate. Our results demonstrate that even in a simple model system designed to minimize ecological complexity, subtle ecological interactions between closely related genotypes create frequency-dependent selection that can fundamentally alter evolutionary dynamics. ### Competing Interest Statement The authors have declared no competing interest.