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Diarrhea caused by enterotoxigenic Escherichia coli (ETEC) is a common threat to humans and animals. Clinical ETEC strains display host tropism, usually driven by adherence and toxin-host interactions in the gut. Other virulence factors, metal acquisition mechanisms, and immune evasion strategies may also influence host specificity. Using publicly available genomes, we performed a pangenomic and functional comparison of 77 human- and swine-derived ETEC strains. Sequence types ST10 and ST4 found in both hosts, suggesting potential cross-host transmission. Phylogenetic analyses showed clustering mainly by ST, regardless of host or geography. Additionally, most functional domains were shared between hosts; however, human-derived strains carried exclusive domains related to adhesion, transposition, and toxins, whereas swine-derived strains harbored domains linked to stress response and metal binding. Notably, the PF09075 domain from the STb enterotoxin occurred only in swine-derived strains, despite previous reports in humans. Comparative analyses of STa (PF02048) and STb revealed no structural or functional homology but confirmed the swine-specific association of STb and greater sequence STa variability in human-derived strains. These findings indicate that sequence and functional differences in these enterotoxins contribute to ETEC host tropism, while ST patterns reveal possible epidemiological links between human and swine infections.
Abstract Dominance is a widespread feature of genetic variants which affects life-history traits and fitness. Although dominance is generally thought to be an intrinsic property of genetic variants, it can sometimes evolve, as in the classic case of melanism in the peppered moth. The broader question of how likely dominance is to evolve is, however, controversial, because conditions favouring dominance evolution are often restrictive. Here, we revisit Haldane’s classic hypothesis that dominance might evolve during the spread of beneficial mutations to fixation (i.e., during selective sweeps). We first confirm results of earlier models that sweeps of unconditionally beneficial mutations generate little potential for dominance to evolve, even in cases where modifier alleles segregate prior to selective sweeps. However, when sweeping beneficial alleles trade off between different environments — which we explore with the illustrative case of sexually antagonistic selection — the scope for dominance evolution expands. This occurs because modifier alleles can alter dominance separately in each environment, increasing the mean fitness of heterozygotes, prolonging the sojourn time of the sweep, and generating more heterozygosity upon which the modifier can act. In extreme cases, beneficial mutations that were initially destined for fixation can undergo a “dominance reversal” as a result of dominance evolution, converting them to balanced polymorphisms. We quantify how regularly dominance reversals of sweeping sexually antagonistic alleles can be expected to evolve. Overall, our results highlight conditions that allow the dominance of beneficial mutations to evolve, which we discuss in light of data on the frequency of selective sweeps, standing genetic variation for modifiers, and plasticity of modifier effects.
Abstract The evolution of recombination is thought to be influenced by many factors, including the mating system. Here, we provide an experimental test of how self-fertilization (selfing) affects the evolution of a recombination modifier. We used experimental populations of Caenorhabditis elegans segregating for the recombination modifier rec-1 , a mutant that redistributes crossovers from the genetically diverse chromosome arms toward the less diverse central regions. By evolving populations under varying selfing rates, we show that increasing selfing reverses selection acting on the rec-1 mutant, from positive to negative. Simulations show that this reversal can be explained by an expansion of the genomic region over which the modifier remains associated with the genetic combinations it creates. These results demonstrate that selfing can fundamentally alter the evolutionary fate of recombination modifiers and reveal a mechanism not predicted by previous theoretical models of recombination evolution under different mating systems, which assumed uniform recombination landscapes.
One of the most prevalent and bioavailable glycans in marine systems is the β-glucan laminarin. Members of the phylum Bacteroidota are particularly well adapted to degrade this and other polysaccharides. Although recent research has provided detailed insights into the enzymatic breakdown of marine glycans by this phylum, the regulatory mechanisms that govern their utilization remain largely unexplored. Here, we describe a novel membrane-spanning one-component system that regulates laminarin utilization in marine Bacteroidota. We analyzed this β-glucan utilization regulator (BguR) type in the marine model bacterium Formosa agariphila KMM3901T. Deletion of the regulator gene abolishes growth on laminarin, whereas the wild type exhibits more than 80-fold induction of the associated genomic gene cluster, indicating the regulator's role as a transcriptional activator for laminarin utilization. Structural predictions show that its periplasmic sensor domain resembles those of hybrid two-component systems (HTCSs), although the absence of phosphorylation domains and distinct architecture indicate a completely different, ATP-independent mode-of-action. Comparative genomics show that this regulator is widespread among Bacteroidota, exhibiting lineage-specific distribution patterns similar to hallmark features such as tandem SusCD-like pairs. BguR is frequently found in close proximity to β-glucan-targeting PULs in the genome, implying a defined substrate preference that extends beyond laminarin. These findings suggest a novel regulatory mechanism for glycan sensing in marine bacteria, shedding light on an important facet of the marine carbon cycle. ### Competing Interest Statement The authors have declared no competing interest. Deutsche Forschungsgemeinschaft, https://ror.org/018mejw64, SCHW 595/10-3 and Project-ID 542264307
Deep-sea chemosynthesis-based ecosystems in the Mariana region include hydrothermal vents on the Mariana Arc and in the Mariana Back-Arc, and serpentinite-hosted seeps on the Mariana Forearc—with the latter being by far the least studied. Here, we surveyed the biodiversity of the serpentine seep on the Quaker Seamount. At first glance, the area appears to be devoid of fauna, but examination of the carbonates revealed dense aggregations of animals dominated by the limpets in genera Bathyacmaea and Pyropelta on brucite-carbonate chimney structures exhibiting visible fluid venting. Sorting of recovered material yielded a total of 14 species and together with an observation of a Munidopsis squat lobster we report a total of 15 species; six of these are considered endemics of chemosynthetic ecosystems. The occurrence of taxa such as the snail Lurifax cf. japonicus and the limpet Pyropelta ryukyuensis, otherwise only known from distant vents in the Izu-Ogasawara Arc and Okinawa Trough off Japan, strengthens the hypothesis that serpentine seeps may function as dispersal stepping-stones across arc and back-arc systems.
