Polyphyletic origins and lineage-specific expansion of chemosensory systems in phylum Actinomycetota
Phylum Actinomycetota (syn. Actinobacteriota or Actinobacteria), one of the largest well-studied bacterial phyla, is renowned for complex morphology, non-motility, ecological versatility, and secondary metabolite production. Chemosensory systems (CSS) are among the most sophisticated prokaryotic signal transduction architectures, yet their evolutionary distribution in this phylum remains poorly understood. Here, a comparative genomic analysis of 693 high-quality genomes spanning the phylum resolves the diversity, distribution, and evolutionary trajectories of CSS. Despite extensive genomic complexity and ecological versatility, only 6.9% of genomes encode complete CSS clusters, confined to a subset of lineages within a few classes, indicating lineage-specific retention or loss. In contrast, two-component systems (TCS) are ubiquitously abundant, with hybrid kinases enriched in CSS-deficient classes, suggesting functional compensation for the absence of dedicated chemosensory pathways. Basal lineages retain simplified CSS (F1 or F5), whereas Actinomycetes show lineage-specific expansion into multiple classes (F5, F8, F9, and ACF), often co-occurring, indicative of enhanced sensory complexity. The phylogeny of CheA homologs shows a polyphyletic distribution, supporting a mosaic evolutionary history shaped by multiple independent acquisitions, diversification, and gene loss. Collectively, our findings show that, unlike motile bacteria, members of Actinomycetota rely predominantly on TCS-driven regulatory networks, with CSS retained only in select lineages where ecological demands require enhanced environmental sensing. This work provides a phylum-wide evolutionary framework for the emergence, diversification, and functional specialization of CSS in non-motile bacteria.