Study Finds Last Bacterial Common Ancestor Had a More Complex Flagellum Than Previously Thought
A new preprint study concludes that the last bacterial common ancestor (LBCA) possessed a flagellum encoded by approximately 52 genes — far more than the 24 previously inferred to be universally conserved. Researchers used large-scale computational searches, gene-tree clustering, and genomic neighborhood analysis to identify 28 additional ancestral flagellar gene families. The findings challenge the long-held view that the ancestral bacterial flagellum was genetically simple, suggesting instead that many modern bacteria have undergone lineage-specific gene loss.
A preprint posted to bioRxiv presents evidence that the last bacterial common ancestor encoded a flagellum of substantially greater genetic complexity than prior reconstructions suggested. While model organisms require more than 40 genes to build and operate a functional flagellum, only 24 flagellar genes had previously been identified as broadly conserved across bacteria, leading many researchers to hypothesize that the ancestral flagellum was a simpler, partial structure later elaborated independently in different lineages. The new study applied profile- and sequence-based homology searches across a comprehensive bacterial genome dataset, supplemented by conserved sequence signatures and gene-neighborhood evidence, to identify 28 additional flagellar gene families with evolutionary histories consistent with an origin predating major bacterial diversification. The resulting 52-gene ancestral flagellum is inferred to have included all major functional modules: the secretion and export apparatus, basal body, axial filament components, motor-force generators, and regulatory checkpoints. This implies the LBCA could already build and operate a fully functional, contemporary-style flagellum. Rather than progressive assembly of complexity, the dominant evolutionary dynamic in flagellar evolution appears to have been gene loss across many bacterial lineages. The work has significant implications for models of early bacterial evolution and for longstanding debates about the origins of complex molecular machines.
What's missing
As a preprint, this study has not yet undergone formal peer review. Key limitations and open questions include: the accuracy of the inferred bacterial species tree used to root ancestral reconstructions, the possibility that some of the 28 newly identified gene families were acquired via horizontal gene transfer rather than vertical inheritance from the LBCA, and uncertainty about whether genomic absence in extant lineages definitively reflects gene loss versus other explanations. The study also does not directly address the evolutionary origin of the flagellum itself (e.g., its relationship to the type III secretion system), only its complexity at the LBCA stage.
What different sources said
- bioRxivCenter
The last bacterial common ancestor encoded a complex flagellum
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