Post-LECA Origin and Diversification of an Axonemal Outer Arm Dynein Motor

Dyneins were present in the last eukaryotic common ancestor (LECA) and play key roles in eukaryotic biology. Axonemal dyneins form the inner and outer arms that power ciliary beating, and it has long been recognized that outer arms in some organisms contain two different heavy chain motors, whereas those from other species contain a third unit that imparts enhanced motive force during ciliary beating. Previous phylogenetic analyses suggested that this third motor derived from a gene duplication event in the LECA, followed by the subsequent replacement of the N-terminal assembly domain with one formed from kelch and immunoglobulin repeats. Here I revisit the origin and organization of this dynein, combining the increased breadth of sequence information now available, AlphaFold modeling, and the recent recovery of a robustly rooted eukaryotic tree-of-life. This analysis confirms the third outer arm dynein HC arose in a common ancestor of the Diaphoretickes, with a basic N-terminal domain consisting of a β-propeller structure followed by two immunoglobulin folds. However, this region has undergone further diversification in some groups, gaining an additional full or partial β-propeller located immediately adjacent to the AAA motor domain. Thus, three variant forms of this N-terminal segment are discernable in extant eukaryotes.