Fig 1. Structure of sperm flagella and respiratory cilia and ODA composition in both cell types.

(A) Cross section schemes of a sperm flagellum and multiciliated respiratory epithelial cell. Both cell types carry a microtubule-based 9+2 axoneme and multiprotein complexes serving as functional modules. Nine outer microtubule doublets surround the central pair and are connected by the nexin-link dynein regulatory complex. Radial spokes form the connecting bridge to the central pair and its associated protein complexes, the central sheath. Each outer doublet carries outer (ODA) and inner (IDA) dynein arms, important for flagellar and ciliary beat generation and regulation. Sperm flagella carry accessory structures that define specific flagellar regions. The mid piece contains a mitochondrial sheath and outer dense fibers surrounding the axoneme. In the sperm flagellar principal piece, the mitochondrial sheath is substituted by a fibrous sheath. In the flagellar end piece, the axoneme is surrounded solely by the plasma membrane. (B) ODAs of the green algae Chlamydomonas reinhardtii present a three-headed structure with an α-, β-, and γ-heavy chain (HC). Mammals present different ODA types, all characterized by a two-headed structure. Respiratory cilia carry ODA type 1 in the proximal and ODA type 2 in the distal ciliary compartment. In comparison to Chlamydomonas, both mammalian ODA types carry the γ-HC ortholog DNAH5, while the β-HC orthologs DNAH11 and DNAH9 are specific to type 1 and 2, respectively. ODA type 3 is specific to sperm flagella and carries DNAH8 as γ-HC and DNAH17 as β-HC ortholog. DNAI1 and DNAI2 are orthologs of Chlamydomonas intermediate chains (IC1 and 2) and are present in all three ODA types [17,18].