Outer arm dynein of sperm flagella and cilia in the animal kingdom
It has been established that the outer arm dynein molecule of flagella and cilia in Protozoa such as Chlamydomonas and Tetrahymena has a three-headed structure, whereas that of sperm flagella in sea urchins, fish and mammals exhibits a two-headed structure. All the latter animals belong to Deuterostomia. The outer arm dynein from sperm flagella of the oyster, a member of Protostomia, is also two-headed. Investigation has begun of the situation in other animals, especially in Cnidaria and Porifera. In Chlamydomonas, there is a mutant whose outer arm dynein lacks one head. Electron microscopy revealed that the outer arm of the wild type shows a pistol-like shape in the cross-section of the flagellar axoneme, whereas that of the mutant exhibits a hook- or fist-like shape. This suggests that we may predict the number of heads of outer arm dynein by observing the cross-sections of flagella or cilia. Examination of the cross-sections of sperm flagella and cilia in various animals, including mammals, tunicates, echinoderms, molluscs, annelids, arthropods, flatworms and sea anemones, indicated that their outer arms were hook- or fist-like, in contrast to the pistol-like shape in Paramecium and Tetrahymena. The former was true with choanocyte flagella of the sponge and metazoan cilia. The reduction in the number of heads of outer arm dynein molecule would have occurred during the evolution from Protozoa to Metazoa (and Mesozoa). Alternatively, the outer arm dynein in Protozoans was specialized to support their peculiar behaviour.