title: The evolution of the sperm tail.
abstract: In this paper the evolution of the sperm tail is discussed. The primitive motile apparatus is assumed to be a conventional '9 + 2' axoneme which persists in all aquatic phyla having external fertilisation. Where internal fertilisation has evolved in association with terrestrial life, the sperm tail has a '9 + 9 + 2' pattern: it has acquired new accessory proteins and has become enormously elongated. A subsequent trend is towards diminished motility, owing perhaps to the excessive development of skeletal structures and sophisticated copulatory organs. This is marked by unusual axoneme patterns and a lack of dynein arms. Aflagellate spermatozoa seem to represent a high evolutionary level. Finally, it appears that in the sperm of some groups motility has been regained. However, the axoneme never reappears: motility is produced instead by the spermatid manchette or an actin system.
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004398:Dyneins / Q000502:physiology; D005306:Fertilization; D006801:Humans; D008297:Male; D008854:Microscopy, Electron; D010802:Phylogeny; D013081:Sperm Motility; D013082:Sperm Tail / Q000502:physiology* / Q000648:ultrastructure; D013094:Spermatozoa / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D004398:Dyneins