ULTRASTRUCTURE OF OSTRICH (STRUTHIO-CAMELUS) SPERMATOZOA .1. TRANSMISSION ELECTRON-MICROSCOPY

The origin and relationships of the tinamous (Order Tinamiformes), rat ites (Order Struthioniformes, Rheiformes, Casuariiformes, Apterygiform es) and birds of the order Galliformes and Anseriformes is the subject of much debate and it has been suggested that the ultrastructural ana lysis of a wide variety of avian sperm may provide information relevan t to this problem. This paper describes the fine structure of ostrich sperm and compares the results with published information for other no n-passerine birds. Ostrich sperm display a short, conical acrosome whi ch covers the tapered tip of the long, cylindrical nucleus. A nuclear invagination housing an acrosomal rod extends deep within the karyopla sm. A centriolar complex is situated beneath the head and consists of a short proximal centriole and a long (3,0 mum) distal centriole which extends the complete length of the midpiece. The central cavity of th e distal centriole contains a pair of microtubules embedded in a rod o f electrondense material. The midpiece is surrounded by a mitochondria l sheath. Concentrations of fine granular material are present between the mitochondria. The principal-piece of the tail is demarcated from the midpiece by a distinct annulus and characterized by a ribbed fibro us sheath enclosing a typical axoneme. Rudimentary coarse fibres are o bserved between the fibrous sheath and the doublet microtubules of the axoneme in the proximal region of the principal-piece. The end-piece contains a disorganized collection of axonemal microtubules. Ostrich s perm differ in a number of respects from that of other non-passerine b irds (the absence of a typical perforatorium; the presence of a ribbed fibrous sheath; a deep nuclear invagination; the structure and length of the distal centriole) but show a close similarity to sperm of the rhea and crested tinamou, both representatives of primitive avian fami lies. These observations add further support to the theory that the ra tites and tinamous constitute a monophyletic group. The evidence prese nted also reinforces the hypothesis that the ratites were the first gr oup to branch off from the main avian stem, to be followed by the Gall iformes & Anseriformes. Although it was impossible to determine whethe r the sperm of the tinamou are more ''primitive'' than those of the os trich or rhea, it is clear that ostrich and rhea sperm are closely all ied and distinct from tinamou sperm.