Abstract:
The origin and relationships of the tinamous (Order Tinamiformes), ratites (Order Struthioniformes,
Rheiformes, Casuariiformes, Apterygiformes) and birds of the order Galliformes and Anseriformes is
the subject of much debate and it has been suggested that the ultrastructural analysis of a wide variety
of avian sperm may provide information relevant to this problem. This paper describes the fine
structure of ostrich sperm and compares the results with published information for other non-passerine
birds. Ostrich sperm display a short, conical acrosome which covers the tapered tip of the long,
cylindrical nucleus. A nuclear invagination housing an acrosomal rod extends deep within the
karyoplasm. A centriolar complex is situated beneath the head and consists of a short proximal
centriole and a long (3,0 μm) distal centriole which extends the complete length of the midpiece. The
central cavity of the distal centriole contains a pair of microtubules embedded in a rod of electron-dense
material. The midpiece is surrounded by a mitochondrial 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 fibrous sheath enclosing a typical
axoneme. Rudimentary coarse fibres are observed 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 sperm differ in a number of respects from
that of other non-passerine birds (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
families. These observations add further support to the theory that the ratites and tinamous constitute
a monophyletic group. The evidence presented also reinforces the hypothesis that the ratites were the
first group to branch off from the main avian stem, to be followed by the Galliformes & Anseriformes.
Although it was impossible to determine whether the sperm of the tinamou are more "primitive" than
those of the ostrich or rhea, it is clear that ostrich and rhea sperm are closely allied and distinct from
tinamou sperm.
