Spermiogenesis in the cattle egret (Bubulcus ibis).

Avian species comprise more than half of all vertebrates yet there is a dearth of information regarding spermatid development in this class of animals. This report of spermiogenesis in the cattle egret, Bubulcus ibis, is the first in the order Pelecaniformes. Five sexually mature and reproductively active male cattle egrets were captured in the wild, humanely euthanized, the reproductive organs dissected out, and tissues from the testes routinely prepared for transmission electron microscopy. Twelve steps of spermatid development, using the step-wise system, were determined. Acrosomogenesis in the egret results in a relatively short, solid, bullet-shaped acrosome that ends bluntly anteriorly and flat posteriorly or basally. The nucleus displays remarkable morphological changes, with the anterior end of the mature spermatid becoming flat, lacking a rostrum and an endonuclear canal. A perforatorium does not develop. It is noteworthy that a longitudinal, but not a circular, manchette develops during spermiogenesis in this bird. The proximal centriole is attached to the nucleus, at the implantation fossa, by means of well-formed, electron dense struts of material. An amorphous fibrous sheath develops in the principal piece. The interesting development and peculiar features of the acrosome and nucleus, as well as the absence of the perforatorium and circular manchette in the spermatozoon of the cattle egret, may be of phylogenetic significance.

Sperm structure of the cattle egret (Bubulcus ibis).

It remains a major concern that sperm structure has continued to be poorly investigated and reported in avian species. To our knowledge, sperm structure in the order Pelecaniforme has not been reported. Although McFarlane (1963; Proceedings of the XIII International Ornithological Congress; Ithaca, NY; American Ornithologists' Union) reported the study of spermatozoa in two genera and two species of the family Ardeidae, he did not provide an account, or the names of the species examined. The present report on the sperm structure of the cattle egret, Bubulcus ibis, is, thus, the first in the order Pelecaniformes (this bird has been placed variably under the order Ciconiiformes, or the order Pelecaniformes). Five sexually mature and reproductively active male cattle egrets were obtained from the wild, humanely euthanized, the reproductive organs dissected out, and tissues from the ducti deferentia were prepared for transmission electron microscopy. The sperm structure of this bird is generally similar to that described for most non-passerine birds. However, the acrosome is a short, conical or bullet-shaped, blunt-ending organelle that lacks a perforatorium. The base of the acrosome is flat and makes contact with the nucleus along, a correspondingly flat plane. The nucleus, thus, ends anteriorly in a flat plane devoid of a concavity or a rostrum, and an endonuclear canal. The acrosomal and nuclear features of this bird are, therefore, main deviations from the situation in the non-passerine clade of birds.