Spermiogenesis in Seison nebaliae (Rotifera, Seisonidea): further evidenceof a rotifer-acanthocephalan relationship

The spermatozoa of Seison nebaliae are filiform cells about 70 mu m long wi th a diameter of 0.6 mu m. They have a slightly enlarged head, 2.5 mu m lon g, followed by a long cell body, The flagellum starts from the head, and ru ns parallel to the cell body, contained in a groove along it, The head cont ains an acrosome, two large, paired para-acrosomal bodies, the basal body o f the flagellum and the anterior thin extremity of the nucleus, The cell bo dy contains the main portion of the nucleus, a single mitochondrion located in its distal portion, and many accessory bodies with different shapes, Th e flagellum has a 9 + 2 axoneme, The study of spermiogenesis shows the Golg ian origin of the acrosome and the para-acrosomal bodies and reveals some p eculiarities: a folding of the perinuclear cisterna is present between the proacrosome and the basal body of the flagellum in early spermatids and the flagellum runs in a canal inside the spermatid cytoplasm, The basal body m igrates anteriorly, These characters are shared partly by the Rotifera Mono gononta. and, to a large extent, by the Acanthocephala studied so far. Many details of the spermiogenetic process are identical to those of Acanthocep hala, thus suggesting that the processes in the two taxa are homologous. (C ) 1999 Harcourt Publishers Ltd.