Spermatozoon ultrastructure and its systematic implications in fishes

Abstract

We have studied approximately 280 species spread over more than 100 families. New data on 80 species are reported. The spermatozoon of Chondrichthyes is characterized by two synapomorphies in the midpiece and the flagellum. Within the Chondrichthyes the subclass Holocephali is characterized by two spermatic apomorphies that distinguish it from the Elasmobranchii. The Osteichthyes, on the other hand, show great spermatic diversity. In each of the most primitive among them, the Actinistia, Dipnoi, Cladistia, Chondrostei, and Ginglymodi, the spermatic model is different, constituting many autapomorphies. The neopterygians possess a common character that distinguishes them from the other fishes: the lack of an acrosome. We propose a structure for the ancestral model of the spermatozoon of Neopterygii that resembles the gamete of aquatic invertebrates employing external fertilization. This simplified gamete exhibits several derived forms in teleosteans. It is mainly in the primitive Teleostei that these evolved forms are of taxonomic interest, as we demonstrate in the Mormyroidea and the Elopomorpha. In the evolved Teleostei we define a spermatozoon of the "perciform type." The distribution of this gametic type within the Perciformes may provide useful data for use with this ill-defined group whose phylogenetic interrelations are so little known.