PROTAMINES, NOT HISTONES, ARE THE PREDOMINANT BASIC-PROTEINS IN SPERM NUCLEI OF SOLITARY ASCIDIAN TUNICATES

In this work we study the protein content in sperm nuclei from 10 species of ascidian tunicates. The 7 species that we have investigated cytochemically show type 3 intermediate sperm basic proteins, as observed previously for the solitary tunicate Styela montereyensis (Chiva, Rosenberg, and Kasinsky, J. Exp. Zool., 253:7-19, 1990). We have focused this study on two specific points: the presence of protamines and the presence of somatic histones. In 4 species belonging to the suborder Phlebobranchiata, sperm nuclei contain only a protein more basic than histones (protamine). This protamine has an electrophoretic mobility similar to core histone H4. The nuclei of sperm in the species belonging to the suborder Phlebobranchiata are also characterized by their complete lack of somatic histones. In 5 species from the suborder Stolidobranchiata, sperm nuclei contain a similar protamine. However, 2 species from the genus Styela contain additionally other protamines with different electrophoretic mobilities. For all species belonging to the suborder Stolidobranchiata, we also have found very small quantities of residual somatic histones. We conclude that during the process of spermiogenesis in ascidian tunicates protamines replace histones either completely (Phlebobranchiata) or almost completely (Stolidobranchiata). In this respect spermiogenesis in tunicates (class Ascidiacea) is similar to that in vertebrates which, in general, also display the substitution of somatic histones by protamines. On the other hand, this model is different from that in echinoderms, where the protein content of sperm nuclei consists mainly of somatic histones and sperm-specific variants of histones.