Studies on the colonial ascidian Botryllus schlosseri. A review of some lines of research

Different lines of research on inland cultures of B. schlosseri have been pursued in our laboratory. Some of them are reviewed here. An understanding of the colony as a physiological unit both in space and time was gained by studying: a) how a vascular network develops which connects rhe zooids of coexisting generaltions, those of each generation being at the same stage, so that protogyny suffices to prevent selfing; b) rhe relationships between succeeding blastogenic generations as expressed by the gradual attainment of sexual maturity, and the transfer of male and female germ cells from one generation to those following; c) the ability of the colony to regulate the number of buds per zooid within the generations, and the number of coexisting generations, according to naturalJy or experimentally altred environmental conditions; d) the transmission through generations of epigenetic characters like the experimentally induced reversal of situs viscerum. Developmenr and differentiation of the heart in the blastozooids, the steps in their regression and the cells involved in their resorption, at the change of generation, have been investigated . The intestinal epithelium differentiares into various celI types which have been characterized by their ultrastrucrure and their role in the extra- and intracellular food digestion and absorption. A comparative study of the intercellular junctions in the alimentary tract of different species has shown the constant presence of tight and gap junctions and the absence of septate junctions, while scalariform junctions and zonulae adhaerentes were also at times recognized. Special attention has been paid to specialization of the ciliated stigmatal cells, which probably influences the metachronal coordination of ciliary beating. The blood celi types of B. schlosseri have been characterized and the polychromatism in which they are involved has been genctically analyzed. The ovoviviparity of this species has been compared with that of two other species as regards egg enve!opes, ovulation, egg retention in the atrial wall in Botryllus and Botrylloides and egg segregartion within the tunic in Diplosoma. The genetic analysis of histocomparibility has shown that, unlike B. primigenus, in B. schlosseri selfing and intercrossing of identical genotypes are feasible and segreg tion distortions in the progeny from compatible colonies are rather unusual.