Formal Genetics of Ascidians

SYNOPSIS. Our knowledge of ascidian genetics is reviewed. The paper is primarily concerned with the author's past and current work on the colonial species Bolryllus schlosseri. Five Mendelian loci account for most of its impressive polychromatism. Breeding experiments have substantiated the hypothesis of a single multiallelic locus for each of three enzymes (MDH, SOD, PGI) suggested by electrophoretic patterns. The nuclei of three linkage groups have been revealed. Self—fertilization entails a severe inbreeding depression. A specific self, nonself recognition, expressed by fusion or repulsion of contacting colonies, occurs in this species also. At variance with Botryllus primigenus, fusible colonies of B. schlosseri are completely interfertile. This has allowed a more direct genetic analysis of the phenomenon, confirming the alleged control by a single multiallelic locus. In order to fuse, the confronted colonies must share at least one allele. Young buds grafted in the tunic after removal of all the zooids develop a new colony at the host's expense only if donor and host are fusible. This means that fusibility and histocompatibility are strictly correlated. Chimerical colonies, obtained either in this way or following the resorption of one of two fused colonies, are now being investigated for their recognition specificity and electrophoretic pattern. Preliminary data indicate that both can be durably altered, suggesting that the allogeneic cell populations are persistent and renewing.