The colonial ascidian Botryllus schlosseri model for developmental and evolutionary studies.

The colonial ascidian Botryllus schlosseri is a cosmopolitan organism, easy to find in the field and rear in the laboratory, and able to reproduce in captivity, both sexually and asexually. Colonies are formed of numerous, genetically identical individuals (zooids) and undergo cyclical changes of blastogenetic generations in which all the adult zooids die and are replaced by their maturing buds. Embryogenesis is rapid and larvae for dispersal of new colonies can be obtained within a week from internal fertilization and brooding. The organism has emerged as a model species for the study of asexual reproduction, natural apoptosis and clearance of effete cell, allorecognition, immunobiology and regeneration. It was introduced as a laboratory animal more than 50 years ago by Armando Sabbadin, at the University of Padova (Italy) who, for the first time, set up conditions for the permanent culture of this species, which had only occasionally been reared before, mainly due to interest in its budding. Since then, the species has been reared in other laboratories and studied with particular attention to developmental biology and immunobiology, thanks to the possibility of subdiving colonies in clonal reproductive fragments, cross colonies and collect offspring under controlled conditions. A shared staging method of zooid development is a necessary prerequisite in order to compare data coming from different laboratories. We re-examine Sabbadin’s (1955) staging method and re-propose it as a simple tool for in vivo recognition of the main morphogenetic events and recurrent changes in the blastogenetic cycle, as this method refers in detail to the developmental stages of buds and adults which are intimately correlated each other.