Observations on reproduction and regeneration in Aeolosoma viride

It is known that Aeolosoma viride reproduces for paratomic fission with pygidial budding. The reproductive process leads to the formation of a chain of zooids in which the first position is occupied by the parental zooid followed by the filial zooids in the reverse order of their degree of differentiation (the oldest filial zooid, that is about to fission, occupies the last position of the chain and is preceded by the other zooids, the youngest of these being contiguous with the parental zooid). For a study of the reproductive activity we analyzed the production of filial zooids, the articulation of the chain during the life cycle and the metamers produced from the parental zooid and from the filial zooids that are being formed in the chain. We individuate two processes that interact: the production of the initial metamer of new zooids from the parental zooid and the morphogenesis of new metamers during the differentiation of the filial zooids. In the parental zooid the production of new zooids (filial zooids) originates from the last metamer that contains a reproductive blastema (bR). This reproductive blastema produces new metamers in the caudal direction with a rate of about 1.4 metamers per day. These metamers assume the value of initial metamers of the new filial zooids. They contain the morphogenetic blastema (bM). Each morphogenetic blastema produces, in the cephalic direction, the metamers that form the body of the filial zooids at a rate of about 2.5 metamers per day. The morphogenesis of the filial zooid is completed by the differentiation of the cephalic region that occurs just before the fission. Experiments for induced regeneration in the cephalic and caudal direction have highlighted the phenomenon of partial repair of the parental zooid that however recovers quickly the reproductive activity. This is carried out by a bR that is regenerate in the anterior stumps, which are involved in caudal regeneration, but preserved in the posterior stumps, which are involved in cephalic regeneration. We emphasize the importance of pygidial blastema that in the filial zooids of the chain perform the morphogenetic processes, while, following fission, perform the reproductive function in those that have become parental zooids. We suggest that the changes of role in the pygidial blastema, during the life span of Aeolosoma viride, are correlated with important variations of gene expression and enzymatic processes.