Branching in Colonial Hydroids

Abstract

Cnidarians are primitive multi-cellular animals whose body is constructed of two epithelial layers and whose gastric cavity has only one opening. Most cnidarians are colonial. Colonial hydroids with their branched body can be regarded as a model for the whole phylum and are the most-studied cnidarian group with respect to developmental biology. Their colonies are constructed by repetition of limited number of developmental modules. The new modules are formed in the course of activity of terminal elements—growing tips of stolons and shoots. The growing tips of cnidarians, in contrast to those of plants, lack cell proliferation and drive morphogenesis instead by laying down and shaping the outer skeleton and formation of new colony elements. Cell multiplication takes place proximally to the growing tips. Branching in colonial hydroids happens due to the emergence of the new growing tip within the existing structures or by subdivision of the growing tip into several rudiments. Marcomorphogenetic events associated with different variants of branching are described, and the problems of pattern control are discussed in brief. Less is known about genetic basis of branching control.