![]() In contrast to the three other medusozoan taxa, medusae of hydrozoans (‘hydromedusae’) are generally formed in a lateral budding process from polyps. Staurozoans are unusual as they form ‘stalked medusae’ by apical metamorphosis of the polyp but do not subsequently undergo fission. ![]() While most authors interpret the cubozoan medusa formation as a complete metamorphosis from polyp to medusa, others understand it as a modified form of strobilation. Also in cubozoans, medusae are generated by apical metamorphosis from polyps (Fig. A process called strobilation, where medusae form by apical metamorphosis of the polyp body followed by horizontal fission, is found in scyphozoans. The generation of medusae occurs in fundamentally different ways in different medusozoan taxa. A polyp-like form has been suggested by many authors as the ancestral cnidarian adult body plan, and the medusa life stage a later inserted secondary derivative, although other scenarios have also been proposed (e.g. Cubozoa, Scyphozoa and Hydrozoa generally show a triphasic life cycle with a succession of a larva, a sessile polyp form and a pelagic, sexually active medusa stage (Fig. Cnidaria, the sister group of the Bilateria, represent the oldest of all animal lineages with a complex life cycle. The wide distribution of complex life cycles shows that such phenomena constitute an evolutionary advantage in virtually all eukaryotic phyla. The emergence of complex life cycles is driven by the exploitation of different ecological niches and seasonally available resources. We propose that the evolution of a new life stage may be facilitated by the adoption of existing developmental genes.Ĭomplex life cycles involve a succession of life stages with drastically divergent body forms, behaviours and ecological habitat. The results challenge prevailing views about polyp medusa body plan homology. ![]() Our data represent the first comparative gene expression analysis of developing medusae in two representatives of Scyphozoa and Hydrozoa. Unexpectedly, however, polyp tentacle marker genes were consistently expressed in the developing medusa bell, suggesting that the bell of medusae corresponds to modified and fused polyp tentacle anlagen. We monitored the expression patterns of conserved developmental genes in developing medusae of Clytia hemisphaerica (Hydrozoa) and Aurelia aurita (Scyphozoa) and found that developing medusae and polyps share similarities in their morphology and developmental gene expression. To this end, a thorough understanding of the correspondence of polyp and medusa is required. It is therefore unclear whether medusa formation has evolved independently in different medusozoans. ![]() Interestingly, scyphozoans and hydrozoans generate medusae by apparently fundamentally different processes. The metagenesis of sessile polyps into pelagic medusae in cnidarians represents one of the most ancient complex life cycles in animals. ![]()
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