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Background  The metagenesis of sessile polyps into pelagic medusae in cnidarians represents one of the most ancient complex life cycles in animals. Interestingly, scyphozoans and hydrozoans generate medusae by apparently fundamentally different processes. It is therefore unclear whether medusa formation has evolved independently in different medusozoans. To this end, a thorough understanding of the correspondence of polyp and medusa is required.    Results  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. 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.    Conclusions  Our data represent the first comparative gene expression analysis of developing medusae in two representatives of Scyphozoa and Hydrozoa. The results challenge prevailing views about polyp medusa body plan homology. We propose that the evolution of a new life stage may be facilitated by the adoption of existing developmental genes.    Electronic supplementary material  The online version of this article (doi:10.1186/s13227-015-0017-3) contains supplementary material, which is available to authorized users.

Adoption of conserved developmental genes in development and origin of the medusa body plan