The advent of the genomic era has provided important and surprising insights into the deducted genetic composition of the common ancestor of cnidarians and bilaterians. This has changed our view of how genomes of metazoans evolve and when crucial gene families arose and diverged in animal evolution. Sequencing of several cnidarian genomes showed that cnidarians share a great part of their gene repertoire as well as genome synteny with vertebrates, with less gene losses in the anthozoan cnidarian lineage than for example in ecdysozoans like Drosophila melanogaster or Caenorhabditis elegans. The Hydra genome on the other hand has evolved more rapidly indicated by more divergent sequences, more cases of gene losses and many taxonomically restricted genes. Cnidarian genomes also contain a rich repertoire of transcription factors, including those that in bilaterian model organisms regulate the development of key bilaterian traits such as mesoderm, nervous system development and bilaterality. The sea anemone Nematostella vectensis, and possibly cnidarians in general, does not only share its complex gene repertoire with bilaterians, but also the regulation of crucial developmental regulatory genes via distal enhancer elements. In addition, epigenetic modifications on DNA and chromatin are shared among eumetazoans. This suggests that most conserved genes present in our genomes today, as well as the mechanisms guiding their expression, evolved before the divergence of cnidarians and bilaterians about 600 Myr ago.
Keywords: Cnidaria; Genome evolution; Hydra; Nematostella; Transcriptome; cis-Regulation.
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