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Evol Dev. 2003 Jul-Aug;5(4):372-8.

Expression of the 22 nucleotide let-7 heterochronic RNA throughout the Metazoa: a role in life history evolution?

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  • 1Molecular Biology Section, Division of Biology 0368, University of California, San Diego, 9500 Gilman Drive, Bonner Hall, Room 2214, La Jolla, CA 92093-0368, USA.


The 22 nucleotide let-7 small temporal RNA has been found consistently in samples from diverse bilateria but not from sponge or cnidarians. Here we further examine the phylogenetic distribution of this regulatory RNA by sampling representatives of diverse metazoan lineages. The 22 nucleotide let-7 RNA is detectable in triclad and polyclad platyhelminths, nemertean, and chaetognath but not ctenophore or acoel metazoans. These results support recent arguments that acoels are distinct from other acoelomate platyhelminths. We argue that let-7 is not a bilaterian or triploblast synapomorphy but instead evolved later in metazoan evolution, perhaps in association with complex life history traits.

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