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microRNAs Touted as Key Players in Vertebrate Evolution

NEW YORK (GenomeWeb News) –New research suggests microRNAs, tiny stretches of non-coding nucleic acids, are behind the evolution of animals with backbones from their spineless predecessors.
In contrast to the notion that genome duplication events alone resulted in vertebrate evolution, research by scientists at Dartmouth College and the University of Bristol in the UK suggests microRNAs — which are many and varied in vertebrates but scarce in invertebrates — may be behind the anatomical and genetic complexity associated with vertebrates. The paper appeared online today in the early edition of the Proceedings of the National Academy of Sciences.
“There was an explosive increase in the number of new microRNAs added to the genome of vertebrates,” lead author Alysha Heimberg, a biologist at Dartmouth University, said in a statement, “and this is unparalleled in evolutionary history.”
New miRNAs spring up all the time in the genomes of different animals. And once they do, they usually stick around. Consequently, the well-conserved miRNAs from animals alive today can provide information about their ancestors’ miRNAs as well.
Heimberg and her colleagues exploited this conservation using Northern analyses and genomic queries of miRBase and Blast searches with unassembled genomes to compare miRNAs across several species — from early chordates to sharks, fish, and mice — and to try to trace the history of 129 “chordate-specific” miRNA families.
Two of these families, it seems, go all the way back to early chordates, the closest vertebrate precursors. This suggests that rapid miRNA acquisition occurred in two periods in vertebrate evolutionary history — once at the base of vertebrates and once along the stem-lineage leading to eutherian mammals — and are closely tied to vertebrate evolution, according to the researchers.
And while the team’s work does not completely rule out the possibility that genome duplication, which also increases the number of miRNAs, contributed to vertebrate complexity, they said it does provide a plausible alternative.
“No good evidence has been marshaled in support of the much-vaunted hypothesis that [genome duplication events] can confer increasing organismal complexity,” the authors wrote. “We suggest that changes in the global transcriptional status of the vertebrate genome … led to the dramatic increase in organismal complexity in this one metazoan lineage.”

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