miRNA Profile of a Triassic Common Mammalian Ancestor and PremiRNA Evolution in the Three Mammalian Reproductive Lineages

摘要

MicroRNAs (miRNAs) are major factors in the regulation of gene expression. Recent evolutionary studies ofmiRNAs indicate that important biological innovations, such as the advent of bilateral symmetry and placental reproduction,are accompanied by bursts of miRNA creation which are subsequently conserved via purifying selection. The emergenceof eutherian (placental) mammals followed by as much as fifty million years the appearance of the first true mammalsin the late Triassic, some 230 million years ago. We have utilized microRNA inventories of eutherian, metatherian(marsupial), monotreme (platypus), and chicken genomes to assemble a minimal microRNA profile of the last commonancestor of all mammals consisting of 171 miRNAs. This profile suggests that the rise of placental reproduction launcheda more than three-fold expansion of microRNAs. In addition to expansion of the microRNA repertoire, the conserved microRNAsfrom five mammalian and one avian genome show evidence for conforming to a canonical phylogenetic historyas well as dramatic deviations from the assumptions of molecular clock-like rates and the equality of substitution ratesamong lineages. We also show that many of these basal mammalian miRNAs are highly expressed in eutherian placentathus creating an opportunity to gain insight on how microRNAs acquire new targets and new functions.