The fact that over half of the known miRNA families in Arabidopsis are not conserved in poplar or rice suggests that miRNA genes evolve rapidly in plants. It also suggests that homology-based miRNA gene discovery will result in gross underestimation of miRNA gene numbers. Deep sequencing of small RNAs under normal and various stress conditions will be necessary to uncover the full complement of miRNA genes in any plant species. To understand the evolutionary history of miRNAs in any species, it will also be necessary to obtain the full complements of miRNA genes in a number of species closely related to the species in question. Finally, uncovering miRNAs from key representative species spanning the entire evolutionary distance from unicellular plants to angiosperms will be necessary to provide a comprehensive picture of miRNA evolution in plants. It is worth noting that miRNAs have recently been identified from the unicellular green alga C. reinhardtii (Molnar et al. 2007; Zhao et al. 2007).
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