MicroRNAs (miRNAs) are short, single stranded RNA molecules that regulate the stability and translation of messenger RNAs in diverse eukaryotic groups. Several miRNA genes are of ancient origin and have been maintained in the genomes of animal and plant taxa for hundreds of millions of years, playing key roles in development and physiology. In the last decade, genome and small RNA (sRNA) sequencing of several plant species have helped unveil the evolutionary history of land plants. Among these, the fern group (monilophytes) occupies a key phylogenetic position, as it represents the closest extant cousin taxon of seed plants, i.e. gymno- and angiosperms. However, in spite of their evolutionary, economic and ecological importance, no fern genome has been sequenced yet and few genomic resources are available for this group.

In a recent study, researchers from Universidad de Buenos Aires sequenced the small RNA fraction of an epiphytic South American fern, Pleopeltis minima (Polypodiaceae), and compared it to plant miRNA databases, allowing for the identification of miRNA families that are shared by all land plants, shared by all vascular plants (tracheophytes) or shared by euphyllophytes (ferns and seed plants) only. Using the recently described transcriptome of another fern, Lygodium japonicum, they estimated the degree of conservation of fern miRNA targets in relation to other plant groups.

These results pinpoint the origin of several miRNA families in the land plant evolutionary tree with more precision and are a resource for future genomic and functional studies of fern miRNAs.

Size distribution of P. minima sRNA sequences

The graph displays the raw number of sRNA reads in the range of 18 to 25 nt. The percentage of reads in each size category in the range is indicated. A large peak at 21 nt and a smaller peak at 24 nt are noticeable.

 

Reference
F. Berruezo, F. S. J. de Souza, P. I. Picca, S. I. Nemirovsky, L. M. Tosar, M. Rivero, A. N. Mentaberry, A. M. Zelada (2017) Sequencing of small RNAs of the fern Pleopeltis minima(Polypodiaceae) offers insight into the evolution of the microrna repertoire in land plants PLoS ONE 12(5): e0177573. doi: 10.1371/journal.pone.0177573 [article]

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