Independent recurrent evolution of MICRORNA genes converging onto similar non-canonical organisation across green plant lineages is driven by local and segmental duplication events in species, family and lineages

Abstract

The relationship between evolutionary history, organisation and transcriptional regulation of genes are intrinsically linked. These have been well studied in canonically organised protein-coding genes but not of MIRNAs. In the present study, we investigated the non-canonical arrangement of MIRNAs across taxonomic boundaries from algae to angiosperms employing a combination of genome organization, phylogeny and synteny. We retrieved the complete dataset of MIRNA from twenty-five species to identify and classify based on organisational patterns. The median size of cluster was between 2-5 kb and between 1-20 % of all MIRNAs are organized in head-to-head (with bidirectional promoter), head-to-tail (tandem), and overlapping manner. Although majority of the clusters are composed of MIRNA homologs, 25% of all clusters comprises of non-homologous genes with a potential of generating functional and regulatory complexity. A comparison of phylogeny and organizational patterns revealed that multiple independent events, some of which are species-specific, and some ancient, in different lineages, are responsible for non-canonical organization. Detailed investigation of MIR395 family across the plants revealed a complex origin of non-canonical arrangement through ancient and recent, segmental and local duplications; analysis of MIR399 family revealed major expansion occurred prior to monocot-dicot split, with few lineage-specific events. Evolution of "convergent" organization pattern of non-canonical arrangement originating from independent loci through recurrent event highlights our poor understanding of evolutionary process of MIRNA genes. The present investigation thus paves way for comparative functional genomics to understand the role of non-canonical organization on transcriptional regulation and regulatory diversity in MIRNA gene families.

Keywords: Brassica; Brassicaceae; MICRORNA genes; green plants; non-canonical organization.