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MicroRNAs are ubiquitous in plant genomes but vary greatly in their abundance within and conservation among plant lineages. To gain insight into the evolutionary birth/death dynamics of microRNA families, we sequenced small RNA and 5′-end PARE libraries generated from two closely related species of Gossypium. Here, we demonstrate that 33 microRNA families, with similar copy numbers and average evolutionary rates, are conserved in the two congeneric cottons. Analysis of the presence/absence of these microRNA families in other land plants sheds light on their depth of phylogenetic origin and lineage-specific loss/gain. Conserved microRNA families in Gossypium exhibit a striking interspecific asymmetry in expression, potentially connected to relative proximity to neighboring transposable elements. A complex correlated expression pattern of microRNA target genes with their controlling …