An X chromosome MicroRNA Cluster in the Marsupial Species Monodelphis domestica

MicroRNAs (miRNAs) are an important class of posttranscriptional gene expression regulators. In the course of mapping novel marsupial-specific miRNAs in the genome of the gray short-tailed opossum, Monodelphis domestica, we encountered a cluster of 39 actual and potential miRNAs spanning 102 kb of the X chromosome. Analysis of the cluster revealed that 37 of the 39 miRNAs are predicted to form thermodynamically stable hairpins, and at least 3 members have been directly cloned from M. domestica tissues. The sequence characteristics of these miRNAs suggest that they all descended from a single common ancestor. Further, 2 distinct families appear to have diversified from the ancestral sequence through different duplication mechanisms: one through a series of simple tandem duplications and the other through a recurrent transposon-mediated duplication process.