Cis- and trans-duplication of C/D snoRNA paralogs in platypus. Cis-duplications of three paralogs of a C/D snoRNA were detected in the gene for ribosomal protein S13 (RPS13). One of these was found in the cDNA library (snoRNA Oa2156); the second was detected by BLAST in the neighboring intron and is probably functional (oval between exons 4 and 5; intact box motifs, conserved between platypus and humans), and the third, also detected by BLAST, is diverged and most likely nonfunctional (based on structural requirements) in platypus but functional in human, mouse, and cow (oval between exons 3 and 4). Trans-duplication of one of these paralogs occurred to the 70-kDa heat-shock protein 8 gene (HSPA8) or vice versa. Within the HSPA8 gene, we also found two paralogs in our cDNA library that evolved via cis-duplications (snoRNAs Oa1989 and Oa1927). Three additional, potentially functional paralogs were found by BLAST search and are conserved between platypus and humans. (Filled ovals) snoRNAs in our platypus cDNA library; (open ovals) snoRNAs located by BLAST search with the library sequences; (hatched oval) a nonfunctional snoRNA in platypus.
