Human and mouse protein-noncoding snoRNA host genes with dissimilar nucleotide sequences show chromosomal synteny

snoRNAs are small protein-noncoding RNAs essential for pre-rRNA processing and ribosome biogenesis, and are encoded intronically in host genes (HGs) that are either protein coding or noncoding. mRNAs of protein-noncoding HGs differ in their nucleotide sequences among species. Although the reason for such sequential divergence has not been well explained, we present evidence here that such structurally different HGs have evolved from a common ancestral gene. We first identified two novel protein-noncoding HGs ( mU50HG-a and mU50HG-b ) that intronically encode a mouse ortholog of a human snoRNA, hU50 . The sequences of mU50HG mRNA differed from that of hU50HG . However, a chromosome mapping study revealed that mU50HG is located at 9E3-1, the murine segment syntenic to human 6q15, where hU50HG is located. Synteny is a phenomenon whereby gene orthologs are arranged in the same order at equivalent chromosomal loci in different species; synteny between two species means it is highly likely that the genes have evolved from a common ancestral gene. We then extended this mapping study to other protein-noncoding snoRNA-HGs, and found again that they are syntenic, implying that they have evolved from genes of common ancestral species. Furthermore, on these syntenic segments, exons of adjacent protein-coding genes were found to be far better conserved than those of noncoding HGs, suggesting that the exons of protein-noncoding snoRNA-HGs have been much more fragile during evolution.