rDNA subtypes and their transcriptional expression in zebrafish at different developmental stages.

Eukaryotic 18S, 5.8S and 28S rRNAs are processed from a single transcript transcribed from the 45S rDNA gene, which is normally tandemly arrayed over hundred copies in a genome. Recently, a maternal (M) subtype and a somatic (S) subtype of rDNA were identified in zebrafish, with M-subtype on chromosome 4 and S-subtype on chromosome 5. It appears that the M-subtype is only expressed in eggs whilst the expression of the S-subtype is coupled with the initiation of zygotic gene expression. In this report, we identified three novel but transcriptionally inactive 18S variants in zebrafish genome with chromosome location different from the M- and S-subtype, suggesting translocation of 18S rDNA fragment during zebrafish evolution. Furthermore, we confirmed that the unfertilized eggs only have the M-subtype transcripts while brain, heart and liver have only the S-subtype transcripts. Both the M- and S-subtype transcripts were detected in female gonad. Our results support that the expression of different subtypes of rDNA is differentially regulated to meet the requirement for 'specialized ribosomes' during different developmental stages.