Interspecies variation of 45S ribosomal DNA in vertebrates

Ribosomal DNA (rDNA) remains one of the least resolved components of complex genomes despite its central role in cellular function and evolutionary inference. Using chromosome-scale assemblies from the Vertebrate Genomes Project, we present a comparative analysis of the 45S rDNA locus across vertebrates. rDNA organization varies widely among clades, with lineage-specific differences in chromosomal distribution, copy number, and positional bias. Mammals and birds typically restrict rDNA to few, often terminal loci, whereas amphibians and several fish lineages exhibit highly dispersed architectures, including extreme cases spanning dozens of chromosomes. Copy number varies by more than an orde