18S58S28S

18S58S28S refers to the contiguous arrangement of the eukaryotic nuclear ribosomal RNA genes 18S, 5.8S, and 28S within the ribosomal DNA repeats. In most eukaryotes, these genes are part of a single transcription unit that is transcribed by RNA polymerase I as a single precursor rRNA, which is processed into mature 18S, 5.8S, and 28S rRNAs after removal of the internal transcribed spacers (ITS1 and ITS2) and any flanking external spacers. The complete unit is typically organized as a tandemly repeated array in the genome, often with hundreds of copies per haploid genome.

The 18S, 5.8S, and 28S rRNA genes together encompass regions that contain both highly conserved and variable sequences. The 18S gene is among the most conserved eukaryotic genes and is widely used for broad phylogenetic placement, while the 28S gene contains regions that provide greater resolution at deeper taxonomic levels. The 5.8S gene lies between ITS1 and ITS2 and is relatively short. Because the entire 18S–5.8S–28S unit can be amplified with universal primers, this region is commonly used in molecular systematics, environmental sequencing, and DNA barcoding to identify eukaryotes or to study evolutionary relationships.

Challenges include intragenomic variation among repeats and the presence of multiple rDNA copies that can differ within a single genome, as well as potential biases from concerted evolution and biased amplification. Despite these caveats, the 18S–5.8S–28S region remains a foundational marker set in many phylogenetic and biodiversity studies.