The evolution of eukaryotic ribosomal DNA.

Mutations occur randomly throughout the ribosomal DNA (rDNA) sequence. Molecular drive (unequal crossing-over, gene conversion, and transposition) spreads these variations through the multiple copies of rDNA. Forces of selection act upon the variants to favor and fix them or disfavor and eliminate them. Selection has not permitted changes in regions within rRNA vital for its function; these sequences are evolutionarily conserved between diverse species. Possible functions for some of these conserved sequences are discussed. The secondary structure of rRNA is also highly conserved during evolution. However, eukaryotic rRNA is larger than prokaryotic rRNA due to blocks of "expansion segments". Arguments are put forward that expansion segments might not play any functional role. Other examples are reviewed of rDNA sequence insertion or deletion, including introns and the internal transcribed spacer 2.