SIMPLICITY-CORRELATED SIZE GROWTH OF THE NUCLEAR 28S RIBOSOMAL-RNA D3EXPANSION SEGMENT IN THE CRUSTACEAN ORDER ISOPODA

The expansion segments within the eukaryote nuclear 23S-like ribosomal RNA molecule are now well characterized in many diverse organisms. A different base compositional bias, a higher propensity for size variab ility, and an increased evolutionary rate distinguish these regions fr om the universally conserved ''core'' regions of the molecule. In addi tion, some expansion segments of higher eukaryotes exhibit significant sequence simplicity which is hypothesized to occur by slippage-mediat ed mutational processes. We describe the discovery of extreme size var iation of the D3 expansion segment in the crustacean order Isopoda. Am ong 11 species D3 varies in size from 180 to 518 nucleotides but maint ains a homologous secondary structure. The D3 size is significantly po sitively correlated to relative simplicity factor (RSF), indicating th at growth is most likely by insertion of simple sequences. D3 size and RSF correlate approximately with a morphology-based phylogeny, and wi thin oniscideans RSF increases as more recent divergences occur. The D 3 of Armadillidium vulgare, with an RSF of 1.87, is the highest value recorded for any known expansion segment. Regions of high sequence sim plicity in nuclear ribosomal RNA were previously only known from the h igher vertebrate Lineage. Here we demonstrate that this phenomenon occ urs in a more extreme condition within a monophyletic invertebrate lin eage. The extreme size changes identified could indicate that expansio n segments are an extraneous element in the functioning ribosome.