INTRASPECIFIC AND INTERSPECIFIC VARIATION IN 5S RNA GENES ARE DECOUPLED IN DIPLOID WHEAT RELATIVES

5S RNAs form part of the ribosome in most organisms. In some, e.g., pr okaryotes and some fungi, the genes are part of the ribosomal operon, but in most eukaryotes they are in tandem arrays of hundreds to thousa nds of copies separate from the main ribosomal array. 5S RNA genes can be aligned across kingdoms. We were therefore surprised to find that, for 28 diploid species of the wheat tribe (Triticeae), nucleotide div ersity within an array is up to 6.2% in the genes, not significantly d ifferent from that of the nontranscribed spacers. Rates of concerted e volution must therefore be insufficient to homogenize the entire array . Between species, there are significantly fewer fixed differences in the gene than would be expected, given the high within-species variati on. In contrast, the amount of variation between species in the spacer is the same as or greater than that within individuals. This leads to a paradox. High variation within an individual suggests that there is little selection on any particular gene within an array. But conserva tion of the gene across species implies that polymorphisms are periodi cally eliminated at a rate approximately equal to or greater than that of speciation. Levels of intraspecific polymorphism and interspecific divergence are thus decoupled. This implies that selective mechanisms exist to eliminate mutations in the gene without also affecting the s pacer.