Clustering of rDNA containing type 1 insertion sequence in the distal nucleolus organiser ofDrosophila melanogaster: implications for the evolution of X and Y rDNA arrays

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Abstract
Summary: The ribosomal RNAs produced by the multigene families on theXandYchromosomes ofDrosophila melanogasterare very similar despite the apparent evolutionary isolation of theXandYchromosomal rDNA.X–Yexchange through the rDNA is one mechanism that may promote co-evolution of the two gene clusters by transferringYrDNA copies to theXchromosome. This hypothesis predicts that the proximal rDNA ofXchromosomes will beY-like. Consequently, rDNA variants found only on theXchromosome (such as those interrupted by type 1 insertions) should be significantly clustered in the distalXnucleolus organizer. Proximal and distal portions of theXchromosome nucleolus organizer were separated by recombination between the inverted chromosomesIn(1)scv2(breakpoint in the centre of the rDNA) andIn(1)sc4Lsc8R(no rDNA). Molecular analyses of the resulting stocks demonstrated that rRNA genes containing type 1 insertions were predominantly located on the chromosome carrying the distal portion of theXrDNA, thus confirming a prediction of theX–Yexchange hypothesis for the co-evolution ofXandYchromosomal rDNA. Distal clustering is not predicted by the alternative hypotheses of selection or gene conversion.