MULTIGENE FAMILY OF RIBOSOMAL DNA IN DROSOPHILA-MELANOGASTER REVEALS CONTRASTING PATTERNS OF HOMOGENIZATION FOR IGS AND ITS SPACER REGIONS - A POSSIBLE MECHANISM TO RESOLVE THIS PARADOX

The multigene family of rDNA in Drosophila reveals high levels of with in-species homogeneity and between species diversity. This pattern of mutation distribution is known as concerted evolution and is considere d to be due to a variety of genomic mechanisms of turnover (e.g., uneq ual crossing over and gene conversion) that underpin the process of mo lecular drive. The dynamics of spread of mutant repeats through a gene family, and ultimately through a sexual population, depends on the di fferences in rates of turnover within and between chromosomes. Our ext ensive molecular analysis of the intergenic spacer (IGS) and internal transcribed spacer (ITS) spacer regions within repetitive rDNA units, drawn from the same individuals in 10 natural populations of Drosophil a melanogaster collected along a latitudinal dine on the east coast of Australia, indicates a relatively fast rate of X-Y and X-X interchrom osomal exchanges of IGS length variants in agreement with a multilinea ge model of homogenization. In contrast, an X chromosome-restricted 24 -bp deletion in the ITS spacers is indicative of the absence of X-Y ch romosome exchanges for this region that is part of the same repetitive rDNA units. Hence, a single lineage model of homogenization, coupled to drift and/or selection, seems to be responsible for ITS concerted e volution. A single-stranded exchange mechanism is proposed to resolve this paradox, based on the role of the IGS region in meiotic pairing b etween X and Y chromosomes in D. melanogaster.