Unequal crossing-over accounts for the organization of Drosophila virilis rDNA insertions and the integrity of flanking 28S gene

Abstract

The majority of ribosomal DNA (rDNA) repeats in Drosophila melanogaster and Drosophila virilis have an insertion in the 28S rRNA coding region, and such repeats are essentially inactive in transcription¹. There are two unrelated types of insertion in D. melanogaster rDNA, one of which (type 1) has some homology with insertions in D. virilis², and is located in precisely the same position as that in the D. virilis rDNA^3–6. D. melanogaster type 1 rDNA insertions fall into three size classes of 5, 1, 0.5 kilobases (kb), and sequences of shorter insertions comprise the downstream end of longer ones^4,5. Indeed, the points at which the sequences of longer type 1 insertions depart from shorter ones are oligonucleotides that are closely related to a 14-base pair (bp) rRNA coding sequence directly repeated at the flanks of D. virilis³ and some D. melanogaster⁴ rDNA insertions. It has been suggested that shorter D. melanogaster type 1 insertions derived from longer ones by unequal crossing-over that involved these homologies⁵, and I show here that long D. virilis rDNA insertions may have arisen from shorter ones in the same way. Also, I suggest that such unequal crossing-over can generate intact rDNA repeats from those with insertions.