Extended open reading frames in Drosophila associated with small introns are a useful genomic tool for the identification of rapidly evolving coding sequence and splice junctions. Robert C. Eisman, Thomas C. Kaufman. Dept Biol, Jordan Hall A505, Indiana Univ, Bloomington, IN.

   Genome reduction in the genus Drosophila relative to many other insects, is primarily due to the deletion of significant regions of intergenic and intronic sequence. In this study of the evolution of orthologous centrosomin genes within several insect Orders, we show the additional loss of many small introns (<100 bp) in Drosophila and two mosquitoes has resulted in exon fusions. Exon fusions appear to be due to the imprecise loss of introns and are associated with rapidly evolving protein sequence. Interestingly, many of the remaining small introns in Drosophila cnn are either in the same reading frame as adjacent coding exons or are covered by long overlapping reading frames of two adjacent coding exons. Our data from Drosophila and other insects suggest these extended reading frames may arise as an intermediate step when introns are reduced in size and may help buffer against the potentially deleterious effects expected when the fusion of coding exons includes small intronic fragments. These effects are also buffered by simple protein folds encoded by extended open reading frames and a relaxed splicing mechanism. Additionally, cnn-like extended reading frames are present in approximately 3% of the genes in D. melanogaster and are useful tools for the identification of rapidly evolving protein coding regions and changes in the intron-exon structure of genes in the genus Drosophila, as well as other insect Orders.