HIGH INTRINSIC - RATE OF DNA LOSS IN DROSOPHILA

PSEUDOGENES are common in mammals but virtually absent in Drosophila(1 ). All putative Drosophila pseudogenes show patterns of molecular evol ution that are inconsistent with tile lack of functional constraints(2 -5). The absence of bona fide pseudogenes is not only puzzling it also hampers attempts to estimate rates and patterns of neutral DNA change . The estimation problem is especially acute in the case of deletions and insertions, which are likely to have large effects when they occur in functional genes and are therefore subject to strong purifying sel ection. We propose a solution to this problem by taking advantage of t he propensity of retrotransposable elements, without long terminal rep eats (non-LTR) to: create non-functional, 'dead-on-arrival' copies of themselves as a common by-product of their transpositional cycle(6-8). Phylogenetic analysis of a non-LTR element, Helena, demonstrates that copies lose DNA at an unusually high rate, suggesting that lack of ps eudogenes in Drosophila is the product of rampant deletion of DNA in u nconstrained regions, This finding has important implications for the study of genome evolution in general and the 'C-value paradox'(9) in p articular.