Abstract
Evolution of axes of development are major transitions in the history of animal life. Though fifty different maternal genes are involved in setting up the dorso-ventral and antero-posterior axis of development in Drosophila, the existence of a left-right axis of development in insect is still unclear. It has been suggested that small differences of consistent direction between the left and the right side of bilateral characters (/Directional Asymmetry/) attest for the presence of a left-right axis of development in insect. Because the development of the animal body plan is controlled by large gene regulatory networks, patterns created by these genes are expected to be strongly conserved during evolution. Using different selection regimes on the wing shape in /Drosophila/, we observed a strong lability of directional asymmetry in wing size across selection regimes. This evolutionary lability of DA was confirmed by a review of studies on DA in insect wing, where we found that patterns of directional asymmetry are extremely unstable at the species or even at the population level. Furthermore, we found that the coefficient of phenotypic variation in DA is several order of magnitude higher than most morphological or life history characters. The precision with which directional asymmetry is expressed and its evolutionary lability suggest that DA can hardly be interpreted as an adaptation. This further questions the interpretation of DA as evidence for the existence of a left-right axis of development in insects.
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