The development and evolution of crossveins in insect wings

The formation of crossveins in Drosophila was an important early case study in understanding the role of the environment in the development and evolut ion of morphological structures. More recent work has shown that signalling processes play a crucial role in the formation of crossveins in Drosophila and that the interaction of a heat shock factor, Hsp90, with components of signal transduction pathways may account for the sensitivity of these stru ctures to environmental perturbations. A new model for the development of c rossveins is presented that divides the formation of crossveins into 3 sepa rate stages. First, the number and placement of the crossveins is determine d by signalling along the proximal-distal axis of the wing. This signal may involve the cdc42 gene product and the Jun-N-terminal Kinase signal transd uction pathway. Then, during the second stage, an inductive signal from the dorsal wing epithelium is sent to the ventral wing epithelium at locations specified by the first signal. The second signal appears to involve the BM P-like signalling pathway. Finally, in the third stage, a domain of vein co mpetent cells is defined by the signalling from the EGF-receptor-Map Kinase signal transduction pathway, and the exact location of the veins is eventu ally determined within that domain by Notch-Delta signalling interactions. By altering components of these 3 stages, insects can independently regulat e the presence or absence, the number and placement, and the thickness and flexibility of the crossveins. This capacity for the modulation of crossvei n structure in many different ways may have contributed to the evolution of different modes of insect flight.