Insights into the molecular mechanisms underlying diversified wing venation among insects.

Insect wings are great resources for studying morphological diversities in nature as well as in fossil records. Among them, variation in wing venation is one of the most characteristic features of insect species. Venation is therefore, undeniably a key factor of species-specific functional traits of the wings; however, the mechanism underlying wing vein formation among insects largely remains unexplored. Our knowledge of the genetic basis of wing development is solely restricted to Drosophila melanogaster. A critical step in wing vein development in Drosophila is the activation of the decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signalling pathway during pupal stages. A key mechanism is the directional transport of Dpp from the longitudinal veins into the posterior crossvein by BMP-binding proteins, resulting in redistribution of Dpp that reflects wing vein patterns. Recent works on the sawfly Athalia rosae, of the order Hymenoptera, also suggested that the Dpp transport system is required to specify fore- and hindwing vein patterns. Given that Dpp redistribution via transport is likely to be a key mechanism for establishing wing vein patterns, this raises the interesting possibility that distinct wing vein patterns are generated, based on where Dpp is transported. Experimental evidence in Drosophila suggests that the direction of Dpp transport is regulated by prepatterned positional information. These observations lead to the postulation that Dpp generates diversified insect wing vein patterns through species-specific positional information of its directional transport. Extension of these observations in some winged insects will provide further insights into the mechanisms underlying diversified wing venation among insects.