The Drosophila trio plays an essential role in patterning of axons by regulating their directional extension

We identified the Drosophila trio gene, which encodes a Dbl family protein carrying two Dbl homology (DH) domains, each of which potentially activates Rho family GTPases. Trio was distributed along axons in the central nervou s system (CNS) of embryos and was strongly expressed in subsets of brain re gions, including the mushroom body (MB). Loss-of-function trio mutations re sulted in the misdirection or stall of axons in embryos and also caused mal formation of the MB. The MB phenotypes were attributed to alteration in the intrinsic nature of neurites, as revealed by clonal analyses. Thus, Trio i s essential in order for neurites to faithfully extend on the correct pathw ays. In addition, the localization of Trio in the adult brain suggests its postdevelopmental role in neurite terminals.