Mutations in the Rho1 small GTPase disrupt morphogenesis and segmentation during early Drosophila development

Rho GTPases play an important role in diverse biological processes such as actin cytoskeleton organization, gene transcription, cell cycle progression and adhesion. They are required during early Drosophila development for pr oper execution of morphogenetic movements of individual cells and groups of cells important for the formation of the embryonic body plan. We isolated loss-of-function mutations in the Drosophila Rho1 (Rho1) gene during a gene tic screen for maternal-effect mutations, allowing us to investigate the sp ecific roles Rho1 plays in the contest of the developing organism. Here we report that Rho1 is required for many early events: loss of Rho1 function r esults in both maternal and embryonic phenotypes. Embryos homozygous for th e Rho1 mutation exhibit a characteristic zygotic phenotype, which includes severe defects in head involution and imperfect dorsal closure. Two phenoty pes are associated with reduction of maternal Rho1 activity: the actin cyto skeleton is disrupted in egg chambers, especially in the ring canals and em bryos display patterning defects as a result of improper maintenance of seg mentation gene expression. Despite showing imperfect dorsal closure, Rho1 d oes not activate downstream genes or interact genetically with members of t he JNK signaling pathway, used by its relatives dRac and dCdc42 for proper dorsal closure. Consistent with its roles in regulating actin cytoskeletal organization, we find that Rho1 interacts genetically and physically with t he Drosophila formin homologue, cappuccino. We also show that Rho1 interact s both genetically and physically with concertina, a G alpha protein involv ed in cell shape changes during gastrulation.