Morphogenesis in the absence of integrins: Mutation of both Drosophila β subunits prevents midgut migration

Two integrin β subunits are encoded in the Drosophila genome. The βPS subunit is widely expressed and heterodimers containing this subunit are required for many developmental processes. The second β subunit, βν, is a divergent integrin expressed primarily in the midgut endoderm. To elucidate its function, we generated null mutations in the gene encoding βν. We find that βν is not required for viability or fertility, and overall the mutant flies are normal in appearance. However, we could observe βν function in the absence of βPS. Consistent with its expression, removal of βν only enhanced the phenotype of βPS in the developing midgut. In embryos lacking the zygotic contribution of βPS, loss of βν resulted in enhanced separation between the midgut and the surrounding visceral mesoderm. In the absence of both maternal and zygotic βPS, a delay in midgut migration was observed, but removing βν as well blocked migration completely. These results demonstrate that the second β subunit can partially compensate for loss of βPS integrins, and that integrins are essential for migration of the primordial midgut cells. The two β subunits mediate midgut migration by distinct mechanisms: one that requires talin and one that does not. Other examples of developmental cell migration, such as that of the primordial germ cells, occurred normally in the absence of integrins. Having generated the tools to eliminate integrin function completely, we confirm that Drosophila integrins do not control proliferation as they do in mammals, and have identified αPS3 as a heterodimeric partner for βν.