Morphogenesis in the absence of integrins: mutation of both Drosophila beta subunits prevents midgut migration.

Two integrin beta subunits are encoded in the Drosophila genome. The betaPS subunit is widely expressed and heterodimers containing this subunit are required for many developmental processes. The second betasubunit, betanu, is a divergent integrin expressed primarily in the midgut endoderm. To elucidate its function, we generated null mutations in the gene encoding betanu. We find that betanu is not required for viability or fertility, and overall the mutant flies are normal in appearance. However, we could observe betanu function in the absence of betaPS. Consistent with its expression, removal of betanu only enhanced the phenotype of betaPS in the developing midgut. In embryos lacking the zygotic contribution of betaPS, loss of betanu resulted in enhanced separation between the midgut and the surrounding visceral mesoderm. In the absence of both maternal and zygotic betaPS, a delay in midgut migration was observed, but removing betanu as well blocked migration completely. These results demonstrate that the second beta subunit can partially compensate for loss of betaPS integrins, and that integrins are essential for migration of the primordial midgut cells. The two beta 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 alphaPS3 as a heterodimeric partner for betanu.