Differential effects of a labial mutation on the development, structure, and function of stomach acid-secreting cells in Drosophila melanogaster larvae and adults

The differentiation of copper cells, which secrete stomach acid in Drosophi la larvae, has been shown previously to be sensitive to the labial(k3) muta tion. Here we found that stomach acid secretion in adults was insensitive t o lab(k3). The basis for this stage-specific effect was elucidated by chara cterizing the development, structure, and function of the adult midgut. Fir st, we demonstrated by copper-dependent fluorescence and morphology that co pper cells were present in the adult stomach. Fine-structure analysis of ad ult copper cells led to the identification of a previously unrecognized pla sma membrane domain: apicolateral contacts between copper cells and their n eighbors consisted of smooth septate junctions that were enriched in alpha beta -spectrin and ankyrin. Second, we demonstrated that adult copper cells were present in lab(k3)/lab(vd1) (conditional/null) adults. The labial pro tein was expressed in adult lab(k3)/lab(vd1) copper cells, but not in larva e. Thus the labk3 mutation had a stage-specific effect on midgut labial exp ression, but did not appear to affect protein function. Surprisingly, stoma ch acidification was dispensable during larval development, since lab(k3)/l ab(vd1) mutant larvae that lacked midgut acidification developed into ferti le adults.