Fig. 1. lin is epistatic to drm. Whole-mount embryos were stained with anti-Crb (A-H,U-X) or anti-En (I-L), hybridized in situ with probes for upd (M-P) or hh (Q-T), or fixed and sectioned transversely (E-H). The wild-type hindgut (A) consists of three morphologically distinct domains: small intestine (SI), large intestine (LI), and rectum (RE). Boundary cell (BC) rings, labeled strongly with anti-Crb, separate the SI and LI anteriorly and the LI and RE posteriorly. Two boundary cell rows also run the length of the LI dividing it dorsoventrally, as seen in transverse section (two arrows in E). In drm embryos (B), the SI is missing, the cells of the LI are columnar, and the BC rows are duplicated (four arrows in F). In both lin (C,G) and drm lin (D,H) embryos, the LI is missing, the cells are cuboidal, and the BC rows are absent. In wild-type embryos, En is expressed in the dorsal portion of the LI (I, black arrowhead). In drm embryos (J), En is expressed throughout dorsal hindgut (black arrowhead), but is not expressed in the rectum. In lin (K) and drm lin (L) embryos, En-expressing cells are not present (white arrowhead). In wild-type embryos (M, arrowhead), upd is expressed within the SI. In drm embryos (N), upd is not expressed (white arrowhead). In lin (O) and drm lin (P) embryos, expression of upd is expanded throughout most of the hindgut (wide arrowhead), but not the RE. In wild-type embryos, hh is expressed in both the SI and RE (Q, arrowheads). In drm embryos (R), hh is expressed in the RE, the ventral portion of the LI, and at the junction between the hindgut and midgut (arrowheads). In lin (S) and drm lin (T) embryos, expression of hh is expanded throughout most of the hindgut (wide arrowhead), but not the RE. In Crb-stained wild-type embryos (U), the proventriculus (PV) forms a multi-layered valve-like structure between the esophagus (ES) and anterior midgut (arrows in U-X indicate junction with anterior midgut). In drm embryos (V), the PV does not fold properly. In both lin (W) and drm lin (X) embryos, the entire foregut appears shorter and wider, and the PV does not form (arrows).

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