Fig. 3. Most ventral and lateral somatic muscle founders are recruited from cells expressing early Poxm. (A-E) Early and late Poxm expressions in the somatic mesoderm are regulated by different enhancers. Whole-mount transgenic Poxm1.8-Gal4/UAS-lacZ (A,B) and Poxm8.4-Gal4/UAS-lacZ (C,D) embryos were stained with rabbit anti-ß-galactosidase antiserum. A map of the Poxm upstream region (B, BamHI; R, EcoRI; X, XbaI), which delimits the 1.8 kb and 8.4 kb fragments used as enhancers in combination with the hsp70 minimal promoter to drive Gal4 expression in the Poxm-Gal4 transgenes, is shown in E. Overviews of late stage 11 embryos (A,C) and enlarged ventral and lateral views of abdominal segments A2-A4 (B) or A4-A6 (D) of stage 16 embryos are shown with anterior to the left and dorsal up. Muscle patterns (B,D) were visualized from the interior (B) or exterior (D) after staining for ß-gal, by dissecting the embryos in halves along the dorsal and ventral midlines, removing tissue below the muscles, and mounting the ectoderm with the attached muscles for bright-field microscopy in a Zeiss Axiophot. The moderate to low ß-gal levels observed at late stages after early activation by the 1.8 kb enhancer result from perdurance (B), whereas the high ß-gal levels observed after activation by the 8.4 kb enhancer mimic late stage Poxm expression (D). For muscle nomenclature, see Bate (Bate, 1993) or Fig. 4J,N. (F,G) Absence of late Poxm expression of a Poxm transgene driven by the early enhancer. Homozygous Poxm³⁶¹ embryos, rescued by the um1-2-Poxm transgene that includes only upstream cis-regulatory sequences up to the XbaI site (E) and no intron, exhibit a wild-type early Poxm pattern (stage 11; F) but no late Poxm expression (stage 16; G). Confocal micrographs of embryos with their anterior to the left and dorsal side up are shown. (H-J) Cells expressing early Poxm give rise to most ventral and lateral muscle founders. Cells expressing early Poxm were labeled by nuclear GFP (H) and their fate was followed by confocal microscopy in rP298-lacZ; Poxm1.8-Gal4/UAS-GFPnls; lmd¹ embryos, in which founders are marked by ß-gal (I) and their fusion with FCMs is blocked. Most ventral and lateral muscle founders that are labeled by ß-gal are also marked by GFP (J), many of which are marked by white arrowheads in two of the three abdominal segments of a stage 15 embryo shown in H and I.