Fig. 5. MS lineage defects in tbx-35(-) embryos confirmed by in situ hybridization. (A) pha-4 transcripts in a wild-type onefold embryo have an intense pharynx component (with ABa- and MS-derived regions) and a weaker endoderm-specific component. All stained embryos (n=37) demonstrated this type of staining. (B) Expression of the pharyngeal myosin gene myo-2 in a wild-type late-stage embryo. All (n=37) stained embryos displayed a pattern similar to the one shown. (C) Expression of the body muscle myosin gene myo-3 mRNA in a twofold embryo. Anterior muscles (descendants of MS) are shown by arrows. (D) Zygotic activation of pal-1 in the early C lineage. Among 31 embryos at this stage, four showed no staining, while the remaining 27 (87%) showed expression only in the C lineage, as shown here. (E) Eighty-eight percent (n=25) of stained tbx-35(-) embryos displayed a reduced anterior region of high pha-4 expression, similar to the embryo shown here. Twelve percent of embryos displayed apparent wild-type staining (not shown). (F) Detection of myo-2 in only the anterior half of the pharynx in tbx-35(-). (G) Absence of anterior MS-derived muscle (arrows) in a tbx-35(-) embryo stained for myo-3. Additional staining is present in the center of the embryo (^*). (H) Detection of zygotic pal-1 mRNA in both the MS and C lineages in a tbx-35(-) embryo, evidence of an early MS to C transformation. Among 36 embryos from the MS516 strain, three showed no staining and 33 showed strong expression in the C lineage. Of these, six also showed strong signal in the MS lineage, and two showed weak MS signal. As the rescuing array in MS516 is inherited by 60% of embryos (n=94), we estimate that 30% of tbx-35(-) embryos express pal-1 in the MS lineage.