Figure 6. ELT-1 represses eff-1 expression in seam cells to preserve stem cell-like identity and maintain apical junction integrity.

(A) Hermaphrodite carrying eff-1p::gfp and ajm-1::mCherry transgenes, subjected to elt-1 RNAi. Within the bracketed region a seam cell is in the process of losing ajm-1 expression, and only has vestiges of AJM-1::GFP remaining (arrowheads). This cell is now expressing eff-1p::gfp. In contrast, cells retaining their AJM-1 border never express eff-1p::gfp (white arrows). (B) eff-1 mutant hermaphrodite expressing scm::gfp and ajm-1::gfp. scm::gfp expression in the “true” seam is shown with white arrows. Anterior daughters of seam divisions sometimes retain expression of scm::gfp (white arrowheads), particularly where they remain in contact with the true seam (presumably these are the most recent daughters). Other anterior daughters do not express scm::gfp but retain their AJM-1 boundary (asterisks). In order to quantify inappropriate fusion in these worms we scored animals with one or more breaks in the seam line. In eff-1 mutants 3% of animals had breaks in the seam (n = 31). (C) eff-1 mutant hermaphrodite expressing scm::gfp and ajm-1::gfp, subjected to elt-1 RNAi. In these animals, no AJM-1 breakdown is observed, either around seam cells (white arrows) or around anterior daughters of asymmetric seam divisions (white arrowheads), demonstrating that the elt-1 (RNAi) induced fusion of seam cells with hyp7 is suppressed in eff-1 mutants. In order to quantify inappropriate fusion in these worms we scored animals with one or more breaks in the seam line. In eff-1 mutants subjected to elt-1 RNAi 0% of animals had breaks in the seam (n = 20), compared with 68% in elt-1(RNAi) animals alone (n = 44, Figure 5A, 5C, 5D). (D) Seam-specific RNAi strain hermaphrodite subjected to RNAi of ajm-1. In this experiment the RNAi deficient mutant rde-1 was used, rescued with rde-1 cDNA expressed under the control of the seam-specific SCM promoter, as described in detail in the Text S1. The bracketed cell has lost its AJM-1 border and the SCM::GFP has become much fainter. (E) eff-1 hermaphrodite carrying dpy-7p::yfp and ajm-1::gfp transgenes. Anterior daughters of seam divisions retain their AJM-1 borders but never express dpy-7 (asterisks). dpy-7 expression is only observed in the syncytial hypodermis surrounding these cells (white arrowheads). Scale bar, 25 µm.