Figure 1.
egl-20 Regulates Q Cell Migration and Is Expressed in the Posterior. (A) Q neuroblast lineage. The left and right Q cells (QL and QR, large circles) generate identical cell lineages to produce three neurons (diamond and two small squares) and cells that undergo apoptosis (crosses).(B) Migrations of the QL and QR descendants in the wild type and representative
egl-20 mutants. Positions of the cells in the Q lineage are shown relative to lateral epidermal cells called the V cells (gray ovals). At hatching, QL and QR are located in approximately the same anteroposterior position. A few hours after hatching, the QL cell migrates posteriorly a short distance and then begins to express the Hox gene
mab-5. The QL cell then divides and both descendants continue to express
mab-5 (filled square and filled triangle). The QL.ap descendant migrates into the tail (filled diamond), but the two descendants of the QL.pa cell (filled small squares) do not migrate substantially. Similarly, soon after hatching, the QR cell migrates anteriorly a short distance and divides. Its descendants, which do not express
mab-5, continue to migrate anteriorly. In
egl-20 mutants, the QL cell still migrates posteriorly, but does not express
mab-5. Consequently, its descendants migrate into the anterior. In addition, the QR.p descendants stop migrating prematurely and are located in positions posterior to the wild-type final positions.(C) Confocal image of a wild-type L1 larva carrying the integrated array muIs49(
egl-20::gfp). The earliest expression of
egl-20::gfp is detected in the tail region of the embryo at pretzel stage (around 600 min). At the time of the Q lineage migrations,
egl-20::gfp is expressed in the anal depressor muscle (mu anal) and in the postembryonic blast cells P9/10, K, F, U, and B. muIs49 rescues cell migration defects (QL, QR, and HSN) and the egg-laying defect of
egl-20(
n585) animals. The
egl-20(
n585) allele contains a mutation in a highly conserved cysteine and behaves as a strong reduction-of-function mutation by genetic criteria (Harris et al. 1996). An identical expression pattern and rescued phenotype were produced by a second independent integrated array carrying
egl-20::gfp. Dorsal is up and anterior is left. Scale bar is 10 um.(D) Diagram of the cells expressing
egl-20::gfp during the L1 larval stage.(E) Positional model for the asymmetric activation of
mab-5 expression (blue shading) in the Q lineage (see text).