The first expression of the
pha-1:lacZ hybrid gene is detectable at the 100 cell stage of embryogenesis (130 minutes of embryonic development; all developmental times refer to time after the first cleavage at 20 degree) in cells that have positions in the embryo corresponding to the pharyngeal and body wall muscle precursors. Expression of the
pha-1:lacZ hybrid gene at around 200 minutes of development is detected in about 25 cells that form a cylinder in the anterior part of the embryo. These
pha-1:lacZ expressing cells are the pharyngeal precursors. The lineage of most of these cells (26 of 27) is restricted to pharyngeal fate and only the MSaaaaa cell produces pharyngeal cells and non-pharyngeal cells (two neurons of the ring ganglion). As soon as these 27 pharynx precursor cells divide (230-250 minutes), expression of the
pha-1:lacZ gene in the pharyngeal precursors ceases and only a few cells in the presumptive pharynx region express the hybrid protein. At 420 minutes of development (2 fold stage) no
pha-1:lacZ gene expression in the pharyngeal precursor cells is detectable.
pha-1:lacZ expression was also observed in body wall muscle cell precursors. Expression of the
pha-1 hybrid protein starts early in all body wall muscle cell precursors and is detectable throughout embryonic development in these cells.