The collections of Drosophila m. Canton S wild-type embryos
were dechorionated in a solution of 5% sodium hypochlorite in
water for about 1.5 min in a small basket. Embryos were thoroughly washed with 0.07% NaCl/0.1% TritonX-100. The embryos
were then fixed with 4% paraformaldehyde in 10 mM phosphate
buffer containing 0.1 M NaCl and heptane (1/1: v/v) with gentle
shaking for 20 min at 25 C. After fixation, the aqueous phase
was completely removed and embryos were vigorously shaken in
a 1:1 mixture of heptane and methanol so that they were devitellinized. Embryos were then washed with methanol twice and
stored in 100% methanol at 85 C.26 Prior to WISH using fixed embryos, the supernatant, methanol, was exchanged with PBT solution (10 mM phosphate buffer, pH 7.0/0.1 M NaCl/0.1% Tween
20), and embryos were stored at 25 C for 15 min. After exchanging
PBT solution, the probe solution (30 lM OMUpy2 in PBT) was then
added to the fixed embryos, followed by incubation at 40 C for
30 min, then at 4 C for 2 h. The embryos treated with probes were
then washed with PBT once. Embryos were mounted on a glass
slide in 70% glycerol in PBS and observed by fluorescent microscopy using a 20 objective lens