CELLULARIZATION IN LOCUST EMBRYOS OCCURS BEFORE BLASTODERM FORMATION

Drosophila intracellular gradients establish the pattern of segmentati on by controlling gene expression during a critical syncytial stage, p rior to cellularization, To investigate whether a similar mechanism ma y be exploited by other insects, we examined the timing of cellulariza tion with respect to blastoderm formation in an insect with extreme sh ort-germ development, the African desert locust, Schistocerca gregaria , Using light and electron microscopic techniques, we show that the is lands of cytoplasm surrounding cleavage nuclei are largely isolated fr om their neighbours, allowing cleavage to proceed asynchronously, With in a short time of their arrival at the surface and prior to blastoder m formation, nuclei become surrounded by complete cell membranes that block the free uptake of dye (10,000 kDa) from the yolk, Our results i mply that the formation of the blastoderm disc involves the aggregatio n of cells at the posterior pole of the egg and not the migration of n uclei within a syncytial cytoplasm, These findings suggest that the pr imary cleavage syncytium does not play the same role in patterning the locust embryo as it does in Drosophila. However, we do identify a syn cytial nuclear layer that underlies the forming blastoderm and remains in continuity with the yolk.