Development of the embryonic segment pattern in the pea-beetle Callosobruchus maculatus. Supracellular,cellular and molecular aspects

Summary

This review deals with the question of how cells in the early embryo of the pea-beetle differentiate into a sequential pattern of segments. Anterior and posterior fragments of an egg have different options for development depending on whether they are exposed, before cellularization, to decaying ooplasm in the complementary fragment. Without such exposure all fragments produce fewer segments than corresponding fragments obtained at cellularization. With exposure a fraction of anterior and posterior fragments produces considerably more segments than corresponding fragments obtained at cellularization. In addition, posterior fragments are uniquely different from anterior ones in that they also produce reversal of segment sequence which can be restricted to longitudinal strips of the larval cuticle.

The difference in reaction to decaying ooplasm between anterior and posterior fragments suggests an asymmetry in the control of metamerization. Lateral inhibition by an asymmetric gradient of a diffusible morphogen can describe these observations 18] except for the restriction of reversal to longitudinal strips. The latter requires either that morphogen transport be polarized, possibly by a voltage gradient in the egg, or that the interpretation of cell position is polarized. The induction of double abdomens with UV-light and RNase suggests that RNA is part of the control mechanism. This and strip-restricted reversal are features shared by eggs of Coleoptera and Diptera.