Convergence and Extension during Germband Elongation in Drosophila Embryos

Abstract

After the initial infoldings of gastrulation, the ventral region of the Drosophila embryo undergoes a rapid elongation called germband extension. This elongation is produced by intercalation of the more lateral cells as they move toward the ventral midline. In many respects, the process is very similar to the convergent extension which occurs during amphibian gastrulation and to elongation of the archenteron in sea urchins.

Several years ago, Gergen, Coulter and Wieschaus (1986) proposed that the intercalary behavior of cells during germband elongation reflects the adhesive preferences established in individual cells by the anterior-posterior patterning which occurs at the blastoderm stage. This model is formally very similar to the clock model used by French, Bryant, and Bryant (1976) to explain intercalary regeneration in imaginal discs and vertebrate limbs. In that model, positional values within a field are infinitely graded, and cells tolerate only finite differences between themselves and their immediate neighbors. When the discrepancies between adjacent cells are too great, the cells are induced to divide or otherwise fill in the gap. Surgical manipulations and wound healing induce cell proliferation and intercalary regeneration because they juxtapose cells with radically different positional identities.

In our model for germband extension, similarly abrupt juxtapositions of positional values would arise when the graded segmental field is condensed onto the limited number of precursors cells present in each segment at the gastrula stage. In contrast to the clock model, however, inappropriate juxtapositions in the early embryo are not resolved by induced cell proliferation. Instead, cells from the more dorsal regions which by chance have the appropriate intervening positional identities intercalate.

The article below presents a more detailed description of the model for germband extension and describes several tests of the model based on the predicted behavior of cells in embryos with aberrant anterior-posterior and dorsal-ventral patterning.