Establishment and maintenance of parasegmental compartments

Embryos of higher metazoans are divided into repeating units early in devel opment, In Drosophila, the earliest segmental units to form are the paraseg ments. Parasegments are initially defined by alternating stripes of express ion of the fushi-tarazu and even-skipped genes. How fushi-tarazu and even-s kipped define the parasegment boundaries, and how parasegments are lost whe n fushi-tarazu or even-skipped fail to function correctly, have never been fully or properly explained, Here we show that parasegment widths are defin ed early by the relative levels of fushi-tarazu and evert-skipped at stripe junctions, Changing these levels results in alternating wide and narrow pa rasegments. When shifted by 30% or more, the enlarged parasegments remain e nlarged and the reduced parasegments are lost. Loss of the reduced parasegm ents occurs in three steps; delamination of cells from the epithelial layer , apoptosis of the delaminated cells and finally apoptosis of inappropriate cells remaining at the surface. The establishment and maintenance of verte brate metameres may be governed by similar processes and properties.