DROSOPHILA MODE OF METAMERIZATION IN THE EMBRYOGENESIS OF THE LEPIDOPTERAN INSECT MANDUCA-SEXTA

Insect embryos have been classified as intermediate- and short germ embryos, in which posterior segments are thought to be generated sequentially from an uncommitted growth zone, or as long-germ embryos, such as Drosophila melanogaster, which develop primordia for all segments simultaneously. In Drosophila the coordinated activities among a three-tiered cascade of zygotic segmentation genes subdivide the embryo into progressively smaller units along the anterior-posterior axis. The mode of pattern specification in lepidopteran embryos has not been determined, although on morphological grounds they have been characterized as intermediate-germ insects. We have cloned orthologues of Drosophila segmentation genes from the tobacco hawkmoth Manduca sexta and have found that the blastoderm expression patterns of these genes show a molecular prepatterning typical of Drosophila. Thus, successive segment formation in Manduca embryos may not be due to sequential addition but rather may be the consequence of a lateral compression of the embryo proceeding in an anterior-to-posterior progression. These data challenge the view that the classification of insect development according to morphological criteria can serve as a reliable indicator of the molecular mechanisms underlying segmentation.