Commissure formation in the embryonic CNS of Drosophila I. Identification of the required gene functions

In the ventral nerve cord of Drosophila most axons are organized in a simpl e, ladder-like pattern. Two segmental commissures connect the hemisegments along the mediolateral and two longitudinal connectives connect individual neuromeres along the anterior-posterior axis. Cells located at the midline of the developing CNS first guide commissural growth cones toward and acros s the midline, In later stages, midline glial cells are required to separat e anterior and posterior commissures into distinct axon bundles. To unravel the genes underlying the formation of axon pattern in the embryonic ventra l nerve cord, we conducted a saturating ethylmethane sulfonate mutagenesis, screening for mutations which disrupt this process. Subsequent genetic and phenotypic analyses support a sequential model of axon pattern formation i n the embryonic ventral nerve cord. Specification of midline cell lineages is brought about by the action of segment polarity genes. Five genes are ne cessary for the establishment of the commissures. In addition to commissure less, the netrin genes, and the netrin receptor encoded by the frazzled gen e, two gene functions are required for the initial formation of commissural tracts. Over 20 genes appear to be required for correct development of the midline glial cells which are necessary for the formation of distinct segm ental commissures, (C) 1999 Academic Press.