polyhomeotic: a gene required for the embryonic development of axon pathways in the central nervous system of Drosophila

Hypomorphic alleles of the locus polyhomeotic (ph) produce multiple, homeotic-like transformations in adult flies that mimic dominant mutations in the Antennapedia and Bithorax complexes. Analysis of null alleles of ph has revealed a complex, embryonically lethal phenotype that includes cell death of the ventral epidermis and abnormalities in the patterns of expression of homeotic and segmentation genes. There is also a dramatic alteration in the pattern of axon pathways in the central nervous system, such that the wild-type array of segmentally repeated commissures and connectives is replaced by bundles of axons confined to the hemiganglia of origin. It is possible that this axonal phenotype is the result of loss of neuronal identity caused by abnormal homeotic and segmentation gene expression.