SMOOTHENED-MEDIATED HEDGEHOG SIGNALING IS REQUIRED FOR THE MAINTENANCE OF THE ANTERIOR-POSTERIOR LINEAGE RESTRICTION IN THE DEVELOPING WINGOF DROSOPHILA

It is thought that the posterior expression of the 'selector' genes en grailed and invected control the subdivision of the growing wing imagi nal disc of Drosophila into anterior and posterior lineage compartment s, At present, the cellular mechanisms by which separate lineage compa rtments are maintained are not known, Most models have assumed that th e presence or absence of selector gene expression autonomously drives the expression of compartment-specific adhesion or recognition molecul es that inhibit intermixing between compartments, However, our present understanding of Hedgehog signalling from posterior to anterior cells raises some interesting alternative models based on a cell's response to signalling, We show here that anterior cells that lack smoothened, and thus the ability to receive the Hedgehog signal, no longer obey a lineage restriction in the normal position of the anterior-posterior boundary. Rather these clones extend into anatomically posterior terri tory, without any changes in engrailed/invected gene expression, We ha ve also examined clones lacking both en and inv; these too show comple x behaviors near the normal site of the compartment boundary, and do n ot always cross entirely into anatomically anterior territory, Our res ults suggest that compartmentalization is a complex process involving intercompartmental signalling; models based on changes in affinity or growth will be discussed.