THE SPATIAL-ORGANIZATION OF EPIDERMAL STRUCTURES - HAIRY ESTABLISHES THE GEOMETRICAL PATTERN OF DROSOPHILA LEG BRISTLES BY DELIMITING THE DOMAINS OF ACHAETE EXPRESSION

The spatial organization of Drosophila melanogaster epidermal structur es in embryos and adults constitutes a classic model system for unders tanding how the two dimensional arrangement of particular cell types i s generated. For example, the legs of the Drosophila melanogaster adul t are covered with bristles, which in most segments are arranged in lo ngitudinal rows. Here we elucidate the kev roles of two regulatory gen es. hairy and achaete, in setting up this periodic bristle pattern. We show that achaete is expressed during pupal leg development in a dyna mic pattern which changes, by approximately 6 hours after puparium for mation, into narrow longitudinal stripes of 3-4 cells in width, each o f which represents a field of cells (proneural field) from which brist le precursor cells are selected. This pattern of gene expression fores hadows the adult bristle pattern and is established in part through th e function of the hairy gene, which also functions in patterning other adult sense organs. In pupal legs, hairy is expressed in four longitu dinal stripes, located between every other pair of achaete stripes. We show that in the absence of hairy function achaete expression expands into the interstripe regions that normally express hairy, fusing the two achaete stripes and resulting in extra-wide stripes of achaete exp ression. This misexpression of achaete, in turn, alters the fields of potential bristle precursor cells which leads to the misalignment of b ristle rows in the adult. This function of hairy in patterning achaete expression is distinct from that in the wing in which hairy suppresse s late expression of achaete but has no effect on the initial patterni ng of achaete expression. Thus, the leg bristle pattern is apparently regulated at two levels: a global regulation of the hairy and achaete expression patterns which partitions the leg epidermis into striped zo nes (this study) and a local regulation (inferred from other studies o n the selection of neural precursor cells) that involves refinement st eps which may control the alignment and spacing of bristle cells withi n these zones.