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The spatial organization of Drosophila melanogaster epidermal structures in embryos and adults constitutes a classic model system for understanding how the two dimensional arrangement of particular cell types is generated. For example, the legs of the Drosophila melanogaster adult are covered with bristles, which in most segments are arranged in longitudinal rows. Here we elucidate the key roles of two regulatory genes, hairy and achaete, in setting up this periodic bristle pattern. We show that achaete is expressed during pupal leg development in a dynamic pattern which changes, by approximately 6 hours after puparium formation, into narrow longitudinal stripes of 3-4 cells in width, each of which represents a field of cells (proneural field) from which bristle precursor cells are selected. This pattern of gene expression foreshadows the adult bristle pattern and is established in part through the function of the hairy gene, which also functions in patterning other adult sense organs. In pupal legs, hairy is expressed in four longitudinal 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 expression. This misexpression of achaete, in turn, alters the fields of potential bristle precursor cells which leads to the misalignment of bristle rows in the adult. This function of hairy in patterning achaete expression is distinct from that in the wing in which hairy suppresses late expression of achaete but has no effect on the initial patterning 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 zones (this study) and a local regulation (inferred from other studies on the selection of neural precursor cells) that involves refinement steps which may control the alignment and spacing of bristle cells within these zones.

The legs and wings of insects and vertebrates develop from secondary embryonic fields that arise after the primary body axes have been established. In order to understand how the insect imaginal wing field is patterned, we have examined in detail the temporal and spatial expression patterns of, and epistatic relationships between, four key regulatory genes that are specifically required for wing formation in Drosophila. The wingless protein, in a role surprisingly distinct from its embryonic segment polarity function, appears to be the earliest-acting member of the hierarchy and crucial for distinguishing the notum/wing subfields, and for the compartmentalization of the dorsal and ventral wing surfaces. The wingless product is required to restrict the expression of the apterous gene to dorsal cells and to promote the expression of the vestigial and scalloped genes that demarcate the wing primordia and act in concert to promote morphogenesis.

We describe a direct way of measuring contact inhibition of locomotion by analysing the changes in motion of pairs of colliding cells. This allows values to be assigned to each type of cell in mixed collisions and will enable certain hypotheses about the relationship between contact inhibition and invasion in culture to be tested critically. We find that fibrosarcoma (FS9) cells, on colliding with chick heart fibroblasts, show a reversed contact-inhibition response that we call contact promotion of locomotion. We also describe a measure of the lateral changes in motion that result from collisions between cells and show that this is dependent on the type of colliding cell but, unlike contact inhibition, it does not appear to be dependent on the type of cell with which it collides for the types studied here. Finally, we analyse how the total response is dependent on the dispositions and motions of the cells before collision and we find that FS9 cells, on colliding with fibroblasts, tend to turn towards the point of initial marginal contact. We conclude that the FS9 cells show a pronounced response on colliding with the fibroblasts, which is in contrast to the subjective impression that the FS9 cells do not respond much. These findings support the thesis of Abercrombie and colleagues, that the infiltration of a population of normal cells by a population of invasive cells in culture is dependent on the nature of the response of each cell type to collision with the other and that the invasive cells fail to show contact inhibition in these heterotypic collisions; but the findings further suggest that these invasive cells show an active invasionary response as opposed to merely failing to show contact inhibition.

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