The development of the bristles in normal and some mutant types ofDrosophila melanogaster

Abstract

This paper describes the development of the normal macro- and micro-chaetae ofDrosophila, together with that of twelve mutant types. The phenotypes of twenty combinations of these genes have been studied. Each normal bristle is secreted by a single cell, the trichogen, which lies beneath a tormogen cell which secretes a socket. These bristle cells are first distinguishable in the epidermis at about 15 hr. after puparium formation, when they have already divided to form a pair, and are slightly larger than the normal epidermal cells. The secretion of the bristle proceeds most rapidly between 30 and 55 hr., during which time the bristle cells are very large and obviously highly polyploid. The socket, apparently, does not completely enclose the base of the bristle in the earliest stages. The development of the microchaetae is essentially similar to that of the macrochaetae. The actions of the twelve genes can be summarized as follows:Scutecauses a primary absence of certain bristle cells, andextra-bristle-complex-41eandhairythe presence of supernumerary groups.Splitfrequently causes an extra division, so that a group of four cells is formed; these may be arranged as two trichogens and two tormogens, or one trichogen and three tormogens; or the whole group may fail to reach the surface of the epithelium, when no bristle or socket is formed.Dichaetemay produce an effect similar to the last-described ofsplit, and it may also cause an extra division of the trichogen, producing a double bristle in a single socket.Hairlesscauses the trichogens of some bristle groups to lie level with the tormogens, and to develop like them into sockets. InStubblethe tormogens are shifted rather to one side of the trichogens, so that the bristle is less closely invested by the socket, and becomes thicker and shorter. Inshaven-nakedthe trichogen is irregularly displaced, becoming more or less converted into a tormogen; the small bristle which may be secreted is often peculiarly fanned out at the tip, suggesting an effect of the gene on the nature of the material secreted.Spinelessandmorulaslow down the growth of the bristle cells.Singed, forkedandBristleall affect the nature of the bristle secretion, there being some reason to suggest that the effects ofBristleandsingedmay be similar and different to that of forked.