Comprehensive information regarding the bristle-containing sensory organs can be found in the excellent multimedia resources of Lewis Held, on the website of the Society of Developmental Biology: http://www.sdbonline.

org/fly/lewheld/00idheld. htm, section ‘the bristle’.

Note that in this example Notch acts predominantly by inhibiting neural differentiation, but this

is not always the case. In mouse embryonic stem cells it acts to promote neural fates, a finding that underscores yet again the importance of cellular context in

determining the outcome of signalling.62

Scute: from scutum, the Roman legionary’s shield; knee-pan; scale; piece of bony armour in crocodile, sturgeon, turtle, armadillo, etc.

Scute is a sex-linked gene which controls a stage in the development of a set of bristles, the most characteristic of which are the four on the scutellum (see Figure 22.1), which can be used as an index of activity of the scute locus. The number of bristles on the scutellum is what is meant by the ‘scute character’. When scute mutates, the change in phenotype is extensive and clear-cut. Normally there are four bristles on the scutellum, the number dropping to 0.5 (on average) in males and 1.5 in females in Sc1 mutants.60

Achaete: The bristles of the mechanoreceptors are named chaetes. In Drosophila there are macrochaetes and microchaetes (see above), present on the edges of the wings and on the dorsal site (notum) of its thorax. Loss of chaetes is referred to as ‘achaete’.

Notch and the development of the bristle-containing sensory organ

Notch acts at two levels in the development of bristle-containing sensory organs. The first is in the selection of a single sensory precursor cell. Initially all cells of the proneural clusters express both Notch and Delta, but just one cell wins. It possesses a more ‘effective’ Delta that activates the Notch pathway of its neighbours and, through induction of E(spl), blocks expression of chaete and scute, thereby shutting down their neurogenic programme (Figure 22.12a). Once the sensory organ progenitor cell has been established, Notch determines the subsequent cell lineage decisions so as to determine the destination of progeny, turning them into a bristle (shaft), socket, glia, neural, or sheath cells (Figure 22.12b). The first (asymmetric) division of the sensory precursor, leading to the progenitor cells, pIIa and pIIb, has been thoroughly investigated.

In the prophase of cell division, just before the condensed chromosomes are aligned by the microtubule cytoskeleton, three proteins of the sensory organ precursor accumulate on one side of the cell. These are Neuralized (E3–ubiquitin ligase), Numb (adaptor), and the AP-2 complex. The daughter cell possessing the higher content of these three proteins loses its ability to receive signals,

but has an abundant capacity to transmit, because of very effective trafficking of Delta.49 Like epsin (see above), Numb is an almost linear (non-globular) protein. It has an N-terminal PTB domain which does not necessarily require a phosphotyrosine in order to bind to other proteins. Numb binds Notch

and it binds Sanpodo. Its tail attaches to the ear segment of -adaptin (AP-2 complex) (Figure 22.13). As a consequence, the daughter cell that expresses Numb loses cell surface expression of Sanpodo, which now concentrates in early endosomal vesicles due to selective uptake by Numb/AP-2.61 This deprives Notch of Sanpodo and impairs its receptor function. Moreover, Numb binds