Embryogenesis in a Drosophila mutant expressing half the normal segment number.

Insect segments are seriated developmental subunits whose borders largely control intra-segmental patterning. The individual segment owes its specific character to position-dependent instructions given to its early embryonic progenitor cells. In Drosophila, the segment borders first become visible as a series of transverse infoldings on the surface of the germ band. We describe here development in a mutant (prdFR1) which forms half the normal number of transverse infoldings spaced at twice the normal distance, thereby subdividing the germ band into a series of giant domains (Fig. 1 a-c). Each domain develops the tracheal pits of two normal segments (Fig. 1e), thus revealing its composite origin, but the larva expresses only one segment border per domain and the cuticle between these borders is similar to the normal segment in both size and zonation (Fig. 2). We conclude that this course of development reflects a conflict between the seriated instructions specifying local segment character (see Fig. 1b) and controlling influences exerted by the abnormally spaced infoldings and later by the definitive segment borders.