Expression of rho (A-E), pnt (F-O), h (P-Q), or salm (R-T) in the airways before or after invagination. (A–E) rho expression in the trachea. At stage 10 (A), the dorso-central part of each primordium expresses rho (arrowheads). By stage 11 (B,C), when the tracheal cells invaginate, rho expression expands to cover more cells of the central/distal areas (arrowheads). At late stage 11 (E,F), rho expression in the dorsal part of the distal trachea becomes weak (D, arrowheads), while the ventral part of the distal trachea still retains strong rho expression (E, arrowheads). (F–O) pnt expression in the trachea. At early stage 11 (F), pnt is expressed in the dorso-central part of each primordium (arrowheads), which expands to the whole cental/distal areas (G-U). The proximal area expresses less pnt (H, arrowheads) than the distal area (I, arrowheads) as also evident in a section from a ventral view (J, arrowheads). At stage 11, compared to the wild type (K), pnt expression in the primordia is barely detectable in rho mutants (L, arrowheads). In the wild type (M), pnt-lacZ is expressed in the distal trachea, with its expression upregulated in a pan-tip pattern due to the Bnl/dFGF-dependent activation of Btl/dFGFR (arrowheads). In btl mutants (N,O) where this upregulation is absent, pnt-lacZ is expressed in the D-cells (arrowheads in Tr7) but is absent in the P-cells (arrows in Tr7). (P,Q) h expression in the trachea. At stage 10/11 (P), h is expressed in the dorso-central part of each primordium (arrowheads). At stage 13 (Q), expression of h-lacZ is detected in DB, DT, and VB (arroheads in Tr5), whereas TC and SB does not express it (arrows in Tr5). Note that the earlier pair-rule pattern of h-lacZ expression is also evident (asterisks). (R–T) salm expression in the trachea. At stage 10/11 (R), salm is expressed in the dorsal part of each primordium (arrowheads). At stage 13 (S,T), salm-gal4 driven UAS-nGFP labels DB, DT (arrowheads in Tr7), and part of TC and SB (arrows in Tr7). (U,V) Requirements of trh for the P/D fate selection. In trh mutants, expression of the P-fate markers upd (U, stage 13) and P0144-lacZ (V, stage 14/15) and the D-fate marker mab2A12 (V) is abolished (asterisks). Scale bar: 50 μm. DB, dorsal branch; DT, dorsal trunk; SB, spiracular branch; TC, transverse connectives; VB, visceral branch.