At stage 11, just before tracheal branching begins, bnl expression appears in five small clusters of epidermal cells arrayed around the tracheal sac, at positions where the five primary tracheal branches will soon bud. Two more cell clusters begin expressing bnl as expression in the other clusters turns off, presaging the subseuent outgrowth of specific branches. For example, the ganglionic branch, which initially grows toward one cluster, continues toward a cluster that expresses bnl later, finally reaching the central nervous system. A branch of bnl -expressing cells in the head marks the path of the pharyngeal branch. Localized ectopic expression of bnl induces branching toward the ectopic source, while general overexpression disrupts the normal pattern of branching, with fine branches growing out in random directions.
Secondary branches sprout from the ends of primary branches and from a few internal positions.
Effects of Mutation or Deletion branchless was identified as a P transposon-induced mutation that reduces or eliminates tracheal branching, just like breathless mutation. Tracheal cells invaginate and form tracheal sacs normally in branchless mutants, but branches fail to grow out. The development of the posterior spiracles of Drosophila may serve as a model to link patterning genes and morphogenesis. Abdominal-B subdivides the primordia of the posterior spiracles into two cell populations that develop using two different morphogenetic mechanisms. The inner cells that give rise to the spiracular chamber invaginate by elongating into 'bottle-shaped' cells.