Fig. 6

Possible mechanism underlying development and evolution of diverse antennal morphologies through modification of al expression. Ancestral insects may have had simple, rod-shaped antennae without protrusions or branches, and al may not have been expressed in a segmentally reiterated fashion. Protrusions or branch structures may have been acquired by the two-step change in al expression pattern. The first step is acquisition of segmentally reiterated al expression and its ventral restriction, as well as the olfactory epithelium with densely packed sensory organs, before the divergence of Obtectomera. This change in al expression, however, may not have changed antennal morphology. The second step is diversification of al expression pattern within each antennal segment, which may have led to the control of cell proliferation in specific regions and the development of protrusions or lateral branches after Obtectomera subclade had diverged. Arrowheads and dashed lines indicate the ventral midline and the position of cross-section, respectively. Red, the al expression domain; dotted region, the olfactory epithelium. See text for details