Unexpected Evolutionary Divergence of Tachykinin-Positive Neurons Innervating the Central Complex in Hexapods

The central complex comprises an assemblage of midline-spanning neuropils in the brain of insects that play a key role in goal-directed orientation and navigation vector calculation. The central complex consists of layers of tangential input neurons that contact topographically organized columnar neurons which provide outputs to the right and left brain hemispheres. Its anatomical organization and functional role are regarded as highly conserved across insects. In addition to classical neurotransmitters, a wide range of neuropeptides have been detected in the central complex including peptides of the tachykinin family. Because the cellular identity of tachykinin-containing neurons in the central complex has not been determined in most cases, we used antisera against tachykinin I and II from the migratory locust, termed Lom-TKs, to identify the immunolabeled neurons in hexapods ranging from flightless two-pronged bristletails to flies. The data show that LomTK-related peptides are present in the central complex of all studied species except crickets. In most species one or several types of columnar neurons were immunolabeled, sometimes together with certain subsystems of tangential input neurons. The types of immunolabeled columnar neurons, however, are distinctly different between species from different orders, in some cases even between insects within the same order, and comprise cell types that innervate either the upper or lower division of the central body. This high degree of evolutionary divergence of tachykinin-positive neurons, even in closely related groups, may be related to species-specific differences in navigational requirements and calls for caution with respect to homologizing neurons across clades.