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

## TL;DR

This study reveals that tachykinin-positive neurons in the central complex of insects show significant evolutionary divergence, even among closely related species.

## Contribution

The paper identifies the cellular identity and evolutionary divergence of tachykinin-positive neurons in the central complex across hexapods.

## Key findings

- LomTK-related peptides are present in the central complex of most studied hexapod species except crickets.
- The types of immunolabeled columnar neurons vary significantly between species from different orders and even within the same order.
- This divergence may be linked to species-specific navigational requirements.

## Abstract

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.

The central complex is a group of midline‐spanning neuropils in the brain of insects with a key role in goal‐directed orientation and navigation. Immunolabeling in 25 species, ranging from bristletails to flies, shows that neurons containing peptides of the tachykinin family of neuropeptides are present in most species studied. Often, sets of columnar neurons show immunostaining, but the types of labeled neurons differ considerably, possibly related to species‐specific differences in navigational requirements.