Variations on a Theme: Antennal Lobe Architecture across Coleoptera

Abstract

Beetles comprise about 400,000 described species, nearly one third of all known animal species. The enormous success of the order Coleoptera is reflected by a rich diversity of lifestyles, behaviors, morphological, and physiological adaptions. All these evolutionary adaptions that have been driven by a variety of parameters over the last about 300 million years, make the Coleoptera an ideal field to study the evolution of the brain on the interface between the basic bauplan of the insect brain and the adaptions that occurred. In the current study we concentrated on the paired antennal lobes (AL), the part of the brain that is typically responsible for the first processing of olfactory information collected from olfactory sensilla on antenna and mouthparts. We analyzed 63 beetle species from 22 different families and thus provide an extensive comparison of principal neuroarchitecture of the AL. On the examined anatomical level, we found a broad diversity including AL containing a wide range of glomeruli numbers reaching from 50 to 150 glomeruli and several species with numerous small glomeruli, resembling the microglomerular design described in acridid grasshoppers and diving beetles, and substructures within the glomeruli that have to date only been described for the small hive beetle, Aethina tumida. A first comparison of the various anatomical features of the AL with available descriptions of lifestyle and behaviors did so far not reveal useful correlations. In summary, the current study provides a solid basis for further studies to unravel mechanisms that are basic to evolutionary adaptions of the insect olfactory system.

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

“…The association of microglomeruli with olfactory lobes has also been described for the accessory lobes of decapod crustaceans [190,223] and the antennal lobes of distinct insect groups, including orthopterans and coleopterans [214,227,228]. The accessory lobes of decapods are second-order association centres located in the deutocerebrum and always occur in close proximity to the olfactory lobes.…”

“…Amongst others, these higher-order neuropils receive olfactory information from the olfactory lobes via interneurons [230,231]. Similarly, microglomeruli are present in the antennal lobes of two distantly related representatives of coleopterans, the diving beetles and lady bugs [228]. Several coleopteran sub-groups also show a microglomerular compartmentation of glomeruli, as revealed by either immunolabelling for synapsin and/ or tachykinin-related peptides [228].…”

The Sensory Machinery of the Head Louse Pediculus humanus capitis: From the Antennae to the Brain

“…About 22 glomeruli were identified in the ALs of the kissing bug Rhodnius prolixus (Barrozo et al., 2009) and about 50–60 in the mosquitoes Aedes aegypti and Anopheles gambiae (Ignell et al., 2005; Ghaninia et al., 2007). Taking into consideration the multiple studies on insects of different orders, the less developed ALs are most likely to be a convergent adaptation to a similar lifestyle and specific ecological and ethological requirements rather than an intrinsic feature of a given taxon (Kollman et al., 2016). The possession of few or non-defined AL glomeruli would not be direct indicators of absence or poor sensitivity to host-related cues.…”

An atlas of the developing Tribolium castaneum brain reveals conserved anatomy and divergent timing to Drosophila melanogaster

“…The immunogen is a fusion protein made of glutathione S-transferase and parts of the D. melanogaster Synapsin (SYNORF1, Klagges et al, 1996). It has been used in many insect species, including T. castaneum (Dreyer et al, 2010) and other beetles (Immonen et al, 2017;Kollmann et al, 2016), to determine neuropil shape and size as well as general brain anatomy. Its specificity was previously demonstrated for D. melanogaster (Godenschwege et al, 2004;Klagges et al, 1996) and the staining in T. castaneum reflects that pattern.…”

“…Standard neuropils have been mapped in T. castaneum (Dreyer et al, 2010;Koniszewski et al, 2016) and, in another Coleopteran, the dung beetle, the compartments and major fascicles of the adult brain have been determined, which in part may be generalized along Coleopterans (Immonen et al, 2017). In addition, there is a database documenting the volumes of the four brain neuropils in adult Coleopterans (Kollmann et al, 2016). However, very little data exist on the neuropils and microcircuitry of Coleopteran developmental stages (see Farnworth et al, 2020 andWegerhoff &Breidbach, 1992 for details on the larval central complex in Coleoptera, as well as Koniszewski et al, 2016 for a general map of the first larval stage brain in T. castaneum).…”