Olfactory senses in the red flour beetle as a behavioral inducer!

Saptaparna Dasgupta, Bennett University

Insects are highly dependent on their olfactory senses for the translation of the happenings in their environment. In the red flour beetle Tribolium castaneum, during metamorphosis, the neural system generally experiences significant change. The integration of new neurons, and the modification and removal of larval neurons, are all part of this process.

However, in the experiment by Trebels et al., the sensory neurons are reused by the adult antennae from the larval antennae. The glomeruli begin to dissolve in the late stages of the larvae. It was concluded from the study that the formation of the antennal lobe in the glomeruli of adults is common amongst the other chosen species. On the other hand, the formation of sensory appendages or functioning of odorant receptor co-receptor differed.

Olfactory senses in insects:

Olfactory perception in insects often begins at the chemosensory sensilla of the antennae and palps. Chemosensory neurons are contained in the sensilla (CSNs). Olfactory sensory neurons (OSNs) and gustatory sensory neurons (GSNs) are two types of CSNs. The olfactory receptors, either odorant receptors (ORs) or ionotropic glutamate-like receptors (IRs), are found in the membranes of the OSNs. The majority of sensory data acquired by insect OSNs originates from a functional heteromer of particular odorant receptors (ORs) and the odorant receptor co-receptor (Orco). As a result, removing Orco usually results in the loss of most olfactory transduction.

Antennae: the sensory organ:

In Tribolium castaneum, at the time of metamorphosis, the morphology of the olfactory systems gets remodeled. In these insects, the larval stage possesses olfactory appendages, whereas the complexity of these appendages differs from insect to insect. It possesses complex antennae having three segments, known as scape, pedicle, and flagellum. These insects have serial homology amongst their antennae, which is carried on from the larval to the adult stage. The sensory neurons of the antennae were visualized throughout the pupal form until its metamorphosis into an adult beetle.

The study:

Several other insects were considered as the comparison namely, the lepidopteran M. sexta and the hymenopteran Apis mellifera. Also, the mealworm beetle Tenebrio molitor was taken for comparison. Further, the larval stage of Drosophila melanogaster and the hemimetabolous American cockroach Periplaneta americana were taken into the comparison list for the experiment. The formation of the adult glomeruli was visualized using (immune)-histochemistry.

Strong dsRNA injection was used to induce systemic RNA interference and effectively knockdown Orco before the formation of the pupa. Glomeruli link-local interneurons in the ALs to process the olfactory information received from the OSNs (LNs). The inhibitory transmitter gamma amino-butyric acid (GABA), the excitatory transmitter acetylcholine, and many neuropeptides are responsible for the LNs’ olfactory representations.

Results obtained:

The CSNs retract their dendrites during the last larval instar and the first several hours of the prepupal stage. They also migrate into the head capsule, where they are observable in later prepupal stages. At this point, the adult antenna’s gross shape is visible under the larval cuticle, with the CSNs and OSNs indicated at its tip. For OSNs in prepupae, we were unable to create volume rendering images because the partial Orco-Gal4xUAS-DsRed labeled fewer neurons, resulting in a considerably lower signal. It was revealed that a functioning OR/Orco complex is not required for the formation of adult antennal lobe glomeruli. However, it is required for their maturation during the postmetamorphic period.

Discussion and Important observations:

Tribolium castaneum is a red flour beetle with olfactory systems. Important observations include:

• There are three distinct features on the larval antennae, including a trichoid sensillum. This implies that the distal, massive trichoid sensillum has a role in chemosensation.
• The antennal imaginal discs of the red flour beetle are absent. As compared to Drosophila melanogaster, where OSNs are first detected at around 15 percent metamorphosis, this is a substantial difference.
• In the adult red flour beetle, polymorphic larval cells are used to construct the antennae. Orco activity appears to be essential for differentiation following adult eclosion, but not during the initial development of the AL glomeruli.

Data should serve to be cautious when generalizing based on results from a single insect species.

Also read: Older patients of diabetes mellitus have low creatinine levels

Reference:

1. Trebels, B., Dippel, S., Goetz, B., Graebner, M., Hofmann, C., Hofmann, F., Schmid, F.-R., Uhl, M., Vuong, M.-P., Weber, V., & Schachtner, J. (2021). Metamorphic development of the olfactory system in the red flour beetle (Tribolium castaneum, Herbst). BMC Biology, 19(1), 155. https://doi.org/10.1186/s12915-021-01055-8

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Author info:

Saptaparna Dasgupta, currently a B. Tech 3^rd year student, pursuing Biotechnology, is a diligent student and determined in terms of her career goals. Being a budding biotechnologist, she is open to all research fields of her course and passionate about knowledge. She is focused and constantly tries to improve her writing skills, also a project enthusiast and is fond of gaining hands-on experience in laboratories. She believes that all hard work and effort pays off eventually and follows this as the motto of her life.