Avani Dave, Jai Hind College
On average, more than one-half of all the existing insect species are considered to be herbivorous, meaning, these insects feed on plants to derive nutrition, making them an essential contributor to the food chain. Despite their critical herbivorous nature, the terrestrial domain of our world is flourishing from the mossy shores dispersed around. It is difficult to comprehend how the world is vegetatively rich while simultaneously being abundantly occupied by these plant-feeding insects.
What helps in striking the most efficient balance between their existence? How can we account for the ecological success of the vegetation on one hand and the insects on the other? These questions can be partially answered by taking into consideration the factors that control the herbivorous insect populations. The regulating forces act both ways, from the plants (toxins) and the environment (predators and parasites) against the insects that are categorically herbivores.
The existence of secondary metabolites in plants has been known to mankind for centuries, these secondary metabolites do not cater to the basic needs of survival of the plants but instead helps in defence. These toxins help in protecting the plants from the potential attack by predators. However, this defence mechanism comes with its own drawbacks, one being the self-harming effect of some broad-spectrum toxins, for e.g. Isothiocyanates (ITCs) formed by members of Brassicales. ITCs are considered to be insecticides, they form covalent bonds with amino acid residues, namely cysteine and lysine allowing their binding to DNA. These ITCs occur as a result of hydrolysis of glucosinolate, which takes place in a damaged plant cell. The restricted occurrence of glucosinolates in selected plants makes its identification by the herbivorous insects more specific. The insects that specialize in such glucosinolates containing plants, i.e. pierid butterflies show a dependency for egg-laying and feeding, both of which are central requirements for the survival of these insects. Hence, there has to be a way for these butterflies to be able to divert the production of ITCs which could potentially lead to insect mortality.
Expression of GRs to sinigrin
It is known that the gustatory receptors (GRs) in insects are located in the form of dendrites present in the sensory hair throughout the body. Several GRs specifically exist for the identification of the secondary metabolite produced by the host plants. A study conducted by Yang J. et al. demonstrated a gustatory receptor (named PrapGR28) in Pieris rapae butterflies that successfully recognized glucosinolates. This GR is present in the tarsi of both males and females as well as the larvae. PrapGR28 demonstrated high expression and specific response to sinigrin, a glucosinolate present in the Brassicales plants. Although during the initial stages of this study, two different sets of GRs were hypothesized to show high expression (PrapGR15 and PrapGR28), only PrapGR28 was considered for future analysis due to specific expression to sinigrin. The study also included knock-in and knock-out experiments conducted in-vivo, to articulate the role of PrapGR28 in bringing about variations in the behavioural choices towards sinigrin. Loss of PrapGR28 also demonstrated a decrease in sinigrin sensitivity.
Prospects of identifying the GRs
The detection of various other gustatory receptors of the herbivorous insects that efficiently respond to compounds produced by the host plants will lead to a detailed understanding of the underlying neuroethology and sensory ecology. There is still a lot to be discovered in this arena of molecular mechanisms of the GRs and how they help the possessor to either get attracted or averted towards their host plants counterparts. Structured studies that incorporate executed components of electrophysiological, transcriptomic, and functional genetics, will be successful in taking us closer to unravelling the gustatory receptors in the planet’s most supreme animal consortium: voracious herbivores.
Also read: The Art of Dreaming, According to Science
Source: Whiteman, N. K., & Peláez, J. N. (2021). Taste-testing tarsi: Gustatory receptors for glucosinolates in cabbage butterflies. PLOS Genetics, 17(7), e1009616. https://doi.org/10.1371/journal.pgen.1009616
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About the Author: Avani Dave is currently in the final year of her bachelor’s degree, majoring in Life Sciences. Holding a good academic and extra-curricular record, she is on a constant journey of acquiring exposure in her field of interest while simultaneously not limiting herself to just that. Avani likes studying Diseases and Syndromes and everything under this umbrella! That being said, she is adept at working across departments and promises to deliver.
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