Ancient Protostome Origin of Chemosensory Ionotropic Glutamate Receptors and the Evolution of Insect Taste and Olfaction

A mosquito lands on your arm. Somewhere on its antenna, a cluster of neurons fires — it has found you by smell. The proteins responsible for that detection aren't unique to mosquitoes or even to insects. They are ancient, shared across worms, snails, and crustaceans, and they are built on the same molecular scaffold your brain uses right now to form memories. Understanding where they came from and how they work might be the key to stopping the mosquito from finding you in the first place. To understand why that's remarkable, you need to know what ionotropic glutamate receptors are. These are a conserved family of ligand-gated ion channels — proteins that sit in the cell membrane, bind a chemical, and respond by opening a pore that allows ions to flood through, changing the cell's electrical state. In animals, they are best known as synaptic proteins: three subfamilies, AMPA, kainate, and NMDA receptors, handle the vast majority of fast excitatory communication in the brain. NMDA receptors in particular are central to synaptic plasticity — the cellular basis of learning. Their architecture is characteristic: a "Venus flytrap" ligand-binding domain that clamps shut around glutamate, two transmembrane segments plus a pore loop, and a cytosolic tail. They are also ancient — bacterial relatives exist in organisms like Synechocystis, and the plant Arabidopsis encodes twenty related genes. So when Croset and colleagues discovered a variant subfamily of these same proteins sitting in the olfactory neurons of Drosophila melanogaster, the question was immediate: what is a synaptic ion channel doing in an insect's nose? These proteins, which the authors named Ionotropic Receptors or IRs, share the core ion channel architecture of classical ionotropic glutamate receptors — the conserved C-terminal region and the transmembrane topology — but differ in critical ways. Most IRs have highly divergent ligand-binding domains that lack the residues that contact glutamate in canonical receptors. Most lack the amino-terminal domain found in classical synaptic receptors. Two exceptions, IR8a and IR25a, are the most ancestral-looking members. IR25a in particular groups phylogenetically with non-NMDA receptors and shares conserved intron positions with them, suggesting the IR family descended from a non-NMDA ancestor. The experimental evidence linking IRs to olfaction in Drosophila is direct. IRs are expressed in antennal sensory neurons that don't express Odorant Receptors or Gustatory Receptors. IR proteins localize to ciliated sensory endings — the business end of olfactory neurons — rather than to synapses. And misexpressing an IR in the wrong neurons confers novel odor-evoked responses. These are not synaptic proteins moonlighting in a surprising place. They are genuine chemosensory receptors, built on an ion-channel frame. The deeper question was evolutionary: how old is this family, and how far does it extend beyond Drosophila? To find out, Croset and colleagues did something ambitious. They built a Hidden Markov Model from the conserved region of ionotropic glutamate receptor proteins and screened thirty-two eukaryotic genomes and nine hundred and seventy-one prokaryotic genomes. What they found reshaped the story entirely. IRs are present throughout Protostomia — the major branch of the animal kingdom that includes arthropods, nematodes, and molluscs — but absent in deuterostomes, cnidarians, and placozoans. In species after species, these receptors are expressed in olfactory organs. The Aplysia IR25a orthologue, in the sea slug, is expressed in the rhinophore and oral tentacle — sensory structures for detecting chemicals in seawater. The lobster OET-07 gene, an IR25a relative, is specifically expressed in mature olfactory sensory neurons. In the honeybee, reverse transcription polymerase chain reaction amplifies six putative antennal IR orthologues from antennal RNA but not from brain. The pattern is consistent across hundreds of millions of years of evolution. This is where the contrast with Odorant Receptors becomes decisive. Odorant Receptors — the other major olfactory receptor family in insects — are insect-specific. They evolved later, probably from Gustatory Receptor precursors, and appear only in terrestrial insects. IRs, by contrast, predate insects entirely. They represent the ancestral chemosensory solution for Protostomia, present before the lineages that would become flies, worms, and snails had diverged. The phylogenetic analysis distinguishes two subfamilies with very different characters. The antennal IRs are conserved — present across insect orders, slow to change, and expressed in olfactory organs. Across twelve drosophilid species, antennal IRs show a mean amino-acid identity of seventy-six percent, compared to eighty-nine percent for classical ionotropic glutamate receptors. Their median rate of nonsynonymous to synonymous substitutions — the dN/dS ratio, a measure of selective pressure — is 0.11. That's constrained. Evolution is not tolerating much change. The divergent IRs are the opposite. Species-specific, highly expanded in some lineages, with pairwise sequence identities as low as eight and a half percent and a median dN/dS of 0.15. Their expression is different too. Promoter-reporter assays in Drosophila show divergent IRs — IR7a, IR11a, IR100a — expressed not in olfactory neurons but in gustatory neurons: selective cells in the adult labellum, the mouthparts, and in internal pharyngeal sense organs. The same reporters light up gustatory neurons in larvae. Many of these cells co-express IR25a, suggesting the ancient, conserved receptor acts as a broadly shared co-receptor pairing with rapidly evolving, ligand-specific subunits. So the same family handles both smell and taste. One branch conserved across six hundred million years of evolution; the other diversifying fast to match each species' dietary niche. The mechanism of that diversification is written into gene structure. Antennal IRs and classical ionotropic glutamate receptors contain multiple introns — four to fifteen per gene. Most divergent IRs are intronless, single-exon genes. The few introns that remain in divergent IRs are strongly biased toward the front half of the gene — nineteen of twenty-five introns fall in the first fifty percent of sequence. That's the signature of retroposition: an RNA transcript is reverse-transcribed, and the resulting complementary DNA is reinserted into the genome, stripped of its introns. Subsequent duplications by non-allelic homologous recombination then proliferate the copies. Croset and colleagues estimate sixteen gene-gain events and seventy-six gene-loss events across the drosophilid tree, with a death rate five times higher than the birth rate. On the specialist branch leading to Drosophila sechellia, which feeds almost exclusively on Morinda fruit, gene loss accelerates — including pseudogenization of the antennal receptor IR75a. Repertoire change tracks ecological specialization. The evolutionary model the paper constructs is clean. An ancestral ionotropic glutamate receptor-like gene in the common protostome ancestor — somewhere between five hundred and fifty and eight hundred and fifty million years ago — was co-opted into peripheral chemosensation. IR25a, the most ancient-looking member, with a dN/dS of just 0.028, has been conserved since. The intron-rich antennal IRs represent the original scaffold. The intronless divergent IRs are later expansions through retroposition, diversifying into gustatory roles and tracking each species' ecological context. Odorant Receptors arrived much later, in terrestrial insects, as a specialized overlay on top of this older architecture. That architecture has a direct practical consequence. Croset and colleagues recovered IR repertoires in Aedes aegypti, Culex quinquefasciatus, and Anopheles gambiae — three mosquito species collectively responsible for transmitting malaria, dengue, and other diseases to millions of people. Several antennal IRs respond to ligands like carboxylic acids, water, and ammonia — signals these insects use to find hosts. Because antennal IRs are conserved across insects, the paper proposes them as targets for broad-spectrum chemical regulators: compounds that could disrupt host-finding or feeding behavior across multiple pest species at once. But zoom out further and the finding carries a different kind of weight. The proteins your brain uses for learning and memory — the same ion channel architecture, the same Venus flytrap binding mechanism — were repurposed, hundreds of millions of years ago, into the molecular machinery that lets animals smell the world. The two functions are not analogous. They are built from the same parts. That's not a metaphor for how biology reuses solutions. It's the literal evolutionary history, preserved in gene structure, expression patterns, and sequence conservation across half a billion years of animal life. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.