CRISPR/Cas9 ants — designed to lose their ability to use pheromones to communicate

Gene-editing using the new CRISPR/Cas9 technology will be useful for genetic engineering of agricultural animals, farm crops, creation of lab animal knockout lines, AND (eventually) clinical medicine (to remove deleterious Mendelian-trait disorders early in utero). These two papers extend the usefulness of CRISPR/Cas9 techology to insects. Ants live in complex societies and display sophisticated social behavior –– including reproductive division of labor between queens and workers, behavioral division of labor among nurses, soldiers and foragers, formation of adaptive foraging networks, nestmate versus non-nestmate discrimination, and collective nest construction. All of these behaviors are largely mediated via chemical communication using a wide range of pheromones.

In the fruit fly, pheromone receptors have been identified that belong to multiple insect chemosensory receptor families, including odorant receptors (ORs), gustatory receptors (GRs), ionotropic receptors (IRs), and pickpocket channels (PPKs). Ants also have numbers of GRs, IRs, and PPKs that are typical for insects, while their OR repertoire is greatly expanded. This raises the possibility that expansion of ORs specifically, rather than chemoreceptors in general, may underlie the evolution of complex chemical communication in ants. Ants also have exceedingly large numbers of glomeruli in their antennal lobes, which likely reflect their expanded OR gene repertoire.

Insect ORs function as chemosensory receptors by dimerizing with the highly-conserved co-receptor protein ORCO to form ligand-gated ion channels. Orco-null mutants in fruit flies, locusts, mosquitoes, and moths therefore lose OR function and show impaired responses to odorants such as food volatiles and sex pheromones. Thus, ORCO constitutes a prime candidate to test the hypothesis that the expanded OR repertoire of ants is required for chemical communication.

In two back-to-back publications [see attached] authors at Rockefeller University and University of Pennsylvania independently generated mutant ant lines, by disrupting Orco, the pivotal gene required for the function of all ORs. The Orco knockout exhibits severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike that seen in the fruit fly, Orco(–/–) mutant ants also lack most of the ~500 antennal lobe glomeruli found in wild-type ants. The development of genetics in these CRISPR/Cas9 ant lines serves as a model organism to study the complexity of eusociality. These independent studies also illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function might be required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain.

Cell 2o17; 170: 727–735 & 736–747

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