To most ants, smell and taste are everything. But some parasitic ant species have lost the genes that drive these senses – a sensory shake-up that may be due to the way they outsource some tasks to host species.
Interpreting subtle chemical cues through smell and taste help ants hold their societies together. This “chemoreception” is involved in everything from group foraging to recognising nest mates.
But not all species of ant live or interact with each other in the same way. Some ants are social parasites that raid the nests of other, closely related ant species, steal their workers and eventually become dependent upon their captives’ labour.
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To unveil the evolutionary impacts of this parasitic lifestyle, at the University of Münster in Germany and his colleagues delved into the insects’ genomes – their full genetic instructions. The researchers analysed the genomes of eight ant species: three parasites, their three host species and two non-host species.
The team found that the parasitic ants – Harpagoxenus sublaevis, Temnothorax ravouxi and Temnothorax americanus – had half as many taste receptor genes compared with their hosts and the non-host ants. The parasites had also lost about a quarter of their olfactory genes, which are involved in smell.
at Middlebury College in Vermont, who wasn’t involved with the work, says he expected some loss of these receptor genes, but not to the degree revealed in the study. In general, olfactory genes are massively multiplied in ants compared with other insects, implying that they are important for ant survival, he says.
Relying on host ants for a range of tasks that involve heavy use of smell and taste may allow these chemoreceptor genes to wane over evolutionary time.
“If you don’t have to forage and do a lot of the major work where you need chemosensory activity, then you have no pressure to maintain those genes as functional copies,” says Pask. “It turns out if [an olfactory] gene is broken you can get along fine with that, because the host species is able to take care of [the task].”
Bornberg-Bauer thinks that the evolution of sociality requires not only the gain of some functions, but also the suppression of others. For these parasitic ants, he says, “less can be more”.
He says his team expected to find gene losses, but they “were surprised to find that the same receptors were lost over and over again”.
Even though the three parasitic ant species evolved their habits independently of one another, they lost many of the same olfactory genes, hinting that this sensory retreat may assist the parasitic lifestyle. If parasitic ants have a harder time recognising their host workers’ smell, they may be more likely to accept their free labour rather than react with a knee-jerk attack response, suggests Pask.
The three parasitic ant species also lost some taste-related genes shared between them, but the number of those was low enough that these could have been lost due to chance and not by the evolution of the ants’ parasitic lifestyles.
at Oberlin College in Ohio, who wasn’t involved in the work, says that the discovery of the chemoreceptors not needed by parasitic ants could also help us determine “which of these genes are important for different aspects of social behaviours in non-parasitic ants”.
For Pask, having eight more ant genomes from species with varying lifestyles is a boon for understanding how different traits acted in the evolutionary origins of ants’ complex, highly organised societies. “We can start to piece together more of that puzzle,” he says.
Reference: bioRxiv, DOI:
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