The microbes that insects harbour may have played a key role in shaping the diets and driving the diversification of inspect species.
Insects make up half of all animals on the planet and much of their evolutionary success appears to be linked to the microbes they host inside their bodies. Not only have these bacteria led to insects with ecologically unique diets – from blood-slurping bed bugs to wood-nibbling mites – but they have also played a key role in the variety across insect species.
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| Some insects have symbiotic relationships with microbes Chris Hellier / Alamy Stock Photo |
“The microbes themselves are the real driving mechanism behind that process of diversification,” says Lee Henry at Queen Mary University of London.
Researchers have long known that some insects house microbes that provide them with the nutrients they may otherwise lack in their diet. For instance, Tse tse flies have a symbiotic relationship with Wigglesworthia bacteria, supplying them with the B9 and B6 vitamins their blood-based diet lacks.
Henry and his colleagues endeavoured to map out all the microbe-insect bonds they could find, ending up with a whopping 1850 relationships of symbiosis across 402 different insect families.
The researchers found that all the insects in these symbiotic relationships have one thing in common – they all have low levels of B vitamins in their diets, suggesting that this is the common-denominator nutrient that is consistently correlated with the evolution of these microbe-insect bonds. But, interestingly, the research indicates that the symbioses between microbes and insects came first: once that evolved, diets lacking in B vitamins became much more common.
“The data suggests that symbioses evolved first and then insects started specialising in nutrient-imbalanced diets,” says co-author Charlie Cornwallis at Lund University in Sweden.
When the researchers looked at the ancestors of the moderns insects included in the study, they discovered that there were at least 16 independent origins of “obligate” symbiosis – when insects cannot survive without hosting a microbe – dating back as far as 336 million years ago.
“We can use these relationships to understand the major evolutionary transitions that have basically spawned all life on the planet as we know it,” says Henry.
Among insects that have evolved to eat only plants thanks to their microbe guests, diversifying into numerous species seems to come easy – so much so that herbivores now make up about half of all insect species. Weevils, for example, have the most species of any animal family at about 70,000.
Fewer critters have the right microbes for blood- and sap-based diets, on the other hand. It seems this dietary niche is a trap limiting their ability to diversify, says Henry. As a result, there are only a handful of insect species with such unique dietary preferences. But it is still unclear exactly why this evolutionary trap happens.
“The main conclusion that symbiosis is driving diversity, especially for sap-feeding, isn’t surprising to me, so it’s cool to see that supported with data,” says John McCutcheon at Arizona State University.
Not everybody agrees with that conclusion, though. Stephen Blair Hedges at Temple University in Pennsylvania says species adapt to eat different things all the time – both before and after they break off their evolutionary branch and differentiate – so it may be a stretch to link the richness of insect species to microbes.
Journal reference
Nature Ecology & EvolutionDOI: 10.1038/s41559-023-02058-0