Humans are known to have one functioning and three non-functioning chitinase genes — those that make an enzyme that digests the insect exoskeletal chitin. According to a new analysis of mammalian genomes, nearly all placental mammals have one of five different chitinase genes, though not all are functioning and some fragmentary; this suggests that the common ancestor of all placental mammals, living with dinosaurs until 66 million years ago, was an insectivore (insect eater). The study was published in the journal Science Advances.
Many bacteria have genes that produce an enzyme that breaks down insects’ hard, outer shells, which are composed of a tough carbohydrate called chitin.
It’s not surprising that humans and mice have a chitinase gene, since many humans today include insects in their diets, as do mice.
But humans actually have remnants of three other chitinase genes in their genome, though none of them are functional.
Dr. Christopher Emerling from the University of California, Berkeley, and the Université de Montpellier, and co-authors showed that these gene remnants in humans aren’t unique to our species or primates, but instead can be traced to the ancestral placental mammals.
“We are looking at genomes and they are telling the same story as the fossils: that we think these animals were insectivorous and then dinosaurs went extinct,” Dr. Emerling said.
“After the demise of these large carnivorous and herbivorous reptiles, mammals started changing their diets.”
The researchers found five different chitinase enzyme genes by looking through the genomes of 107 species of placental mammals (from shrews and mice to elephants and whales).
They found that the greater the percentage of insects in an animal’s diet, the more genes for chitinase it has.
“The only species that have five chitinases today are highly insectivorous, that is, 80 to 100% of their diet consists of insects,” Dr. Emerling said.
“Since the earliest placental mammals likely had five chitinases, we think that this makes for a strong argument that they were highly insectivorous.”
“As you would expect, ant and termite specialists such as aardvarks and certain armadillos have five functioning chitinase genes.”
“But so do the insect-loving primates called tarsiers. They appear to be the only primates that have so many functional chitinase genes.”
Among the surprises was that the insect-eating-specialist pangolin has only one functional chitinase gene, in contrast to the five in the aardvark and four in the lesser anteater.
All eat ants and termites exclusively, but pangolins may have possibly evolved from carnivores that lost their chitinase genes shortly after taking over the ecological niche opened up when meat-eating dinosaurs died out.
Bison, gibbons and the dromedary camel have only one functional chitinase. Tigers, rhinos and polar bears have none.
“We have many other questions we think chitinases can answer about mammal evolution and physiology,” Dr. Emerling said.
“This is suggesting that there are a lot of these enzymes that might be helping organisms digest their food. This goes from being a simple curiosity — humans have a chitinase, how cool! — to being something that can help us understand how different animals are adapted to their specialized diets.”
“Chitinases are found not only in the gut but the salivary glands, the pancreas and the lungs, where they may be involved in asthma,” he said.
Christopher A. Emerling et al. 2018. Chitinase genes (CHIAs) provide genomic footprints of a post-Cretaceous dietary radiation in placental mammals. Science Advances 4 (5); doi: 10.1126/sciadv.aar6478