Why aren't mammals poisonous

Mammals also have the potential to be poisonous

Poisonous bite: The genes that enable snakes to bite them are also found in our salivary glands, as comparative genome analyzes reveal. Most mammals do not produce toxic saliva, but from a genetic point of view they have the potential to do so. According to the researchers, the basis for spitting poison goes back to the last common ancestors of mammals and reptiles.

Animals use poison in different ways: some release the toxins through the skin, others inject the substances with the help of a spike or shoot with harpoons. However, the most common and one of the best studied is the oral system, in which the poison is transmitted through a bite - for example in snakes. Depending on the species, their poison glands contain a cocktail of various toxic proteins, which usually serve to paralyze the prey and thus prevent it from escaping.

The poisons of snakes have been studied many times. However, since the concrete composition changes rapidly from an evolutionary point of view, such analyzes provide few indications of the evolutionary origins of oral poison systems.

Analyzed basics of the poison network

Instead of concentrating on specific toxins, researchers working with Agneesh Barua from the Okinawa Institute of Science and Technology in Japan have therefore now analyzed genes that are related to the poison system but do not themselves code for poisons. “Many of the toxins found in poison today only emerged after the oral poison system had already been established. We had to look for the genes that were there before the venom was created - genes that allowed the venom system to develop, ”Barua said.

In the genome of the Taiwan Habu snake, a pit viper widespread in Asia, the researchers identified around 3,000 genes that interact with the actual venom genes. As Barua and colleagues found out, these “cooperating” genes primarily regulate the folding and modification of proteins. “Poisons are complex mixtures of proteins. So you need a robust system that ensures that the proteins are folded correctly so that they can function effectively, ”explains Barua.

Similarities with other species

In order to trace the evolutionary origins of this genetic network of poisons, the researchers next analyzed the genomes of various other species, including humans, dogs, mice, chickens, lizards and frogs. "Our comparative analysis showed that gene expression in the tissues shows clear similarities, especially between venom glands in snakes and salivary glands in mammals," the researchers report.

From this they conclude that these glands have an ancient functional core that has been preserved since the lineages were split hundreds of millions of years ago. "This is the first really solid evidence to support the theory that venom glands evolved from early salivary glands," says Barua.

Could mice become poisonous?

A multitude of poisons developed in snakes in the following millions of years. Many of them are based on substances that were found in low concentrations in saliva. By modifying and multiplying the genes responsible for them, they became powerful, concentrated toxins. A similar process can also be found to some extent in mammals: "For example, shrews produce a simple poison that is very similar to saliva," says Barua.

The analyzes suggest that salivary glands can be converted into poison glands in the course of evolution: “If mice that produce more poisonous proteins in their saliva have better reproductive success under certain ecological conditions, then in a few thousand years we could find poisonous mice meet “, speculates Barua. (Proceedings of the National Academy of Sciences, 2021, doi: 10.1073 / pnas.2021311118)

Source: Okinawa Institute of Science and Technology (OIST)

April 9, 2021

- Elena Bernard