A poison like no other for insects
Flannel moths look like cuddly soft toys. However, their caterpillars possess a potent poison. Its origin could be bacteria.
The main purpose of thick hairs on caterpillars is that predators spit out the woolly larvae as soon as they try to eat them or spurn them from the outset. And often enough, the hairs also inject a poison when they are touched. In the larvae of the genus Megalopyge, Andrew Walker from the University of Queensland and his team have discovered that this toxin has an ancient origin - and probably comes from horizontal gene transfer from bacteria, as they report in "PNAS".
The team had studied caterpillars of the southern flannel moth (Megalopyge opercularis) and Megalopyge crispata, which live in North America. Both caterpillars and adult butterflies have a dense coat that breaks off easily in the larval stage and then releases a painful venom that causes a strong burning sensation. Chemically, it differs significantly from that of related species and even from that of all other insects. "We were surprised that the venom of the caterpillars was completely different to anything we had seen in insects before," says Walker.
The team therefore surmised that the toxins may have had a different evolutionary origin. In fact, some of the proteins were very similar to the molecules found in bacterial toxins. Specifically, the caterpillar venom resembles a type of bacterial toxin that binds to the surface of a cell, where it assembles into a ring-like structure that tears holes in the target cells: this effect is known, for example, from toxins produced by E. coli or Salmonella bacteria.
Based on their findings, Walker and Co. assume that the most important components of the caterpillar venom originate from genes that were once transferred horizontally from bacteria to the ancestors of these insects: a process that is said to have taken place 400 million years ago. In horizontal gene transfer, the genetic material is passed on and incorporated outside of sexual reproduction and across species boundaries. Traces of bacterial genes can also be found in our genome.
"Many caterpillars have developed sophisticated defence mechanisms against predators, including cyanide droplets and adhesives that cause severe pain, and we are interested in understanding how they are related in evolutionary terms," says Walker. In the background, there is always the hope that these compounds could perhaps also be used for us humans, for example in the medical field. In the case of flannel moths, this could lead to a method that could be used to introduce drugs inside cells, for example. However, this should then be painless.
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Cover image: © RPFerreira / Getty Images / iStock (detail) Caterpillars of the moth genus Megalopyge sometimes have very potent toxins, which they secrete via their stinging hairs.
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