Australia’s funnel-web spiders are deadly to humans – particularly the males from the species Atrax robustus that calls Sydney home – but how they evolved to do this has been a mystery.
Primates – including humans – weren’t around when these spiders originated around 150 to 200 million years ago, so wouldn’t have featured as predators or prey in their evolution.
Now, research published in the journal Proceedings of the National Academy of Sciences suggests the adult male funnel-webs repurposed toxins used to kill insect prey to defend themselves against vertebrate predators.
The spider’s lethal effects on humans are simply an “unfortunate evolutionary coincidence”, writes a team led by Bryan Fry from the University of Queensland, which included researchers from Australia, India, France, Norway and Belgium.
Fry has been intrigued by this for more than two decades. “I was living in the Dandenong Ranges,” he says, “and in the fall I started getting the cutest, most adorably tiny funnel-web spiders in my pool.”
His little visitors, Hadronyche modesta, are the smallest of all their kin from the family Atracidae, and Fry thought they would make an ideal comparison with their larger cousins.
“If we could find common patterns across the full range of funnel-web spiders,” he says, “that would provide big inroads into understanding the evolutionary processes that have shaped their venoms into such potent chemical cocktails.”
It took 20 years for the researchers to find a broad range of species and raise juveniles to adulthood so they could investigate venom changes, which they did by reconstructing their molecular evolution.
The peptides that make funnel-web spider venom so deadly are a group of neurotoxins called delta-hexatoxins. Previously, only eight of these peptides from five species had been analysed. Fry and team nearly tripled that by profiling 22 from the venom of 10 species.
They found the toxins had originally evolved to kill insects, such as cockroaches and flies, and were used by juvenile males and females of all ages.
The males, once they reach adulthood, venture far and wide to find females, which exposes them to predators. They feed very little during this mating season, and the study supports theories that their venom evolved long ago via natural selection to protect them against hungry vertebrates such as dunnarts, birds, rats and geckos.
“It’s obviously easier to evolutionarily tinker with existing toxins than to develop entirely new chemical forms of defence,” Fry explains.
“The defensive toxins are gene duplicates of the toxins used for insect feeding, but these gene duplicates are more potent for vertebrates than to insects. So the spiders have genes for a myriad of toxins, but they turn different ones on at different life stages.”
This could explain why adult male funnel-web spiders are the deadliest, but why cats and dogs can survive the toxin remains a mystery.
The results seem to apply to males from all species, which have a broader range than the Sydney area, Fry says.
“While the Sydney funnel-web gets the most press, because it bites rich people, the other species are also potential danger. Especially as spreading population centres chop down more bushland, bringing people into closer contact with these spiders.”
These other species are found all along the eastern and southern coasts of Australia, from Rockhampton to Adelaide.
Fry’s team hopes their ecological insights will help unravel funnel-web spider venom’s impact on humans and how it might best be treated.