On the trail of deadly potential: Researchers have clarified why the dangerous funnel-web spiders in Australia have developed a poison that is so threatening for us and why it is mainly the males that produce it. The deadliest component of the poison is therefore not primarily directed against prey insects, but was created as a weapon against predatory vertebrates. The males are apparently more exposed to these enemies in their search for females. The fact that humans are so particularly sensitive to the poison is ultimately an evolutionary coincidence, say the researchers.
The threat is often overestimated – only a few species of spider have a bite that is actually dangerous or even fatal for humans. There are exceptions, however: In addition to the Brazilian wandering spiders, the Australian funnel-web spiders are considered to be the most dangerous representatives of the eight-legged poisoners. It is a group of 35 species that live on the ground and stalk insects. The Sydney funnel-web spider (Atrax robustus) is particularly notorious and can even bite through shoe surfaces with its powerful jaws. Without treatment, the nerve poison can lead to death within a few hours from respiratory and circulatory failure. Fortunately, however, an antidote is available, so that there have been hardly any deaths in the last few decades.
Why so deadly?
It is already known from studies that the deadly potential of the poison is based on special components of its complex mixture: the so-called δ-hexatoxins have a fatal effect on our nervous system. However, it seems surprising that they are particularly damaging to people. Because we can hardly have played a role as prey or enemies in the evolutionary history of these Australian spiders. In addition, it is still unclear why the significantly smaller male spiders in particular produce the particularly potent toxins in the venom. They are responsible for most of the problematic biting attacks on humans – especially when looking for females during the mating season.
The researchers led by Bryan Fry from the University of Queensland in Brisbane have now devoted themselves to a more detailed investigation of the funnel-web spider venom and its evolutionary background. To do this, they took a closer look at the characteristics of the δ-hexatoxins of the different funnel-web spider species. So far only nine δ-hexatoxins of four species have been characterized. As part of the study, the researchers have now examined 22 additional versions of these toxic substances from ten other species. They also recorded the genes that are responsible for their production. The scientists explain that it was possible to draw conclusions about their functions and their development history from the characteristics of the toxins and their genetic blueprints. “The fact that we now had a lot more data helped paint a much clearer picture of these substances and shed light on their interesting evolutionary history,” says Fry.
A male weapon
The test results and sequential comparisons show that the δ-hexatoxins developed from precursor forms that were used to catch prey. Even today, they still show a paralyzing effect on insects such as cockroaches, tests have confirmed. According to the researchers, however, it is clear from the results of the investigation that the δ-hexatoxins have been systematically modified to target vertebrates. In this context, the researchers were able to confirm through experiments that they cause particularly strong pain reactions in mice. It is now becoming apparent that the δ-hexatoxins in the poison cocktail of the funnel-web spiders are less likely to be used to catch prey, but rather are a weapon against predators.
This function appears to be particularly important in the case of the males, the researchers explain: “When they become sexually mature, they leave the security of their den and cover considerable distances in search of females. This migration is dangerous, however, because some vertebrates are known to like to eat spiders, ”says Fry. Typical predators are mouse-like marsupials such as the narrow-footed marsupial mice. It therefore seems plausible that the special poisonous substances have established themselves in the male spiders in order to deter these predators.
The problem: we are also vertebrates
“Our data show that natural selection exerted the necessary pressure to convert an insect-specific poison into a vertebrate-specific defense poison,” summarizes Fry. This also explains the intense effect on humans: “Our bad luck is that we are also vertebrates,” says Fry. Our great sensitivity to these substances is ultimately a coincidence, which results from the evolutionary history of these spiders and their poisons, summarize the scientists.
In addition to the insights into the history of these interesting toxins with a scary factor, the study results could now also be used medically, the scientists say: “We hope that this research will give us a better understanding of what exactly the funnel web spider venom does in the human body . This could benefit the development of treatment strategies for bite victims, ”says Fry.
Source: University of Queensland, Article: PNAS, doi: 10.1073 / pnas.2004516117