For redback spiders (Latrodectus hasselti), mating involves a remarkable acrobatic display: the male performs a somersault, positioning his abdomen directly into the female's fangs. While this might appear to be a suicidal act, it is, in fact, a sophisticated mechanism that facilitates copulation and, at times, allows the male to survive. A recent study has revealed that this spectacular behavior is driven by a relatively straightforward genetic basis.
Redback spiders are found in Australia, while their close relatives, the katipo spiders (Latrodectus katipo), inhabit New Zealand. These species diverged approximately a hundred thousand years ago, a very recent period in evolutionary terms.
Despite this close kinship, male katipo spiders simply escape after mating, whereas redback spiders exhibit a unique "self-sacrificing somersault." Scientists were prompted to investigate why two such closely related species display such distinct behaviors.
To understand this phenomenon, researchers cross-bred the spiders, producing hybrid offspring. They initially bred female katipo spiders with male redback spiders, then mated the resulting hybrid females with males from both species.
The hybrid males were then mated with female katipo spiders, and the entire process was recorded using infrared cameras. Across 104 successful matings, observers noted the manifestation of two key behaviors: the somersault and abdomen contraction, which helps to avoid being bitten.
It was found that these two actions do not always occur simultaneously. Approximately half of the hybrid males performed both behaviors, while the others exhibited only one or the other.
This suggests that the genes responsible for the somersault and contraction are not tightly linked; they are likely located far apart on the same chromosome or even on different chromosomes. The somersault appears to be controlled by a gene or supergene on the X-chromosome, whereas abdomen contraction seems to have a more complex nature and might depend on signals from the female.
This separation of genetic foundations is surprising; it might seem that two behaviors occurring almost simultaneously should be tightly linked. However, as the experiment demonstrates, evolution has taken independent paths for these traits. This highlights the remarkable flexibility with which even the most elaborate mating rituals can change under the influence of minor genetic modifications.
This discovery also carries practical significance. Redback spiders are an invasive species in New Zealand, posing a threat to the native katipo spiders through hybridization. A better understanding of these genetic behavioral mechanisms will aid in assessing the risks of interbreeding and developing conservation strategies for the rare New Zealand species.
Observing spiders serves as a reminder that even nature's most dramatic displays often rely on simple genetic "levers," and studying these mechanisms not only helps us comprehend evolution but also to protect vulnerable ecosystems.
