What was found
Male Australian redback spiders (Latrodectus hasselti) perform one of the stranger mating behaviours known in spiders. During copulation, the male somersaults his abdomen toward the female's fangs, actively placing himself in a position where she can begin to eat him. The behaviour is often described as self-sacrifice because being cannibalized can increase the male's reproductive success.
Redback males also develop a pronounced constriction in the abdomen during mating. This appears to help prevent them from dying too quickly after the female attacks, giving a male a chance to continue mating and potentially use his second palp.
Their close New Zealand relative, the katipō (Latrodectus katipo), does not show these self-sacrificial mating traits. That difference gave researchers a rare opportunity to ask how such an elaborate behaviour is inherited.
The team first crossed L. katipo females with L. hasselti males. Hybrid females were then backcrossed, producing males with different combinations of genetic material from the two species. When those males were observed during mating, close to half of the relevant descendants performed the redback-style somersault.
The inheritance pattern was consistent with the somersault being controlled by a single X-linked genetic locus. The abdominal constriction did not follow the same pattern and appears to have a more complex genetic basis.
What makes that interesting
At first glance, deliberately flipping into a mate's fangs looks like the kind of behaviour that should require a complicated network of genetic changes. The study suggests otherwise. An extremely unusual behavioural trait can apparently be inherited through a comparatively simple genetic mechanism.
Just as interesting is what did not happen. The somersault and abdominal constriction might look like two pieces of one tightly integrated self-sacrifice strategy, but the crossing experiments suggest that they are not genetically linked in that simple way. One can be inherited without the other.
That makes the redback–katipō comparison particularly valuable for understanding how complex behaviour evolves. Evolution does not necessarily have to produce an entire behavioural package at once. Separate components can have different genetic histories and still end up working together as a remarkably specialized mating strategy.
Why it matters
The study gives researchers an unusually clear route into the genetics of spider behaviour. Redbacks and katipō are close relatives, yet their males differ dramatically in what happens during mating. Crossing them allows researchers to move beyond simply describing the behaviour and begin asking how it arose and how quickly it could change during evolution.
It is also a good warning against reducing the result to a "suicide gene." The researchers identified an inheritance pattern consistent with a single X-linked locus for the somersault; they have not yet identified one specific gene that causes the behaviour. Work on the underlying genes is still continuing.
What the study does show is arguably more interesting anyway: even one of the most extreme mating behaviours in spiders may rest on a genetic architecture far simpler than its appearance suggests.