The evolutionary lesson in the prickly genitalia of a beetle

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The sex story of beetles has often been told in a very peculiar way, with the male in the evolving driver’s seat, his hapless companion reluctantly taken for a ride. A quick glance at the insect’s penis makes it easy to see why: the appendix is ​​endowed with hundreds of sharp, hard thorns that make it look like an elaborate lump. This terrifying influx of spikes riddles the female’s reproductive tract with bites and scratches that can leave, as biologist Göran Arnqvist puts it, “pretty massive scars.”

Prickly quills are great for the male, who tends to spawn more offspring when his spines are particularly long. But the female, who is usually penetrated by several partners, can suffer damage so severe that it limits her egg-laying capacities, or heralds a tragic premature death.

To guard against these dangers, females have developed their own defensive arsenal, including a boosted immune system, super-fast healing, and an ultra-thick reproductive tract, a pretty literal girdle of the kidneys. In populations of Callosobruchus maculatus beetles, the thickness of the female tract appears to have increased at the same rate as the length of the penile spines – a kind of genital arms race, what the researchers call a sexually antagonistic coevolution. It sounds like a classic tale of noxiousness and palliative reaction: “Males develop something that is good for males, but bad for females, so females adapt to that,” said to me. Arnqvist. The male grows; the female displays; the species as a whole advances painfully.

But Arnqvist, who has studied beetles for many years at Uppsala University in Sweden, thinks it’s time to reframe. He and his colleagues worked to quantify how the female, too, can benefit from these encounters, reaping rewards from even the most hellishly ornamented males. Its evolution, they argue in a new article, is motivated not only by a avoidance injuries inflicted by the male, but a attraction to the traits a well-endowed suitor could offer. “I think we’ve seen this far too simply in the past,” said Arnqvist, who published the results today in Proceedings of the Royal Society B.

Read: This common butterfly has an amazing sex life.

To study the myriad effects of penile spines on women, the Arnqvist team had to manipulate evolution in real time. Genetics, they knew, played a big part in the length. So they carefully bred two groups of beetles, separating the males with extra-long or extra-short tips until the private parts of the populations were visibly distinct, and then paired them with mates.

The women who struck the ugly with the men of the Longthorn Lineage certainly suffered internal damage – it’s inevitable, Arnqvist said. But their sons had larger and thicker tips, which by proxy helped the genes of the females to endure. Their daughters also inherited a multitude of valuable traits: They had bodies that swelled to larger sizes and laid more eggs before they died.

Johanna rönn

The idea of ​​seeking out high-quality fathers to father high-quality children is of course not new. But the Arnqvist team’s study adds a layer of complexity to our understanding of the evolutionary carrots and sticks faced by female beetles, which must juggle the immediate cost of genital injury with the eventual benefit of producing a excellent brood. “You get that push and pull in different directions,” Ainsley Seago, an entomologist at the Carnegie Museum of Natural History, told me. The two competing pressures operate at different timescales, but the benefits are designed to last, says Yoko Matsumura, a zoologist at the University of Kiel, Germany, who was not involved in the study: by matching his genes to those of a quality male, the female brings her own heritage to life.

There may even be more instant positives for females associating with superlative males. The Arnqvist team found that females that mated with males of the long-thorn lineage lay more eggs in their lifetimes than those that only mated with the short thorns. It is believed that males with naturally longer phallic accoutrements have better access to the female’s hemolymph – the insect equivalent of blood, flowing outside of her reproductive tract – where their seminal fluids can prepare for her. body to reproduction. They could also, thanks to their genes, produce a better quality ejaculate, a premium beetle juice. This rich cocktail can make up to 8 percent of the insect’s total weight (the equivalent of all the human body’s blood, in proportion) and is packed with hundreds of ingredients, many of which are believed to enhance the production of eggs or supply the female body with much needed nutrients. (When they become adults, seed beetles stop eating and focus their efforts only on sex.)

The history of the female beetle is still not really a happy a. But perhaps these findings shed a less terrible light on his fate, says Blake Wyber, an evolutionary biologist at the University of Western Australia, who was not involved in the study. The findings of Arnqvist and his team, Wyber notes, also help put the female perspective at the center of the scene, something largely lacking in animal mating research. Wyber and his colleagues are trying to do the same with their own population of beetles, using new techniques to measure how the reproductive system of females changes over time, perhaps even in a single individual. “It’s not only, Woe to them“Wyber told me.” They can take advantage of this; they’re not just being used. And it’s ‘quite what you would expect,’ says Patricia Brennan, evolutionary biologist at Mount Holyoke College in Mass., Who was not involved in the study. Without the benefits that apply to both sexes, she said to me, “How would the system be maintained?”

Arnqvist and others believe these evolutionary themes will echo in other cases of sexual antagonism as well – among bedbugs, perhaps, whose males use their saber-shaped phalluses to stab the females’ abdomen. and ejaculating in the wound, or ducks, for whom sex involves a ballistic corkscrew penis that explosively deviates from the male. Brennan’s research focused on the latter example, and she showed how women can force men to jump through their own genital hoops. The ducks have concocted extraordinarily labyrinthine vaginas, with twists, turns and dead ends to thwart unwanted phallic advances. (Matsumura is studying another type of beetle whose female genitals are also coquettishly coiled.) The males, in turn, have struggled to lengthen and develop their penises, hoping to keep pace.

Even the female beetle has impressive strategies for ensuring her own success. She’ll sprint away from the males when she’s not in the mood (all the more resolutely, Arnqvist told me, after she has had her first sexual experience), and eagerly kicks the cubs. lotharios trying to cool off. She is also the master of her own violent sexual destiny: her reproductive system is armed with tiny rows of teeth that seem to pierce the packages of semen and seminal fluid deposited by the penis, promoting the gush of treats. On the winding, winding road that led to the jabby sex of the beetles, the females too decidedly took the wheel.


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