Honeybees stress each other out by warning about minor parasites

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European honeybees (Apis mellifera) release warning pheromones in response to parasitic infection

William Mullins / Alamy

European honeybees produce a warning pheromone when parasites infect their hive, yet the social stress this chemical causes might be more devastating than the parasites themselves.

A one-celled fungus called Nosema ceranae can infect the guts of individual bees, causing a disease called nosemosis. Similar to tapeworm infections in humans, nosemosis apparently makes bees hungrier reduces their resistance to pesticides probably viruses, but it isn’t particularly fatal. Yet, nosemosis is one of the top reasons honeybee populations are declining.

Christopher Mayack at Swarthmore College in Pennsylvania suspected this might have something to do with how the fungus affects the bees’ social structures.

“Subtle changes in behaviour can be critical for the honeybee because it’s so highly social,” he says. “If their social harmony – really, their functioning as a group – gets disrupted, it can cause colony collapse, meaning complete dysfunction of the hive.”

Bees, like most social insects, use pheromones to communicate. To learn how those pheromones – hence social communication – alter during an N. ceranae infection, Mayack’s team vacuumed up 100 Apis mellifera forager bees from 30 different hives near Philadelphia, 18 of which were infected by this fungus. They then used a form of spectrometry to measure the bees’ pheromone production their N. ceranae infestation rates.

Compared with bees from uninfected hives, those from infected ones had much higher concentrations of a pheromone the insects produce when threatened by large invaders, like humans bears.

This so-called alert pheromone is commonly released when bees sting, or when they are crushed or killed. Bees also use it to mark flowers where they have already removed the nectar. The chemical seems to serve as an important call to action, whether to repel or to attract.

In the case of N. ceranae infestations, this could mean that the pheromone stimulates the bees to care for their infected hive mates, says Mayack. However, it might also drive them to quarantine sick individuals – or even kill them. Either way, Mayack suspects the bees’ behaviour changes so much, it could destroy their healthy social balance.

“Just like with humans, if there’s too much stress, the social contracts can break down quite easily, the functioning of the group won’t be carried out so well,” says Mayack.

He also wonders whether the pheromone might make infected hives more aggressive towards beekeepers. Even so, more studies are needed to determine exactly what is going on with the chemical’s production in affected hives, he says.

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.210194

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Male parasitic wasps can detect females inside an infected host fly

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A false-colour image of a male parasitic wasp, Nasonia vitripennis

DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY

Males of a species of parasitic wasp can identify potential mates from chemicals they give off, even before the females have emerged from within their host fly.

Jewel wasps (Nasonia vitripennis) are found across North America. Females deposit eggs inside the cocoon-like casings of developing flies, using their ovipositors to inject each fly with a venom that paralyses it. The developing wasps remain in the host as they mature from egg to adult, only eating their way out to mate. Males emerge first, hanging around on the hosts to wait for females to appear.

“Males want to increase their mating success, so would benefit from finding hosts with females,” says Garima Prazapati at the Indian Institute of Science Education Research (IISER) Mohali.

It is possible for these wasps to up their chances. Males develop from unfertilised eggs females from fertilised eggs, so some hosts hold all-male broods, while others house a mixture of males and females.

Prazapati her team collected jewel wasps from the wild bred them. They isolated some females, keeping them from mating so their eggs would go on to create all-male broods. Next, they individually presented 26 male wasps with two Petri dishes: one holding a host containing male female adult wasps, one with a host containing only adult males.

The researchers found that the males spent around four times longer on the host with the females inside.

Analysing the chemical compositions of both hosts, the team found that the one containing female wasps had a higher abundance of nine cuticular hydrocarbons – compounds that cover the wasp exoskeleton – than the host with males inside.

They then dipped adult wasps in a chemical solution that extracts these hydrocarbons and found that adult females also had a higher concentration of them than males.

Prazapati says this suggests that the males must be able to detect the abundance of female-specific chemical cues emanating from within the fly casings. “This is the ultimate mate‑finding strategy,” she says.

They are certainly good at finding the female wasps, says team member Rhitoban Raychoudhury, also at IISER Mohali. “But males being attracted to females isn’t news.”

Given the lifestyle of parasitic wasps, this strategy of searching for mates while they are still within the host is important for males to secure reproduction may also be seen in other species, he says.

Journal reference: bioRxiv, DOI: 10.1101/2021.04.06.438549

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Grasses pass genes from one species to another but we don’t know how

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Some traits of food crops may be due to DNA acquired from other plant species

Jozef Sedmak/Alamy

It’s freecycling, but for DNA. Grasses routinely pass genes from one plant to another, even if they belong to distantly related species.

“We’ve shown that lateral gene transfer is a widespread process in grasses,” says Luke Dunning at the University of Sheffield in the UK. The finding adds to the evidence that DNA can be transferred from one complex organism to another, rather than only being inherited, that this can benefit the recipient.

Biologists have known for decades that single-celled organisms like bacteria can pass genes in this way, a process called lateral gene transfer or horizontal gene transfer. But as recently as 20 years ago, it was thought that this didn’t happen in organisms with more complex cells, known as eukaryotes – the group that includes all animals, plants fungi.

“People thought it was completely restricted to bacteria didn’t happen in eukaryotes,” says Dunning. “It’s probably only been 10 to 15 years that that’s really shifted.” Nowadays many eukaryotic examples are known, such as a plant gene that has crossed into insects.

Most studies of this phenomenon have focused on isolated examples: for example, in 2019 Dunning’s team showed that a grass called Alloteropsis semialata had 59 laterally transferred genes.

To find out how widespread such gene transfer really is, Dunning’s team studied the genomes of 17 grass species, some of which have been evolving independently of one another for 50 million years. These included food crops like Asian rice, common wheat foxtail millet.

The team found that 13 of the 17 species carried laterally transferred genes – indicating widespread transfer. In total 170 genes had been transferred.

“As more more genomes of eukaryotes are sequenced, we’re seeing so many examples of horizontal gene transfer,” says Julia Van Etten at Rutgers University in New Jersey. She co-authored a 2020 study estimating that about 1 per cent of the genes in the single-celled eukaryotes called protists are the result of lateral gene transfer.

For every 10,000 genes in the grass genomes, the team estimates 3.72 are detectably laterally transferred. “But that is a massive underestimate,” says Dunning, because only some transferred genes will be favoured by natural selection become common in a population. “It’s probably an ongoing process happening all the time, then you’re only going to fix one or two.”

The team found that lateral transfer was more common among closely related species, perhaps because their genes are more compatible. But it still happened in the least related ones.

Transfers were also more common in grasses that had rhizomes – underground stems that can send out roots shoots beneath the surface. “They are tissues that allow plants to asexually reproduce,” says Dunning. “If you get any foreign DNA into that rhizome, when the plant regenerates it’s in every cell of that clone, including the flowers, that’s how it gets into the germline.”

“The million-dollar question is to find out how it’s happening,” says Dunning. The grasses aren’t hybridising with each other, as the DNA would look very different if they were. He suggests that in many cases pollination by wind might be a factor. “Potentially you could have illegitimate pollination where you only get a small bit of DNA transferred from an outside species”, instead of a true hybrid, he says.

Although laterally transferred genes only make up a small percentage of eukaryote genomes, they may still be having major impacts on evolution, says Van Etten. For example, she works with red algae that live in hot, toxic environments. “They’re thriving because they’ve horizontally acquired genes for arsenic detoxification mercury detoxification, they’re able to take up sugars so they don’t have to photosynthesise all the time. They’ve changed their entire lifestyle.”

It may be that lateral gene transfers underpin some of the traits found in domestic strains of crop grasses like wheat, says Dunning. That is speculation, but if it is confirmed, it will mean lateral gene transfer has helped us create the crops that now feed us.

Journal reference: New Phytologist, DOI: 10.1111/nph.17328

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