Science

Girl power for wasps

The bacterium that replaces men

There is a bacterium that can determine the sex of entire wasp nests. When a female wasp is infected with this bacterium, she lays eggs that only become female wasps, RUG researcher Elzemiek Geuverink discovered.
By Leoni von Ristok / Photos by Traci White / Translation by Sarah van Steenderen

There is a bacterium that can determine the sex of entire wasp nests.

This is shown by research done by PhD researcher Elzemiek Geuverink at the Faculty of Science and Engineering at the RUG.

Ichneumon wasps do not have any sex chromosomes. The genetic difference between the sexes is determined by the amount of chromosome sets.

Normally, female wasps will only have daughters if they fertilise their eggs. Unfertilised eggs produce sons.

When the female is infected with a particular bacterium, unfertilised eggs only produce daughters.

The bacterium manipulates the female in such a way that she lays eggs containing the activated version of a gene that causes them to develop into female wasps.

Ichneumon wasp females lay their eggs in the larvae of fruit flies, which are then eaten from the inside out by the ichneumon wasp larvae.

Ichneumon wasps can be used to control fruit fly infestations this way. Infected nests are more effective at this because they have more females.

Reading time: 7 minutes (1,275 words)

A bacterium that determines the sex of descendants and makes men obsolete. Geuverink admits that it sounds a little weird and unnatural. But, she says, it does happen in nature.

‘There are entire wasp nests that consist of only females’, according to the researcher. ‘Some species have even completely done away with their genitalia over the course of thousands of years.’ The result? Nothing but females producing females without help from a single wasp male.

Chromosomes

That this is even possible is because of the way these wasps procreate and determine the sex of their descendants. In contrast to human beings, wasps do not have specific sex chromosomes. The genetic difference between the sexes is determined by the amount of chromosome sets.

Male wasps have one chromosome set in each cell, meaning one of each chromosome. This is called haploid. Wasp females have two sets of chromosomes, or two of each chromosome. They are diploid.

To be clear, we are talking about minuscule ichneumon wasps, approximately the same size as fruit flies. These wasps do not live in social structures with a strict division of labour such as bees or ants, for example. ‘The males are completely useless. The only thing they do is mate with the females’, Geuverink laughs. She received her PhD for her research at the Faculty of Science and Engineering in late March.

Female boss

In ichneumon wasp society, the female determines the sex of her descendants: after intercourse, the wasp female stores the sperm she received in a built-in pouch. That allows her to fertilise her eggs. But she can also decide to not do that. Unfertilised eggs are haploid and will automatically become sons. When the female wasp fertilises an egg she adds a second set of chromosomes to the egg. This then leads to the egg producing a daughter.

But the fact that females are distinguished from males by having two sets of chromosomes while males only have one does not explain why a diploid egg produces a daughter.

Feminizer

How do two identical chromosomes together have a different effect than one? To put it simply, it is because genes in a chromosome are not only present or absent; they can also be switched on or off. And sometimes genes can only be switched on when there are two of them. It is kind of like Down syndrome, also known as trisomy 21. Trisomy 21 is caused by the presence of three chromosome 21 rather than just two in the cells of people with Down syndrome.

A gene present in one chromosome, for example, can switch on its ‘parallel gene’ on the other chromosome. And sometimes it is only the combination of two genes that are able to produce a functional protein that actually influences development. Conversely, an abundance of protein can lead to abnormal development.

The gene present in many insects that causes them to develop as female is appropriately called a feminizer. In a ‘single’ chromosome, the feminizer is turned off, which then produces a son. When a single chromosome is combined with a second, the feminizer is turned on, and the egg develops as female.

Hostile takeover

There are bacteria, called Wolbachia, that screw up or even completely take over the entire natural process of sex determination. When they infect a female, that female is only able to produce daughters. It does this without sperm cells or fertilisation, by the way. In other words, asexually. ‘Wasps that are asexual have no interest in sex and simply fly away from males’, Geuverink says. So the bacterium takes over the role of males needed to produce daughters.

The bacterium does not do this by adding a second set of chromosomes to the eggs as you might expect, but by manipulating the female on a different level. It makes the female lay nothing but diploid eggs with an activated version of the feminizer gene. These unfertilised eggs then produce nothing but daughters. Males are no longer produced, but then again, they have become obsolete.

Infected mothers pass on the bacteria and produce infected daughters who in turn produce only daughters. This creates entire nests of wasps consisting of only women.

Gruesome practices

So why is this an issue, exactly? The females in infected wasp nests are all clones of each other. That means there is no genetic variety and the entire nest run the risk of being wiped out, for instance by a disease they have no resistance to, according to Geuverink. If the wasps in a nest have more variation, a few of them might be able to fight off the disease, allowing the nest to survive.

But infected wasp nests can also be useful. For example, they can help control insect infestations. Wasp females lay their eggs in fruit fly larvae. When the eggs hatch, the wasp larvae eat the fruit fly larvae from the inside out. ‘That is a fairly gruesome system’, Geuverink says. ‘There are different wasp species that are parasitical to various other insects. But the species I study only attack fruit fly larvae.’

Because of this particular insect parasitism, fewer (or no) fruit flies develop. Male wasps lay no eggs and are therefore useless to pest control. ‘Males have no impact on their host. Only the females lay eggs, thereby impacting the ecosystem.’ A population of only females will eat many more fruit fly larvae and are therefore more effective in controlling fruit flies.

Right now, Europe is overrun by the drosophila suzukii, the spotted-wing fruit fly, a Japanese relative of our native fruit fly. Native fruit flies eat overripe fruit and can be annoying, but they cause no major problems. Their Japanese relative, however, attacks fresh fruit and is therefore capable of destroying entire fruit harvests. The spotted-wing fruit fly is also stronger, which means it can defend itself better from wasp larvae that eat fruit fly larvae. Our native wasps, therefore, are unable to control this infestation.

Fortunately, our native wasps also have stronger, asexual relatives in Japan. It might be tempting, then, to use a Japanese type of wasps. Or to infect the native wasps with the bacterium, making them asexual and setting them loose among the Japanese fruit fly population. But it is not as easy as all that.

Wasps on a strip

Infected ichneumon wasps are a promising alternative to pest control. It is hoped that eventually we will be able to use not only wasps on fruit flies, but also other insects and mites to control other insect infestations. It is currently possible to order native ichneumon wasps as pupae on a strip to release in your garden. ‘But we shouldn’t release too many Japanese species in the Netherlands. Introducing infected wasps, and especially exotic species, must be done very carefully’, Geuverink warns. ‘It sounds great in theory, but sometimes that works in a lab might work out completely differently in nature.’

There are plenty of examples of exotic species used to control native infestations. Only for these species to radically alter the ecosystem because they found another prey in their new environment. Because exotic species often do not have any natural enemies in their new habitat, they can actually become an infestation themselves. Before we can use these manipulative bacteria as pest control in any targeted manner, researchers will have to spend quite a lot more time in their labs.

Nederlands

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