In autumn, as green hues give way to yellows and oranges, some leaves develop mysterious green islands, where life apparently holds fast against the usual seasonal decay. These defiant patches still continue the business of photosynthesis long after the rest of the leaf has withered. They aren’t the tree’s doing. They are the work of tiny larval insects that live inside it – leaf-miners.
The larvae were laid within the leaf’s delicate layers by their mother. They depend on it for shelter and sustenance, and they can’t move away. If their home dies, they die, so they have a vested interest in keeping at least part of the leaf alive. These are the miniature landscape architects that create the green islands, and they don’t do it alone – to manipulate the plant, they wield bacteria.
Wilfried Kaiser and scientists from Rabelais University discovered this partnership after realising that some bacteria and fungi can also cause green islands. He reasoned that microbes might be helping insects to achieve the same ends. So he searched for them in one particular species, a tiny moth called the spotted tentiform leaf-miner, Phyllonorycter blancardella. Its larva makes its home in the leaves of apple trees.
Kaiser found that the leaf-miners are host to just one detectable type of bacteria – Wolbachia. That’s hardly surprising. Wolbachia infects around 60% of the world’s insect species, making it a strong candidate for the title of world’s most successful parasite. Without exception, every leaf-miner that Kaiser tested, from all over the Loire Valley, carried Wolbachia in their tissues.
This is an updated version of the first post I wrote this year. The scientists in question were looking at ways of recruiting bacteria in the fight against mosquito-borne diseases, such as dengue fever. They’ve just published new results that expand on their earlier experiments.
Mosquitoes are incredibly successful parasites and cause millions of human deaths every year through the infections they spread. But they are no match for the most successful parasite of all – a bacterium called Wolbachia. It infects around 60% of the world’s insect species and it could be our newest recruit in the fight against malaria, dengue fever and other mosquito-borne infections.
Wolbachia doesn’t usually infect mosquitoes but Scott O’Neill from the University of Queensland is leading a team of researchers who are trying to enlist it. Earlier this year, they published the story of their first success. They had developed a strain that not only infects mozzies, but halves the lifespans of infected females. Now, as the year comes to an end, they’re back with another piece of good news – their life-shortening bacteria also guard the mosquitoes from other infections.
It protects them against a species of Plasmodium, related to the parasite that causes malaria in humans, as well as the viruses responsible for dengue fever and Chikungunya. Infected insects are less likely to carry parasites that cause human disease, and those that do won’t live long enough to spread them. It’s a significant double-whammy that could have a lot of potential in controlling mosquito-borne diseases.
The drawers of the world’s museums are full of pinned, preserved and catalogued insects. These collections are more than just graveyards – they are a record of evolutionary battles waged between animals and their parasites. Today, these long-dead specimens act as “silent witnesses of evolutionary change”, willing to tell their story to any biologist who knows the right question to ask.
This time round, the biologist was Emily Hornett, currently at UCL, and her question was “How have the ratios of male butterflies to female ones changed over time?” You would think that the sex ratios of insects to mirror the one-to-one proportions expected of humans but not if parasites get involved.
The bacterium Wolbachia is arguably the world’s most successful parasite, infecting around 20% of all insects, themselves an extraordinarily successful group. It can infect eggs but not sperm, which means that females can pass the bacteria on to their offspring, but males cannot. As a result, Wolbachia has it in for males – they are evolutionary dead-ends, and the bacterium has many strategies for getting rid of them. It can kill them outright, it can turn them into females and it can prevent them from mating with uninfected females. As a result, populations infected with Wolbachia can be virtually male-free.
To study the effect of Wolbachia on butterfly populations, Hornett (great name for an entomologist)turned to collections of the blue moon butterfly (Hypolimnas bolina). This beautiful species was heavily collected by entomologists between 1870 and 1930 and their efforts have stocked the museums of the world with specimens. While these were long dead, Hornett found that many of them contained viable DNA and she used them to develop a genetic test for Wolbachia infections.
She validated her Wolbachia test by using butterflies collected by the entomologist H.W.Simmons in Fiji over 70 years ago. Simmons carefully recorded the numbers of males and females in his butterflies and noted some very unusual all-female brood. Sure enough, Hornett confirmed that only mothers who tested positive for Wolbachia produced these skewed clutches, while those that were infection-free gave birth to the standard bisexual broods.
Satisfied that her test was accurate, Hornett cast her net further. She looked at specimens collected from five populations of blue moon butterflies collected from the Phillippines, Borneo, Tahiti, Fiji and Samoa between 73 and 123 years ago. The butterflies are well studied to this day, so Hornett could compare the proportion of Wolbachia infections then and now. In butterfly time, this represents a gap of 500 to 1000 generations separating the specimens from their modern descendants.
The results show that the butterfly and the bacterium have been engaging in a heated evolutionary battle throughout the Pacific. The male-killer’s dominance has fluctuated greatly, rising in some areas and falling in others, while the butterfly has repeatedly evolved to resist its sex-skewing antics.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
A humble species of fruit fly is the genetic equivalent of a Russian doll – peer inside its DNA and you will see the entire genome of a species of bacteria hidden within.
The bacteria in question is Wolbachia, the most successful parasite on earth and infects about 20% of the world’s species of insects. It’s a poster child for selfishness. To further its own dynasty, it has evolved a series of remarkable techniques for ensuring that it gets passed on from host to host. Sometimes it gives infected individuals the ability to reproduce asexually; at other times, it does away with an entire gender.
Now, Julie Dunning-Hotopp from the J. Craig Venter Institute and Michael Clark from the University of Rochester have found an even more drastic strategy used by Wolbachia to preserve its own immortality – inserting its entire genome wholesale into that of another living thing.
Among bacteria, such gene swaps are run-of-the-mill. Humans and other multi-celled creatures must (mostly) contend ourselves with passing our genes to our young but bacteria have no such limits. They can exchange genes as easily as we exchange emails and this free trade in DNA, formally known as ‘horizontal gene transfer’, allows them to swap beneficial adaptations such as drug resistance genes.
Gene transfer between bacteria and eukaryotes is rare but if any bacteria was well placed to do it, it would be Wolbachia. It infects the developing sex cells of its hosts and gets passed on from mother to child in the egg itself – a prime location for integrating its genes into those of the next generation.
Other labs had already managed to detect traces of Wolbachia genes in a species of beetle and a nematode worm. To discover the full extent of its genetic infiltration, Dunning-Hotopp and Clark decided to search for Wolbachia genes in a wide range of invertebrates.
This is the third of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
In our world, there is (roughly) one man for every woman. Despite various social differences, our gender ratio remains steadfastly equal, so much so that we tend to take it for granted. Elsewhere in the nature, things are not quite so balanced.
Take the blue moon butterfly (Hypolimnas bolina). In 2001, Emily Dyson and Greg Hurst were studying this stunningly beautiful insect on the Samoan islands of Savaii and Upolu when they noticed something strange – almost all the butterflies were females. In fact, the vastly outnumbered males only made up 1% of the population.
The cause of this female-dominated world was an infection, an inherited bacterium called Wolbachia. Wolbachia is a strong candidate for the planet’s most successful parasite for it infects a huge proportions of the world’s arthropods, themselves a highly successful group. And it does not like males.
Wolbachia has an easy route of infection – it can be passed to the next generation through the eggs of an infected female. But it can’t get into sperm, and for that reason, male insects are useless to it and it has a number of strategies for dealing with them. Sometimes it allows females to reproduce without male fertilisation. At other times, it forces males to undergo sex changes to become females. But in cases like the blue moon butterfly, it simply kills the males outright before they’ve even hatched from their eggs.
In 2001, Dyson and Hurst noted that the islands with the fewest males were the ones with the most prevalent Wolbachia infections. But by 2005, things had changed. Sylvain Charlat from University College London, along with Hurst and others, found that males were increasing in number all around Upolu Island. A year later, a formal survey confirmed the males’ amazing comeback.
On Upolu, they equalled the females in number. Within just 10 generations, the male butterflies had gone from being outnumbered a hundred to one to an equal footing with the females. “To my knowledge, this is the fastest evolutionary change that has ever been observed,” said Charlat. In just ten generations, they evolved resistance to the parasite – a dramatic example of natural selection in action.
The mosquito Aedes aegypti sucks the blood of people from all over the tropics, and exchanges it for the virus that causes dengue fever – a disease that afflicts 40 million people every year. The mosquito has proven to be a tough adversary and efforts to drive it from urban settings have generally failed in the long-term. So how do you fight such an accomplished parasite? Simple – use a better parasite. In fact, try the most successful one in the world, a bacterium called Wolbachia.
Wolbachia‘s success rests on two traits. First, it targets the most diverse group of animals on the planet, the insects, infecting the majority of species and about one in eight individuals. Second, it spreads like wildfire by using several extremely self-serving strategies, all of which screw over male insects in some way or other. Wolbachia passes from one generation to the next in the eggs of infected females. But without similar access to sperm, males are useless to it and has evolved a number of ways of dealing with that. Sometimes it kills males outright before they’re even born; sometimes it turns them into females.
In other subtler cases, it ensures that infected males can only mate successfully with infected females. If they try to breed with uninfected ones, the embryos die at an early stage of development. This strategy is known as “cytoplasmic incompatibility” and while it’s still unclear how it works, there’s no doubt that it does. It gives infected females (who can mates with any male they like) a competitive advantage over uninfected females, who are restricted to uninfected males. With this upper hand, massive swathes of a given population eventually become Wolbachia-carriers.
Conor McMeniman and colleagues from the University of Queensland have found a way to use that to their advantage. They have found a strain of Wolbachia that can halve the lifespan of the Aedes mosquito and that induces complete cytoplasmic incompatibility. If introduced into a natural population, it should invade with tremendous zest.
Shortening a mosquito’s lifespan may seem like a flimsy victory, but McMeniman recognises it as an important one. Only old mosquitoes really pose a threat to human health because it takes about two weeks for an individual to become infectious after it first sucks up a mouthful of infected blood. The virus first need to reproduce in its gut before travelling back to its salivary glands, where it can spread further. Because mozzies are short-lived anyway, most die before they reach that point, which means that any technique that slashes their already limited lifespan will have a huge impact on controlling the diseases they carry.