In Robert Louis Stevenson’s classic story, Dr Henry Jekyll drinks a mysterious potion that transforms him from an upstanding citizen into the violent, murderous Edward Hyde. We might think that such an easy transformation would be confined to the pages of fiction, but a similar fate regularly befalls a common fungus called Fusarium oxysporum.
A team of scientists led by Li-Jun Ma and Charlotte van der Does have found that the fungus can swap four entire chromosomes form one individual to another. This package is the genetic equivalent of Stevenson’s potion. It has everything a humble, Jekyll-like fungus needs to transform from a version that coexists harmlessly with plants into a Hyde-like agent of disease. In this guise, it infects so many plant species so virulently that it has earned the nickname of Agent Green and has been considered for use as a biological weapon. It can even infect humans.
These disease-making chromosomes came to light after Ma and van der Does sequenced the genome of a variety of F.oxysporum called lycopersici (or ‘Fol’), which infects tomatoes. Its genome was unexpectedly massive, 44% bigger than its closest relative, the cereal-infecting F.verticillioides. Looking closer, Ma and van der Does found that most of this excess DNA lies within four extra chromosomes, which Fol has and its relative lacks. Together, they make up a quarter of Fol’s genome.
Ma and van der Does demonstrated the power of this extraneous quartet by incubating a harmless strain of Fol with one that causes tomato wilt. Just by sharing the same space, the inoffensive strain managed to acquire two of the extra chromosomes found in the virulent one. And, suddenly, it too could infect tomatoes. In a single event, the fungus had been loaded with a mobile armoury and changed into a killer. It seems that the fungus needs just two of the four chromosomes to cause disease; the others probably act as accessories, boosting its new pestilent powers.
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.
For centuries, farmers have been genetically modifying their plants without even knowing it. That’s the message from German scientists who found that grafting, a common technique used to fuse parts of two plants together, causes the two halves to swap genes with each other.
Grafting can involve fusing the stem of one plant (the scion) to the roots of another (the stock), or a dormant bud to another stem. There are many reasons for this – sometimes it’s the most cost-effective way of cultivating the scion, sometimes the stock has properties that the scion lacks including hardiness or sturdiness. The vessels of the two halves eventually merge but people have long believed that they keep their genetic material to themselves. It turns out they were wrong.
Sandra Stegemann and Ralph Bock from the Max-Planck Institute tested the theory by grafting two strains of genetically engineered tobacco plant. A Samsun NN strain had its main genome loaded with a gene that produced a glowing yellow protein, and another that made the plant resistant to the antibiotic kanamycin. The second Petit Havana strain was engineered to produce a glowing green protein, and be resistant to spectinomycin, another antibiotic. These genes were shoved into the genome of its chloroplast, the small structures that allow plant cells to photosynthesise and that contain their own separate genetic material.
Once the plants had merged, Stegemann and Bock found that the point of fusion was rife with cells that produced both glowing proteins and shrugged off both antibiotics. They cut slices from the plant and grew them in liquid that contained both kanamycin and spectinomycin for a month. While chunks that were taken from other parts of the plant fared poorly under these conditions, many of those from the graft site thrived, even producing fresh shoots.
This is the seventh of eight posts on evolutionary research to celebrate Darwin’s bicentennial. It combines many of my favourite topics – symbiosis, horizontal gene transfer, parasitic wasps and viruses.
Parasitic wasps make a living by snatching the bodies of other insects and using them as living incubators for their grubs. Some species target caterpillars, and subdue them with a biological weapon. They inject the victim with “virus-like particles” called polydnaviruses (PDVs), which weaken its immune system and leave the wasp grub to develop unopposed. Without the infection, the wasp egg would be surrounded by blood cells and killed.
The wasps’ partners in body-snatching are very different to all other viruses. Once they have infected other cells, they never use the opportunity to make more copies of themselves. They actually can’t. To complete their life cycles, viruses need to package their genetic material within a coat made of proteins. In most cases, the instructions for building these coats are encoded within the virus’s genome, but polydnaviruses lack these key instructions entirely. Without them, the virus is stuck within whatever cell it infects.
It’s such a weird set-up that some scientists have questioned whether the polydnaviruses actually count as viruses at all or whether they are “genetic secretions” from the wasps themselves. Where on earth are those missing coat genes?
Annie Bezier form Francois Rabelais University has found the answer and it’s an astonishing one. The viruses’ coat genes haven’t disappeared – they’ve just been relocated to the genomes of their wasp hosts.
In this way, the wasps and the viruses have formed an unbreakable alliance, where neither can survive without the other’s help. Without the virus, the next generation of wasps would be overwhelmed by the defences of their caterpillar larders. Without the wasp, the virus would never be able to reproduce. Some viruses may be able to live happily alongside their host with little ill effect; others may even be beneficial in some way. But this is the first example of a virus co-evolving with its host in a compulsory binding pact.