Sunday, October 3, 2010
Sunday Spinelessness - How the aphid got its blush
Once upon a time, a green aphid lived on a plant somewhere. Like most of her kind, this particular green aphid didn't have wings so her whole life was going to be spent on this one plant, sucking away at the plant's sweet sap. You might be quite jealous of this aphid's life, since for an aphid a plant's sap really is an Exceptional Confection and she spends her time doing nothing but drinking it. But her life was harder than you might imagine, at the moment she was pregnant and her one goal in life was to make sure her babies would have a safe life here on this plant. This was long before gardeners came in to the world with pyrethrum and soap spray, but she had plenty of other enemies that could make life very hard for her and her babies. The scariest of all of the aphid's enemies were the wasps. The wasps would fly from plant to plant, smelling the air with their manically twitching antennae and searching for little green bodies with their eyes. This was very bad for the aphids, because theses wasps, just like the Ichneumon you already know about, would like nothing better than to use an aphid's body as an incubator for the their own babies.
And here comes the magic bit of our story. Our aphid wasn't feeling very well, she was infected by some fungus or other which was spreading through her body. You know that when you feel ill that you get hotter and you might swell up, and that's because you body is fighting back against your illness. The aphid's body was fighting back too, breaking up the fungus cells and stopping their spread. The aphid didn't know that her body was fighting fungi, all she knew was that she was starting to feel better, but something amazing was happening inside here. All those broken up fungal cells in her body had left their genes floating around, and one of those genes found its way into one of the aphid's babies.
A few weeks later the aphid was feeling much better, and it was time for her to give birth to her babies. Can you imagine her surprise when one of her babies wasn't green, like every other aphid that had ever existed until now, but was bright pink! She was staggered, but she always called her pink baby her Best Beloved and she never cared for her less than her green babies. And when the wasps came, with their twitching antennae and their giant scanning eyes, they always missed the pink bodied aphid. So the pink bodied aphid grew up and had pink bodied babies, and they got missed by the twitching, scanning wasps too so they had even more babies. So now, all these years after our aphid lived on her plant somewhere every pink aphid in the world can thank her, and her fungal infection, for their lovely colour.
No, I can't write the whole thing as a Just So Story, and I certainly can't write the poem that should finish it. But I do want to talk a little more about these guys:
I guess for most people a rose covered in little pink aphids is one of the less welcome signs of spring. I'd like to see the rose do well and produce lots of flowers, but I was also happy to see such a wonderful illustration for one of the most interesting pieces of evolutionary genetics to be published this year:
Lots of aphid species come in pink and green flavours, but just how the pink ones keep up their colouration has been a bit of a mystery. It's been known for a long time that the pink pigments are caratenoids, the same family of chemicals that make carrots orange and tomatoes red. These chemicals are important parts of lots of biochemical pathways, but animals can't make their own and have to rely on what they can eat. Except for aphids. Aphid caratenoids are slighltly different than those of their host plants, so they must be making their own but just how they can do that had been a mystery until this year. Mike Bok at Arthropoda and Ed Yong at Not Exactly Rocket Science covered the story when it broke, so here's the short version:
The pea aphid, Acyrthosiphon pisum, has become something on an unlikely model organism recently, and has even had its genome sequenced. Nancy Moran and Tyler Jarvick realised that the release of the pea aphid genome data was their chance to work out how the aphid got its caratenoids. They searched for genes similar to those that make up the caratenoid production pathway in plants, fungi and bacteria in the aphid genome and found some. Even more excitingly, there were able to relate the aphid genes to their relatives across the biological world and conclude that the genes of the aphid catatenoid pathway are most closley related to fungal genes - the aphids got their pink colouration from a fungus genome.
The transfer of genes from one organism to another outside of the traditional parent-offspring relationship is called horizontal gene transfer. In recent years it's become increasingly obvious that single celled organisms have been swapping genes in this way since life began, but the aphid case is among the first times horizontal gene transfer has been confirmed in animals. I should say, my little tale up there is a just so story in more ways that one. We really don't know how an aphid acquired fungal genes. Perhaps it was an infective fungus broken down an immune response, maybe it was raw RNA reverse transcribed into the aphid's genome or maybe there was a bacterial conduit. Wild aphid populations have a lot of symbiotic bacteria which can help them adapt to particular host plants and environments, perhaps in the past they had another that used fungal genes to change the aphid's colour and those genes have since migrated to the aphid's own genome.
Moran, N., & Jarvik, T. (2010). Lateral Transfer of Genes from Fungi Underlies Carotenoid Production in Aphids Science, 328 (5978), 624-627 DOI: 10.1126/science.1187113