You’re eating viral DNA?

As a society, we are scared of viruses. They are too small to see, insert themselves into our own cells, and turn our bodies into factories for making more of them. On top of that, they make us feel ill and can be tough to beat. HIV, H1N1, Papaya Ringspot Virus – it’s hard to find anything good to say about the little pseudo-living things. So it comes as no surprise that when people hear that scientists sometimes use DNA from viruses to genetically engineer crops, they get scared.

Viral DNA in food? How nefarious! Well, not really.

35S-promoterThe most common piece of viral DNA in GE crops is the 35S promoter from the Cauliflower Mosaic Virus. Promoters are like “on switches” that tell the cell when and how strongly to express or “turn on” a gene. The 35S CaMV promoter is a very well-described one that tells the cell to always leave the light on. Though part of those inserted genes came from a virus, it doesn’t make viruses of any kind.

Another way you might find viral DNA involved in genetic engineering is in developing virus resistance. A protein that coats the Papaya Ringspot Virus was engineered into Papayas to block where the virus attaches to the cell and prevent infection. Later on, as we started to understand more about some really cool aspects of genetics, it was discovered that the viral gene that was inserted also does a little gene silencing.

Susceptible papayas on the left, resistant on the right

Termed RNA Interference, or RNAi, you can stick a piece of viral DNA that the cell uses to recognize infecting viruses so it can destroy them – kind of like a vaccination for plants. In the case of inserting a viral coat protein, a virus-infected papaya will also have this harmless protein everywhere in the fruit, and in the case of the latter, RNAi doesn’t even produce proteins and carries very few safety concerns.

Nevertheless, the prospect of eating viruses still sounds scary. Well guess what, you are yourself made partly from viruses, and are eating things made from viruses. The complete sequence of the B73 Maize Genome has just been published, and at final count*, it seems that corn itself is mostly virus, if you look at its DNA sequence. Upon reading this, Abbie Smith at ERV says that she has fallen in love with corn all over again. She noticed that corn has a lot of viruses in its genome:

The human genome is made up of about 45% of this stuff.

Corn genome?

84.2%

WHOAAA!

A full 75.6% is Class I retrotransposons! Thats so cool!**

Yes that’s right, the corn genome is made up of mostly viral DNA, and the same is probably true for most of the plants you eat. Heck, we ourselves are almost half virus on the DNA level!

This really puts into perspective focusing on the source of the DNA (virus, plant, animal…) when talking about genetic engineering. If you think that “viral DNA” is unsafe in and of itself, then you might not want to eat anything at all, because it is everywhere and in everything.

For complete coverage of the Corn Genome and its discoveries, take a trip to James and the Giant Corn.

I always find discussions of “plant genes” vs “animal genes” and “viral genes” interesting, because what about the genes that are shared by different organisms? Plants and animals are all eukaryotic cells with nuclei and mitochondria, which share a common ancestor as well as a whole slough of common genes. Can you call a gene shared by both plants and animals a gene that ‘belongs’ to either one? Both? Neither?

Forgive me, but I’m a bit of a nominalist on this topic when it comes down to the nitty gritty philosophical details: my position is that there is no such thing as a plant gene. Or an animal gene, viral gene, etc. There are, instead, genes found in plants, genes found in animals, and genes found in viruses. Genes shared by Eukaryotes are just that – genes in common. Apart from the syntactical preferences of different organisms in how they like their DNA to read, there is nothing about a gene that makes it belong to one lineage or another.

Previous philosophers would think about these things in terms of ‘essence,’ and the term still has its uses. Like essential oils, the essence of something is what is left after you have ‘boiled down’ that something to its most ultimate and fundamental parts. There seems to be a pervasive notion that all genes carry the “essence” of the organism they are found in. When people talk about “fish genes in my tomato” they are expressing a worry that their familiar tomato will have some of its essential characteristics mixed up with those of a fish and that it will cease to be a tomato anymore.

tomatoe_finsThe fish-tomato example is rather ironic, as this meme began when scientists were experimenting with using the “antifreeze protein” from a species of fish to see if they could keep tomatoes from freezing (They were never commercialized). But as it turns out, a similar antifreeze protein in cod evolved out of noncoding DNA – going from useless sequences (sometimes haphazardly referred to as ‘junk DNA’) into a functional and essential gene. If you put this antifreeze gene in a tomato, is it even a fish gene? Or a junk gene? What if it once was a viral gene that got into fish, and eventually became what it was before a genetic engineer stuck it in a fruit, is it still a viral gene?

It’s an antifreeze gene… that evolved in fish. And you would essentially still be eating a tomato.

What should be the final nail in the coffin of the genic essentialism going around in these discussions is the fact that the foods we eat are made up of so much DNA from other species. Corn is 84% virus yet it still manages to be Corn. I also just found references to these issues in a book called Acceptable Genes: Religious Traditions and Genetically Modified Foods. Does a plant with a ‘pig gene’ carry the essence of a pig and present a problem for Jews and Muslims?

We’re entering an age where genetics are going to play a much bigger role in our lives, and in our food, so we need to wrap our collective heads around how our gathered knowledge compels us to change our perspectives. We may be 45% virus ourselves, but at least we could stop acting like them and propogate ideas that help us understand this stuff rather than add to the confusion!

*Nothing in science is truly final, but this is about as close as it gets.

**It is really cool, Abbie. Except when you are searching for a gene and you keep running into broken bits of retrotransposons all over the place! Can’t your people clean up after themselves?

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Karl earned his Ph.D. in Plant Breeding and Plant Genetics at UW-Madison, with a minor in Life Science Communication. His dissertation was on both the genetics of sweet corn and plant genetics outreach. He currently works as a Post Doctoral Research Associate for the USDA in Madison, WI. His favorite produce might just be squash.


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