Tuesday, March 29, 2016

Mutation Breeding

The natural genetic diversity found in a crop species was formed over long periods of time by the slow accumulation of mutations in different lineages. We can simulate this process, in shorter time-frames, by dramatically increasing the mutation rate. This can be done by exposing the limited germplasm we have to ionizing radiation (such as X-rays) or any of the many chemicals that damage or modify DNA, thus causing mutations.

"Taihei" (center) and mutants (ring).
Adapted from fig2 on p259 at [link]
and image at [link].
Once we have this newly-made genetic diversity, we can then select towards whatever our goals are.  (Well, we can if some of those mutants happen to be of the sort we find interesting.) Mutation breeding was used to generate the diversity of colors in chrysanthemum seen at right. Another such project is responsible for the intensely red grapefruit types we've gotten used to.

Really, mutation breeding has been used to improve crops in all sorts of ways.

X-rays and Gamma-rays are extremely energetic types of light. They're so energetic that they break DNA when they make a direct hit. When used for mutagenesis, they tear chromosomes apart. Cells can heal this damage, but errors (such as mutations and chromosomal rearrangements) are often introduced in the process. They're relatively easy to generate (even for a relative amateur), but they're difficult to handle. They'll pass through almost anything. It takes heavy lead shielding to keep them contained.

High-energy particle beams (think neutrons or electrons) wreak havoc to DNA like X-rays. With advances in technology, such particle beams may eventually be easier to come by. However, for now, they are limited to research institutions with much larger budgets than mine.

Lots of chemicals can damage DNA and hence cause mutations. Ethyl methanesulfonate (EMS) is commonly used because it is relatively easy to handle (including neutralization for disposal) and effective in generating mutations. The problem with mutagenic chemicals is... well, that they're mutagenic. Things being mutagenic in the lab often translates to them being carcinogenic (cancer causing) in real life. I don't feel like mutating today. Do you?

If I wanted to do some mutation breeding in my home lab/garden, I would want a way to make mutations that I could turn on and off. It is feasible to build a device to generate X-rays (with some intensive electrical engineering), but ideally I'd want something that would only impact what was in a very small space.

Ultraviolet (UV) light causes mutations. (That is essentially what a sun-burn is, after all.) UV-C in particular is referred to as germicidal-UV because it causes large numbers of DNA breaks, resulting in the destruction of chromosomes and death of cells. This germicidal-UV is used to sterilize surfaces/water/etc. in various situations in the lab, hospital, or restaurant. All it takes to generate UV-C is some slightly fancy fluorescent bulbs. Since these can be turned on and off with a switch and the fancy light they produce can easily be contained in a box, this seems like it might be a way to go.

What isn't clear is if UV-C light can be effectively used to induce mutations in seeds. UV-C is readily absorbed by organic materials, so large doses will be probably be needed to impact the embryo protected within any but the smallest of seeds. There seems to be very little research available on this topic, so I'll have to do some experimenting.