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Project Documentation & Protocols: Maize Gene Discovery Project: Education:
Building a Storehouse of Seeds from Mutated Plants

Contents: Maize Gene Discovery | The Challenge of Maize Genetics | Why Discover Maize Genes? | Finding Genes
Linking Genes to Function | Creating Databases | Building a Storehouse | Accomplishments | What's Next? | Glossary

 

Why Study Mutants

One goal of the Maize Gene Discovery Project (MGDP) has been to produce and catalog mutant plants and save their seeds for future research.

Mutant organisms are Mother Nature's way of letting us learn by her mistakes. When scientists look at how organisms change when things go awry, they can often predict a gene's normal function.

 

Breeding Mutant Plants

Maize is one organism that begs for researchers' attention because it generates ample mutations for study. Many maize breeds are chockfull of transposons that cause new mutations in every generation.

To take advantage of this trait in creating their storehouse of mutant maize seeds, the Maize Gene Discovery project breeds lines of corn containing known transposons (called mutator elements) including a specially designed transposon called RescueMu (learn more).

 

Growing RescueMu Plants

During each year of the project the team sews several fields of 2304 seeds -- 48 rows and 48 columns . of their special transposon stock of maize.

  • Field Plants are tagged with Row & Column Addresses.
  • colored tags are used to aid in collecting samples from the columns (all plants in column #1 have a yellow tag in the example).

Although each of these plants contains newly mutated genes, very few exhibit mutant traits because most mutations (199 out of 200) are recessive. But the mutations will appear in the next generation if the plants are self-fertilized -- creating a one in four chance that a particular seed will carry two copies of the recessive mutation rather than one, as shown in the following table.

Egg Pollen
Normal gene (dominant) mutant gene (recessive)
Normal gene Normal/Normal: ¼
(appears normal)
Normal/mutant: ¼
(appears normal)
mutant gene mutant/Normal: ¼
(appears normal)
mutant/mutant: ¼
(exhibits recessive trait)

So the team hand-fertilizes each ear with pollen from the same plant. Then, when the ear matures, they collect the ears.

 

Cataloguing Mutant Kernels, Seedlings and Adults

First, researchers look for mutant phenotypes among the sibling kernels on the ears themselves. About 10% of the ears exhibit unusual traits such as changed kernel color or indentations. Some of the mutations observed are similar to those causing sweetcorn (failure to convert sugar into starch).

In the following year of the project, researchers grow sixty seeds from each of the self-fertilized ears and catalogue the different mutant traits produced. Two separate strategies are used to study seedling and adult plants as shown in the diagram below.

As the seed families grow, a group of highly skilled botanists carefully examines each plant at each stage of development, looking for and photographing unusual (mutant) traits. The information they gather appears in an online catalog called PhenotypeDB (learn more).

 

Finding the Mutated Gene

There are several methods in molecular biology for identifying which maize gene is mutated by a transposon insertion. The transposon is a "tag" inserted into the gene, and this tag can be selectively recovered from the complex maize genome. The DNA sequence next to the transposon tag is from the maize gene that was "hit" by the transposon. With these gene sequences, the user can look at the whole genome sequence of rice or the available gene sequences of maize to find a "match." The match would define which gene has the transposon insertion. The methods in this area are rapidly improving in speed and accuracy as more and more DNA sequences are available from grasses.

 

The Upshot

In three years, MGDP increased four-fold the number of seed stocks available from the Maize Genetics Cooperation Stock Center, which contained stocks collected since 1928. By project end, MGDP will have increased the stocks eight-fold.

Future researchers can select seeds from the storehouse based on specialized characteristics described in PhenotypeDB (learn more), and then grow mutants of interest. Without these available seed stocks, researchers would have to grow many fields of corn just to find seed for an unusual trait.


Katherine Miller, a freelance science writer, contributed the text for this page to the Maize Gene Discovery Project. You can reach her at .

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