Rationale for seed treatment with chemical mutagens

Because maize has separate male and female germline primordia, it has been recommended in the past to use pollen treatment for effective chemical mutagenesis, which is more efficient and precise. However, in special situations (namely, when seeking dominant kernel and seedling mutants or when screening for recessive kernel and seedling mutants of a type that can easily be recognized) a minimal sample method may make seed treatment practical.

If dry seed is treated with a chemical mutagen before germination the following logic applies. Each kernel contains 4 separate primordial germcells each for the tassel and the ear. These 8 cells carry 16 genomes. Treatment of 10,000 kernels will affect 160,000 genomes and produce 80,000 recessive (50% rate), and 400 dominant mutants. Planting and selfing the resulting M1 plants will produce about 10,000 ears with the above stated mutants present in heterozygotes, but unexpressed except for dominant kernel mutants.

Dominant mutants may be identified by observation (kernel mutants) and by planting and screening as seedlings or plants. Since each kernel will represent 2 genomes and 1/8 of the genomes available, a 107-kernel sample will be required from each ear to save 99.9% of the mutants (22 kernels for 95%). In practice, screening a 100-kernel sample from 10,000 ears will save almost all mutants produced. This number (1 X 106) is feasible for seedling screens but not for older plant characters.

Recessive mutants may be identified using the minimum sample method. A one-kernel sample each from 10,000 selfed ears, when planted and selfed, will test 20,000 genomes and should express 10,000 mutants or roughly 10 recessive mutants for each locus in the genome, each of which will be a completely independent event. A repeated sample of the M1 would produce an additional large number of new mutants, only 1/8 of which should be duplicates of the previous sample. These would be at the cost of 10,000 M1 selfs and 10,000 single-sample M2s for 20,000 total selfed plants to produce 10,000 mutants for a mutant-producing efficiency of .50, which is comparable to pollen treatment.

A labor-saving procedure would be to densely plant the treated seed to reduce ear size. Allow to open pollinate, harvest and shell by machine, mix to randomize sampling, and take repeated single-seed equivalent samples to screen for the dominant mutants; or, use a single-kernel sample to grow, self and test for recessive mutants.

M.G. Neuffer

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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