Oil Composition Variations - Genetics, Genomics and Breeding of Crop Plants - page 262

8.1.4 Types of Mutation

Most frequently, a mutation is a change in the coding DNA sequence (exon)
of an enzyme that induces one amino acid deletion, insertion, or substitution
that makes the enzyme no longer functional. If the mutation is in a non-
coding sequence (regulation sites, intron) and affects the transcription ability,
the transcript of the enzyme will be absent, and therefore the enzyme will
not be synthesized. The mutation may also delete a part of the entire gene
and this will have the same consequence. All these mutations are recessive
since this change does not affect the function of any normal gene in a diploid
heterozygous plant, and is called a cis effect.
In some cases, the mutation may affect a regulatory peptide encoded by
another gene. The regulatory peptide is modified, and the mutation may
affect several functions (syndrome effect), as it has been reported for
transcription factors (Schmidt et al. 1992), and will behave as dominant.
Such a mutation has trans effect. A heterozygous plant for the mutation may
be more or less affected depending on the effect of the normal allele, which
competes with the mutated one.
Due to insufficient natural variation for oil composition in crop plants,
breeders have used artificial mutagenesis, either physical ( - or X-rays) or

chemical (a series of compounds: ethylmethane sulfonate, EMS; diethyl
sulphate, DES; N-nitroso-N-methylurea, NMU) to induce mutations.
Mutations are random and the treatment enhances their rates of appearance.
However, a mutagen may favor some base changes or DNA rearrangements
such as deletions or insertions. They may also activate silenced transposon
or retrotransposon sequences. As a result, the exact type of mutation cannot
be predicted according to the agent and a rigorous study is required to
determine the exact changes for a given induced mutation.
In the past few years, fatty acid pathways have been modified using
transgenic strategies. We can distinguish when the insertion of an extra
copy of a desaturase is in the same direction, because in most cases this
causes overexpression of the gene. The mutation is, therefore, partially
dominant but also its effect largely depends on the cultivar's genetic
background. This, in most cases, will enhance the level of the next product.
In another situation, a transgene can knock out a gene by insertion of another
(extra) copy in the reverse (or same) direction elsewhere in the genome. This
causes post-transcriptional gene silencing (PTGS) and the absence of the
transcript. The level of the product before the blocked step will be enhanced.
The insertion has a trans effect on all copies of the wild gene, and the mutation
will be dominant.