Agriculture Reference
In-Depth Information
new transgenic crops have been released into commer-
cial cultivation. Plant breeders need to be aware of the
concerns as well as the regulations that apply to plants
derived using recombinant DNA techniques as well as to
other forms of gene manipulation (e.g. induced muta-
tions). As well as the general social and environmental
concerns the breeder must consider the following:
that traditionally developed cultivars do. Multiple
transformation events are necessary to ensure that
one transformed plant has the desired level of expres-
sion for the altered trait, plus no deleterious epistasis
interaction with the transformed gene or background
changes.
MOLECULAR MARKERS IN PLANT
BREEDING
Is the level of expression of the genetically engineered
crop plant sufficiently useful to agriculture to merit
the time and resources that has gone into its devel-
opment, and what will need to go into any further
development?
Although plant breeders have practiced their art for
many centuries, genetics is a subject that really only
'came of age' in the 20th century with the rediscovery
of Mendel's work. Since then research in genetics has
covered many aspects of the inheritance of qualitative
and quantitative traits, but plant breeders usually still
have little, or no, information about:
Is there a concentration or dose effect that will further
optimize the effects? Higher or lower level of expres-
sion may be produced by changing the promoter or
with multiple gene insertions. If multiple copies are
necessary for desirable expression then this can cause
problems in the breeding programme as, effectively
another polygenic trait character may have been cre-
ated and will need handling in the normal way for
quantitatively inherited trait.
The locations of many of these loci in the genome or
on which chromosome they reside
The number of loci involved in any trait
Will the wider use of the introduced gene lead to
consequent effects in terms of the very situation it
has been introduced to change. For example, all tra-
ditional plant breeders are aware of the consequent
evolution in disease and pest populations of resistance
to overcome single gene resistance when introduced
into commercial cultivars - there is no need to repeat
the same mistakes!
The relative size of the contribution of individ-
ual alleles at each locus on the observed pheno-
type, except where there is an obvious major effect
(e.g. height and dwarfing genes)
Theory of using markers
Inactivation or silencing of the gene - the causes
and mechanisms of which are currently being inves-
tigated. How stable will this trait be in large-scale
agriculture and how will it interact (over time) with
the plant's original genome?
The idea of associating easily visualized markers in
plants with loci affecting qualitative and quantitative
variation in traits of interest is not new, and was first
proposed by Sax in 1923. Since then a variety of con-
tributions have been made to the general concept and
theory of using mapped genetic markers for identify-
ing, locating and manipulating genes of specific interest.
The basic idea is relatively simple. If a trait or character-
istic is difficult to score for whatever reason (e.g. it shows
continuous variation; assessment is detailed and time
consuming or the trait is only expressed after several
years of growth) an easily scored marker that was deter-
mined by a locus closely associated with that affecting
the character would be an attractive alternative way to
monitor the locus of interest.
The concept therefore, is to use the marker locus as
a point of reference for the chromosomal segment in
the vicinity of the gene that is really of interest. The
Care must be taken to avoid the induction of addi-
tional variation (somaclonal variation) in very genera-
tion of the transgenics. These would include obvious
mutations as well as cryptic ones. ( Note : as pointed
out earlier, most plant mutations are deleterious.)
Finally, it was at first naively believed that plant breed-
ers would be able to take an adapted cultivar and
simply transform it with a specific gene to give an
'instant' new cultivar - one that had all the previous
desirable characteristics but also with the transformed
trait. It is now known that this is not in fact the case.
New cultivars derived through plant transformation
require the same rigorous field-testing prior to release
 
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