Agriculture Reference
In-Depth Information
approach requires that alternative alleles at the marker
locus match the different alleles at the locus of real
interest, thus effectively marking the sections of the
homologous chromosomes containing the locus that
determines the particular expression of the trait we are
trying to select.
The association of these marked chromosome seg-
ments with the expression of specific quantitative
characters can be evaluated while allowing other chro-
mosomal regions in the same individuals to vary at
random. The aim therefore is to obtain marker genes
that are closely associated with the locus determining the
desirable phenotypic expression of polygenic characters
such as yield or quality.
The segregating nature of F
2
populations (resulting
from selfing an F
1
produced by crossing two homozy-
gous inbred lines) often makes this generation ideal for
studying quantitatively inherited characters. Investiga-
tions have also been carried out using BC
1
generations
although the information obtained from this type of
investigation is likely to be reduced (approximately half )
of that obtainable from studies on F
2
s.
With an adequate number of uniformly spaced mark-
ers (a saturated map) it is possible to identify and
characterize the linkage groups, which signify the chro-
mosomes involved. It is also theoretically possible to
construct such a detailed map that the location of
all major genetic factors associated with the quanti-
tative trait might be linked rather easily and thus,
by following the presence/absence of the different
alleles,
epistasis, pleiotropy and the genetic base of heterosis.
So the effective use of mapped genetic markers should
allow advances in cultivar development and selection
procedures.
Genetic markers in plants associated with expres-
sion of morphological characters have been used for
quite a long time and marker maps assembled. They
have been quite well developed in a number of species
(e.g. wheat, maize, peas and tomatoes) but generally had
rather limited usage because of the problems of finding
or generating such markers and their species-specific
nature.
The characteristics of a 'good' marker system are:
•
That the markers are easy, quick and inexpensive to
score the phenotypes expressed
•
The markers are neutral in terms of their pheno-
types, and so have no deleterious effects on fitness and
no effects on any other traits, including undesirable
epistatic interactions with any other traits
There is a high level of polymorphism
•
They are stable in expression over environments
•
Can be assessed early in the development of the plant
(seedling level), and/or in tissue culture. Thus allow-
ing evaluation without the need to grow a plant for
months, or even years before it can be scored
•
The scoring should be non-destructive, so that
desirable individuals can be selected and grown to
maturity
•
•
Codominance in expression of the alternative alle-
les, so that heterozygotes can be differentiated from
homozygous dominant genotypes
to describe their individual and interactive
effects.
Markers in plants could assist plant breeders in the
development of a better understanding of the underly-
ing genes for characters of interest as well as providing
breeders and geneticists with a powerful approach for
mapping and manipulating individual loci associated
with the expression of these traits. In addition, if the
marker genes are tightly linked to other qualitative or
quantitative characters then much of the selection in
a plant breeding scheme could be carried out based
on the identification of specific set of alleles at the
marker loci.
The ability to identify loci which have effects on spe-
cific quantitative trait (termed quantitative trait loci -
QTL) should lead not only to the ability to handle
these loci in a much more deterministic manner but
also to provide a more powerful means of investigating
Types of marker systems
Any type of genetic marker that has the above properties
(or many of them) may be suitable for marker-based
applications in the investigation and manipulation
of quantitative traits, but the question is really how
closely do they conform to the ideal requirements given
above.
The types of markers that can and have been used in
plant breeding include:
•
Morphological markers - which are basically those
that you see by simply looking at a plant's phe-
notype, including characters such as pigmentation,
