K K Vinod
Primary objective of any plant
breeding programme is selection of the best genotype from an array of breeding
lines (genotypes). Conventionally, selection involves various traits that are
expressed in plants (phenotypes) and the selection tools employed by the
breeders involve qualitative (visual or perceivable traits) and/ or biometrical
(quantitative) traits. However, since gene expression is always modified by the
environment due to several adaptive reasons, selection of traits is not always
as successful as it should be. This led to the thinking of selecting directly
for the genes themselves, as the genes are coded on the DNA molecules that are
free of environmental interference. However, selection of genes was not an easy
job as it appeared to be, because of their obscure locations on the genome. Hence,
this is achieved indirectly by selecting detectable DNA variations in the
individual genomes, which are either associated or closely linked to the target
genes, as detected by the co-segregation of these fragments with phenotype. Such
detectable DNA fragments are called markers. Use of these molecular markers has
opened a novel way for selection known as marker assisted selection or marker
aided selection (MAS). Extensive use of molecular markers by the present day
breeders has opened up a separate field of study, the molecular breeding.
Principles of MAS
As the name indicates,
MAS is not a breeding method but markers are used as a selection aid. We
consider that traits may be typically controlled by single or many genes.
Although in strict sense no trait is controlled by a single gene but a group of
genes, the practical consideration of mono, oligo and polygenes revolves around
the number of genes that produce the perceivable quantum effect of the trait.
Any detectable DNA fragment lying around or within these genes can report the
presence of this gene in its carrier. This forms the fundamental idea behind
MAS. MAS therefore can involve in selection for both qualitative and
quantitative characters. However, MAS is not generally advocated for the
selection of easily selectable traits, especially those qualitative traits with
high heritability and penetrance, while for traits with low heritability and low
expressivity MAS may be a good option.
Since markers tag for
genes, MAS is useful mostly in transfer of genes from a donor to a recipient.
Therefore the donor should have a distinguishable marker allele that is linked
to the target gene. This marker allele should be distinctly different from the
allele produced by the same marker in the recipient genotype. Now while
crossing both donor and recipient it is easy to determine which progeny carry
the gene by identifying the linked marker allele. The target allele of the gene
may be either a dominant one or recessive. Most commonly used procedure in transfer
of single gene from donor to recipient is backcross breeding. Since the
recipient genotype is used repeatedly in crossing steps in the backcross
process it is also called recurrent parent (RP). Using markers, the backcross
breeding proceeds by selecting progenies that are carriers of the target allele
until a stable homozygous genotype is obtained which has almost entirely of the
recipient genome carrying the target allele. This is called marker assisted
backcross breeding (MABB). Depending on the transfer of dominant or recessive
allele backcross procedures may suitably vary.
