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Plant mutants have been used in genetic studies and breeding for decades, yet
huge number of mutants remains to be characterized at the molecular level. In recent
years, the information of genome sequence has become available for several plant species
but the functions of many of the genes are not known. The analysis of
well-characterized mutants using saturated mutant populations, coupled with recent methods for the
detection and generation of mutants are bridging the gap between plant genes and their
functions. There are basically two ways to analyze function of a mutated gene: reverse and
forward genetics (Caldwell et al., 2004; Jose and
Joseph, 2006; and Christian et al., 2009). In
reverse genetic approaches, one starts with a (sequenced) gene of interest, selects a mutation
in that gene, and then tries to identify a phenotypic change associated with mutation.
In forward genetic approaches (Caldwell et
al., 2004), one begins with a prediction of specific effect of a mutation for physiological and morphological processes and
then isolates mutants with the predicted phenotype followed by mapping and isolation of
the genetic sequence that determines the above phenotype. Thus, contrary to reverse
genetics (Jansen et al., 1997; Sessions et
al., 2002; Jose and Joseph, 2006; and Christian et al., 2009), forward genetics starts with phenotyping of mutants and later identifies the
gene responsible for the altered phenotype and both the approaches are valuable
and complementary. Once corresponding gene linked to a mutation is identified,
the introgression of the gene in a new cultivar for breeding requires at least two
backcrosses for removal of background mutations and elimination of genotype of donor plant.
Moreover, the essential confirmation that a phenotype of interest results from a given
mutation though can be achieved via complementation testing, which is also used for
determining allelism of recessive mutations, such confirmation is not possible in case of
dominant mutants. In addition, once the mutant is isolated the next requirement is to maintain
the purity of the mutant, i.e., homozygous nature, which is slightly difficult in case of
dominant mutant (Jack et al., 1997), where the mutant shows same phenotype in both
homozygous and heterozygous forms. Likewise, if the population of different mutants was
grown together, there are always chances of cross pollination from neighboring plants
and heterozygous progeny may result. Tomato crop grows well under warm climate,
but cannot be grown under temperate climate. As a consequence in temperate
climate tomatoes are grown densely in green houses (Liu et al., 2004; and Menda et al., 2004).
In such growth conditions, close proximity of plants also increases the chances of
cross pollination leading to heterozygous population.
In mutation breeding experiments, it is essential that the plant materials be
genetically pure and uniform for the traits to be examined and that pollination be rigidly
controlled both prior to and during the experiment to prevent outcrossing. The identification
of homozygous wild type, heterozygous and homozygous mutant plants, can be done
by normal genetic crossing experiments. However, it involves considerable time and
money for genetically screening of mutant. Moreover, in crops, it is difficult to distinguish
between homozygous dominant or heterozygous dominant mutations. In procedures, that
assist in the identification of mutations include growing progeny of suspected mutants
and observing whether segregation occurs or back crossing the suspected mutant to
the parent followed by selfing or sibling of the progeny of a true recessive mutant. |
