Combining Breeding
with Molecular Genetics
|
In today's world where automated sequencing and DNA synthesis are mundane activities, it may seem contradictory to be worrying about saving or using “old genes.” Can't new ones be synthesized to order? Can't we modify a plant at will by introducing a new gene or two into an existing variety? Why should we worry about saving populations of historically valuable genes in millions of living plant specimens at great cost to the tax-paying public?
Perhaps it is not the genes themselves we are now in fear of losing. It is the information they encode in all their combinatorial complexity. After all, we are only at the very beginning of the endeavor to understand the way in which a genotype confers a particular set of attributes to a living organism. The subtleties of phenotypic plasticity in the face of a changing environment and the layers of genetic redundancy that characterize biological systems are largely mysterious. We have only just begun to consider the millions and billions of genetic trials and errors that have been evaluated by nature over evolutionary time. |
We cannot even begin to simulate the selective filters that have provided us with the diversity of form and function in the living world. We do know that living forms of natural diversity are needed to sustain life, and that it would be impossible to replace or recreate that diversity if it were lost at this time.
As plant breeders, we know what to do with living forms of genetic diversity. If we keep our options open and learn to better utilize the reservoirs of natural variation that have been preserved in our gene banks and in the few remaining in situ populations of wild species and landrace varieties, an almost infinite array of novelty can be achieved using traditional, time-proven practices involving crossing and selection of genes that have withstood the test of evolutionary time (Burbank 1914; Hawkes 1958; Rick 1967; Harlan 1975, 1976; Peloquin 1983). By restricting the gene pool, we can readily channel a phenotype into a constrained and predictable outcome. By expanding the gene pool, we can open up many new possibilities for consideration that have not been previously evaluated, would be unlikely to be generated in nature, and would not be readily predicted based on current knowledge
In crosses between wild and cultivated species of inbreeding plants, alleles that were “left behind” during the domestication process may be reintroduced into the cultivated gene pool. This infusion of “new blood” renews and invigorates modern cultivars in surprising and interesting ways. It is not uncommon for some of the inbred progenies derived from these crosses to perform better than the better parent (Frey et al. 1975; Rick 1976, 1983; Tanksley and McCouch 1997). This phenomenon is known as transgressive variation and results from positive interaction between the genotypes of the parents. Today, plant breeders can analyze populations derived from wide crosses using molecular markers to determine which portions of the chromosomes are associated with the transgressive variation of interest. This makes it possible to dissect a complex phenotype and to determine where individual genes or, more correctly, quantitative trait loci (QTLs) map along the chromosomes. Information about DNA markers linked to QTLs represents a powerful diagnostic tool that enables a breeder to select for specific introgressions of interest, a technique referred to as “marker-assisted selection.”
This approach has proven to be extremely successful in several crop species (tomato [Bernacchi et al. 1998], hybrid rice [Xiao et al. 1998], inbred rice [Thomson et al. 2003], wheat [Huang et al. 2003], barley [Pillen et al. 2003], and pepper [Rao et al. 2003]). In China, two introgressions from a wild relative of rice have been associated with a 30% increase in the yields of the world's highest-yielding hybrid rice (Deng et al. 2004). In tomato, yield increases of greater than 50% resulted from introgressing three independent segments from a wild relative, as reported by Gur and Zamir (2004). The effect of these introgressions on yield was stable in different genetic backgrounds and in both irrigated and drought conditions. This work was facilitated by the availability of a library of chromosome segment substitution lines, called introgression lines when the donor is a wild species, that provided the foundation for exploring the interactions among the independent QTLs. Plant geneticists have long recognized the value of exotic libraries (Brassica [Ramsay et al. 1996; Cermankova et al. 1999], millet [Hash 1999], rice [Sobrizal et al. 1996; Ghesquiere et al. 1997; Ahn et al. 2002], tomato [Monforte and Tanksley 2000; Zamir 2001], wheat [Sears 1956; Pestsova et al. 2002, 2003], and Arabidopsis [Koumproglou et al. 2002]). They represent a permanent genetic resource that greatly facilitates the utilization of wild and exotic germplasm in a breeding program, and they are also an efficient reagent for the discovery and isolation of genes underlying traits of agricultural importance.
 |
 |
http://biology.plosjournals.org
|
