Isozymes as Markers for Studying and Manipulating Quantitative Traits

Abstract

Many plant characteristics (e. g., grain and forage yield, time of flowering, stress tolerance) show continuous variation. This usually implies that the inheritance of such traits is complex (quantitative) and probably involves the collective effects of numerous genetic factors. Classically, the activity of these factors has been characterized en masse, using biometrical procedures, and it has usually not been possible to isolate and measure the individual and interactive parameters of single factors (genes) or segments of chromosomes. Identification, mapping, and examination of individual genes affecting quantitative traits should provide knowledge concerning the organization of genomes and insight into the relative contribution of "major" and "minor" genes to such complexly inherited traits. With a better understanding of the inheritance of such traits, it should be possifule to develop new methods for enhancing plant improvement. A powerful approach for studying tHe inheritance of quantitatively inherited traits involves the use of mapped genetic markers. (Consider, for example, the segregating generation produced from selfing the single cross of two homozygous lines. If the transmission of each chromosomal segment could be followed by identifiable genetic markers, the entire genome could be assayed, segment by segment, for genes associated with the variation of any desired quantitative trait. Thus, the effects contributed by individual chromosomal regions could be identified. Theoretically, a detailed map of all major genes associated with the quantitative trait could be constructed, describing the chromosomal locations of the genes and their individual, and interactive effects. The concept of using mapped, monogenic markers in the study and evaluation of quantitatively inherited traits is not new. Early reports of associations of quantitative traits and major genes included those of Sax (1923), Rasmusson (1933), and Everson and Schaller (1955). Studies on the use of genetic markers for the investigation of polygenic characters in Drosophila have been reported by Breese and Mather (1957, 1960), Thoday (1961), and Spickett and Thoday (1966). Law (1967) used intervarietal chromosome substitution lines in wheat (Triticum aes-tivum) to investigate effects associated with four morphological marker loci. Factors influencing four quantitative traits (grain weight, grain number. plant height, and tiller number) were identified and mapped with respect to the marker loci on chromosome 7B. The reports noted above predated the use of isozymes as markers, so the researchers were restricted to the use of morphological genetic markers. Thus. the studies were limited in scope and utility for practical applications such as in plant or animal improvement. Molecular markers, such as isozymes, have a number of inherent properties that allow the theoretical approaches pioneered by these earlier scient', sts to be used very effectively for dissecting and manipulating quantitative variation. Tanksley (1983) stressed that molecular markers are superior to morphological markers for use in studies of quantitative traits for several reasons. (1) Alleles at most molecular marker loci 206 Many plant characteristics (e. g .• grain and forage yield, time of flowering. stress tolerance) show continuous variation. This usually implies that the inheritance of such traits is complex (quantitative) and probably involves the collective effects of numerous genetic factors. Classically. the activity of these factors has been characterized en masse, using biometrical procedures. and it has usually not been possible to isolate and measure the individual and interactive parameters of single factors (genes) or segments of chromosomes. Identification, mapping, and examination of individual genes affecting quantitative traits should provide knowledge concerning the organization of genomes and insight into the relative contribution of "major" and "minor" genes to such complexly inherited traits. With a better understanding of the inheritance of such traits, it should be possible to develop new methods for enhancing plant improvement. A powerful approach for studying the inheritance of quantitatively inherited traits involves the use of mapped genetic markers. Consider, for example, the segregating generation produced from selfing the single cross of two homozygous lines. If the transmission of each chromosomal segment could be followed by identifiable genetic markers. the entire genome could be assayed. segment by segment, for genes associated with the variation of any desired quantitative trait. Thus, the effects contributed by individual chromosomal regions could be identified. Theoretically, a detailed map of all major genes associated with the quantitative trait could be constructed, describing the chromosomal locations of the genes and their individual, and interactive effects. The concept of using mapped, monogenic markers in the study and evaluation of quantitatively inherited traits is not new. Early reports of associations of quantitative traits and major genes included those of Sax (1923), Rasmusson (1933). and Everson and Schaller (1955). Studies on the use of genetic markers for the investigation of polygenic characters in Drosophila have been reported by Breese and Mather (1957, 1960), Thoday (1961), and Spickett and Thoday (1966). Law (1967) used intervarietal chromosome substitution lines in wheat (Triticum aes-tivum) to investigate effects associated with four morphological marker loci. Factors influencing four quantitative traits (grain weight, grain number. plant height, and tiller number) were identified and mapped with respect to the marker loci on chromosome 7B. The reports noted above predated the use of isozymes as markers, so the researchers were restricted to the use of morphological genetic markers. Thus, the studies were limited in scope and utility for practical applications such as in plant or animal improvement. Molecular markers, such as isozymes, have a number of inherent properties that allow the theoretical approaches pioneered by these earlier scientists to be used very effectively for dissecting and manipulating quantitative variation. Tanksley (1983) stressed that molecular markers are superior to morphological markers for use in studies of quantitative traits for several reasons. (1) Alleles at most molecular marker loci D. E. Soltis et al. (eds.), Isozymes in Plant Biology