II.16. Quantitative genetic analysis with qualitative definable loci.
S. Jana, Crop Science Department, University of Saskatchewan, Saskatoon, Canada.
The use of isogenic lines in the study of quantitative variation enables one to determine the various types of gene actions associated with specific loci or tightly linked chromosome segments marked by these loci. Furthermore, changes in genetic effects of individual loci or marked chromosome segment during different stages of growth and development can also be followed in studies with isogenic lines. When we decided to conduct a series of experiments in this area, the first requirement was of course to find suitable marker genes which influenced a metrical character with sufficient heritability so that such an undertaking would be meaningful. We were fortunate to receive a number of homozygous isogenic lines of Atlas 46 barley from Professor C. W. Schaller of the University of California at Davis which appeared to be well suited for our investigation.
A. Two-locus isogenic lines. The four homozygous lines used in this investigation were isogenic for Atlas variety of barley, except for two unlinked loci conditioning lemma awn development. A detailed account of the processes involved in the development of these lines by Briggs and Schaller was given in Qualset et al. (1965). However, in order to avoid possible confusion by the use of incorrect symbols given in Qualset et al. (1965), Professor T. Tsuchiya (personal communication) recommended that we refrain from using the gene symbols to describe the genotypes of the lines. We have designated the loci as A/a and B/b in this report. The four homozygous lines were phenotypically easily distinguishable on the basis of their awn length as full-awned (AABB), half-awned (AAbb), quarter-awned (aaBB) and awnless (aabb). The allele A was dominant over a and the allele B was dominant over b. All 12 possible cross combinations among the four lines were made to obtain a 4 x 4 complete diallel cross. The F1 hybrids and their parents were grown in a twice replicated field experiment at Saskatoon in 1972. Each entry in a block was grown in a plot which consisted of three rows 4.87 meters long. Plants were spaced equidistant 45 cm apart. Data were collected from all plants in a plot excluding end plants. A total of about 27 plants were available in each plot for observation.
Starting in the third week after sowing (second week after emergence), observations were taken on number of tillers and plant height at one-week intervals until maturity. At maturity, number of heads per plant, seed yield and seed size were also recorded. All observations were recorded on a single plant basis and plot means were used for the genetic analysis following the diallel method (Jinks and Hayman, 1953) and the matrix method (Seyffert, 1966). A summary of the results obtained from the analysis by the use of the latter method is given in figure 1. In this figure, summation (ignoring signs) of additive effects of A/a and B/b loci is denoted as A, similar summation of dominance effects is denoted as D and the sum of their four types of interactions as E. The results clearly indicate that the relative magnitudes of various types of gene actions ascribable to the A/a and B/b loci (or the linked gene blocks) change considerably for both plant height and tiller number at various stages in the life cycle of the plant. It is conceivable that the nature and size of genetic effects of individual genes would change with the stage of plant development even when the genes controlling a quantitative character are not individually recognizable. Whether a basis for such changes can be found in the physiological and biochemical transformations associated with the development of the plant is an open question and is indeed an intriguing area of research.
B. Three-locus isogenic lines. In the spring of 1972, professor Schaller kindly made available to us eight homozygous isogenic lines of Atlas barley, which, in addition to the A/a and B/b loci, differed with respect to the alleles at a locus designated as Sp/sp. Faris et al. (1966) described the dominant mutant Sp causing dark brown spots on the first leaf at about three to five leaf stage. The spots appeared progressively on higher leaves of the growing plant and increased in size as the plant developed, ultimately covering most of the leaf surface. The eight homozygous lines were: full-awned spotted (AABBSpSp), half-awned spotted (AAbbSpSp), quarter-awned spotted (aaBBSpSp), awnless spotted (aabbSpSp), full-awned green (AABBspsp), half-awned green (AAbbspsp), quarter-awned green (aaBBspsp) and awnless green (aabbspsp). A complete 8 x 8 diallel set of crosses involving these lines is currently being grown in a controlled environment room under simulated summer field conditions Our aim is to study quantitative variation for a number of physiological and morphological characters at different stages of growth of the plant. We plan to repeat this experiment both in the field and the controlled environment room. Results emanating from these repeated experiments are expected to provide useful information towards a biological elucidation of such important concepts as dominance and epistasis in mathematical genetics. Furthermore, the proposed experiments would evaluate repeatability of the results mentioned in Section A above and those reported by Jana (1973).
References:
Faris, D. G., C. O. Qualset and C. W. Schaller. 1966. Effect of a dominant spotting gene on yield of barley. Barley Newsletter 10:99-100.
Jana, S. 1973. Factorial genetic analysis of a two-locus system in barley. Can. S. Genet. Cytol. 15:473-482.
Jinks, J. L. and B. I. Hayman. 1953. The analysis of diallel crosses. Maize Co-operation Newsletter 27:48-54.
Qualset, C. O., C. W. Schaller and J. C. Williams. 1965. Performance of isogenic lines of barley as influenced by awn length, linkage blocks, and environment. Crop Science 5:489-494.
Seyffert, W. 1966. Die Simulation quantitative Merkmale durch Gene mit biochemisch definierbarer Wirking. I. Ein einfaches Modell. Zuchter 36:159-163.
