III. 1. A proposal for marker facilitated intercultivar gene transfer in spring barley.
J.D. Franckowiak, Agronomy Department North Dakota State University Fargo, North Dakota, 58105, U.S.A.
The genetic resources currently available in barley could reduce the cost and time required to place specific genes into locally adapted breeding material. A genetic marker closely linked to a specific desirable gene can facilitate identification and manipulation of that chromosome segment in segregating materials. Hockett (1981) has demonstrated that two closely linked markers can be backcrossed into a new genetic background together even though only one was screened for regularly. A proposal utilizing closely linked genes and backcrossing to replace a specific gene in elite germplasm is outlined and discussed below.
Procedures:
Steps in the genetic marker facilitated gene transfer include: one,
selection of a suitable genetic marker which maps close to the desirable
gene; two, incorporation of the marker into an adapted cultivar; and three,
replacement of the marker gene with its normal allele and the closely linked
desirable gene. To demonstrate this approach to gene transfer, a brachytic
dwarf mutant, br, and resistance to wheat stem rust, Rpg1,
incited by Puccinia graminis tritici, were selected.
The br locus is on the short arm of chromosome 1 and within 15 crossover
units of the Rpg1 locus (Brookins, 1940; Franckowiak, 1986; Tsuchiya,
1984). The br mutant was chosen because the recessive genotype confers
a phenotype which has acceptable vigor and can be identified in most environments.
To illustrate the sequence of crosses required for gene transfer, the symbols
"b" and "A" are used to represent the br and Rpg1 genes,
respectively. Normal plant height and susceptibility to P.g. tritici
would then be "B" and "a", respectively.
During the gene transfer process, gene b from the genetic stock is backcrossed into the recurrent parent, Figure 1.Then, the b-a chromosome segment of the modified line is replaced by the B-A segment from the donor line, Figure 2. The two series of backcrosses outlined in Figures 1 and 2 do not need to be totally sequential. The incorporation of the B-A segment from donor line may be initiated after 3 or 4 backcrosses for marker substitution. Since recombination can occur at a low frequency in the B-A segment, the second series of crosses should be conducted in 4 to 6 separate lines. Using spring barleys and growing three generations per year, the 10 to 12 crosses required for gene replacement could be completed in less than six years.
Discussion
Backcrossing a desirable dominant gene into a cultivar is the most efficient method of modifying elite materials (Briggs and Allard, 1953; Burton, 1981). However, the backcross or modified backcross methods of plant breeding have not been employed extensively for gene transfer because desirable genes are often difficult to recognize in segregating material. Screening for some traits can be expensive or quantitative expression of a major gene can prevent its identification in heterozygotes (Simmonds, 1979). Exploitation of linked markers can eliminate during the gene transfer process frequent screening of segregating material for the desirable trait. The desirable phenotype needs to be identified only after homozygous modified lines are selected. In the case of the Br-Rpg1 (B-A) segment, over 50% of the lines should be homozygous for resistance to P.g. tritici. The probability of recombination between the Br and Rpg1 genes is less than 7.5% in each segregating generation.
Since the two sets of modified lines produced, one from each backcrossing series, are nearly identical to the recurrent parent, evaluation of the desirable gene in near isogenic lines is possible. If the desirable gene has no adverse effects, the modified lines could be used for rapid conversion of breeding materials to homozygousity for the B-A segment.
Using linked markers for manipulation of a specific desirable gene is not a new concept, but one that has seldom been taken advantage of during the development of improved crop cultivars. However, barley breeders could choose the technique for several traits. Transfer of the gene for wheat stem rust resistance, Rpg1, is illustrated above. Other resistance genes which are closely linked to acceptable markers include the Rh, Rh3, and alleles for resistance to Rhynehosporium secalis (Dyck and Schaller, 1961), the Reg6 gene for resistance to Erysiphe graminis hordei (Jorgenson, 1978), and the Ryd2 gene for resistance to barley yellow dwarf virus (Schaller, 1976). The Rh locus for scald reaction is closely linked to the lzd gene for dwarf plant and the Ryd2 locus is closely lined to sld gene for dwarf plant (Takahashi, 1984). Both groups of linked loci are on chromosome 3. The Reg6 gene for powdery mildew reaction is closely linked to the bl gene for non-blue aleurone on chromosome 4. Marker facilitated gene transfer is not limited to genes for resistance. For example, the Bmy1 gene for -amylase is closely linked to the yh gene for yellow head and is near the Reg6-bl segment on the short arm of chromosome 4 (Nielson et al., 1983).
Utilization of the marker facilitated gene transfer technique by barley
breeders will be limited by several factors. One, crosses need to be made
nearly every generation. Two, the transferred B-A segment may have deleterious
phenotypic effects in its new genetic environment. Three, few desirable
traits have been studied extensively and their controlling elements located
on the barley linkage maps. And four, acceptable markers are not scattered
uniformly across the seven barley chromosomes (Tsuchiya, 1984). Resolution
of these problems will require additional research on desirable genes and
genetic markers. Markers suitable for gene transfer include dwarfs, non-lethal
chlorophyll mutants, and male steriles. Presumably, an array of marker
genes could be identified so that any newly mapped gene is within 15 crossover
units of an acceptable marker. The male sterile mutants are not only acceptable
markers, but they could also facilitate crossing, serve as markers for
linkage studies (Haus, 1984), and aid in mapping new genes (Eekhoff and
Ramage, 1984).
References:
Briggs, F.N. and R. W. Allard. 1953• The current status of the backcross method of plant breeding. Agron. J. 45:131-138.
Brookins, W.W. 1940. Determination of linkage relationship of factors differentiating reaction to stem rust in barley crosses. Ph.D. Thesis, University of Minnesota, St. Paul.
Burton, G.W. 1981. Meeting human needs through plant breeding: Past progress and prospects for the future. In K.J. Frey, ed. Plant Breeding II. Iowa State Univ. Press, Ames. pp. 433-465.
Dyck, P.L, and C.W. Schaller. 1961. Association of two genes for scald resistance with a specific barley chromosome. Can. J. Genet. Cytol. 3:165-169.
Eckhoff, J.L.A. and R.T. Ramage. 1984. Assignment of a short awn mutant to chromosome 4. Barley Genet. Newsl. 14:20-21.
Franckowiak, J.D. 1986. BGS_. Resistance to Puccinia graminis Pers. f, sp. tritici Eriks. & Henn. Barley Genet. Newsl. 16 (to be published).
Haus, T.E. 1984. The use of male sterile stocks in linkage analysis. Barley Genet. Newsl. 14:55-56.
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Nielsen, G., H. Johansen and J. Jensen. 1983. Localization of barley chromosome 4 of genes coding for beta-amylase (Bmyl) and protein 2 (Pazl). Barley Genet. Newsl. 13:55-57.
Schaller, C.W. 1976. BGS 0123. Resistant to barley yellow dwarf virus. (BYDV). Barley Genet. Newsl. 6:123.
Simmonds, N.W. 1979. Principles of Crop Improvement. Longman, New York.
Takahashi, R. 1983. Coordinator's report. Chromosome 3. Barley Genet. Newsl. 1 3:84-93•
Tsuchiya, T. 1984. Linkage maps of barley (Hordeum vulgare L.). Barley Genet. Newsl. 14:81-84.
