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CROP BREEDING, GENETICS & CYTOLOGY

Improvement Strategy for Mature Plant Breeding Programs

^a Dep. of Plant Sci., North Dakota State Univ., Fargo, ND 58105 USA
^b Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108 USA

mpeel{at}ndsuext.nodak.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
In mature plant breeding programs an elite core germplasm forms the backbone of the breeding effort, confronting the breeder with the challenge of maintaining germplasm diversity as genetic distance between the elite core germplasm and donor germplasm widens. This research assessed the effects on grain yield of transferring genes from two-row barley (Hordeum vulgare L.) cultivars to a six-row gene pool. We conducted three cycles of recurrent-type breeding, beginning with crosses involving five two-row donor parents. The crossing scheme led to progenies with theoretical proportions of two-row germplasm of 25 to 50% in Cycle 1, 12.5 to 25% in Cycle 2 and 6.25 to 12.5% in Cycle 3. In Cycle 1, yield of the six-row progeny across the five populations ranged from 86 to 97% of the respective six-row parent. Kernel weight of the six-row progeny was also low in Cycle 1. In Cycle 2, grain yield improved to 99% of the check mean and kernel weight improved to 102% of the check mean. In Cycle 3, sets of lines representing three populations yielded from 112 to 119% of the check mean in 1997 and 1998 and individual lines surpassed `Stander', the highest yielding check in both 1997 to 1998. The highest yielding lines in each cycle were derived from populations having the highest theoretical percentage of adapted recurrent parent germplasm, that is, when six-row gene combinations were predominately left intact. Results from the three breeding cycles support the proposition that a strategy that maintains favorable gene combinations while introgressing relatively small amounts of donor germplasm can lead to incremental yield gains.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
OLDER PLANT BREEDING PROGRAMS routinely have a core germplasm pool. Use of this core germplasm is important as the breeder attempts to create favorable gene combinations. Industry guidelines and market acceptability necessitate a narrow core germplasm for breeding malt barley. Martin et al. (1991) indicated that the malting and brewing industry in the USA has standards for some 22 quality traits. Breeding malting barley in the upper midwestern USA began with a relatively narrow genetic base and the core germplasm has become narrower with time. Horsley et al. (1995) concluded that six major ancestors exist in the Minnesota breeding program and seven in the North Dakota program. The extreme narrowness of successful Midwestern malting barley pedigrees was discussed by Rasmusson and Phillips (1997).

Introgression of genes into the upper Midwestern malting barley gene pool without disrupting quality has been difficult. In a cross to introgress low grain protein, the donor germplasm lowered diastatic power below the acceptable range (Goblirsch et al., 1996). Incorporation of resistance to kernel discoloration required six cycles of breeding to overcome inferior quality and agronomic traits from the donor cultivar (Gebhardt et al. 1992). On the other hand, encouragement for attempting to increase grain yield by introgressing small genomic portions can be found in other crops. In wheat (Triticum aestivum L.) the 1RS.1BL translocation has been credited with enhancing yield and other traits in several genetic backgrounds (Carver and Rayburn, 1994). Introgressing alleles from donor wild species using molecular markers enhanced yield of tomato (Lycopersicon esculentum Mill.) lines (Bernacchi et al., 1998). Following the introduction of unadapted germplasm into elite gene pools of oat (Avena sativa L.) and soybean [Glycine max (L.) Merr.], Murphy and Frey (1984) and Vello et al. (1984) suggested that the value of such germplasm will best be determined following multiple cycles of breeding.

Two-row malting barley from Europe is a potentially valuable germplasm source for six-row barley programs to increase diversity for malting quality, disease resistance, and agronomic traits. Two-row germplasm could enhance yield in subtle ways and by altering the yield component traits. Two-row barley typically has more heads per plant and larger, plumper kernels than six-row barley (Carleton and Foote, 1968). Reports of positive correlations between yield and the components of yield indicate that introduction of diversity for yield component traits can enhance yield (Lambert and Liang, 1952). On the other hand, negative correlations among components, and between components and yield, have been described by Puri et al. (1982) and Rasmusson (1987). Adams (1967) attributed negative correlations among yield components to component compensation and negative association due to competition for plant resources. This hypothesis tends to discourage component breeding.

The overall objective of this research was to assess the effects and benefits of transferring genes from two-row cultivars to a six-row gene pool utilizing three cycles of recurrent-type breeding. The focus was on genetic gain for grain yield. The donor parents were five two-row cultivars and all cycles involved crossing to elite six-row parents. This scheme led to progenies with theoretical portions of two-row germplasm of 25 to 50% in Cycle 1, 12.5 to 25% in Cycle 2, and 6.25 to 12.5% in Cycle 3.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Populations derived from three cycles of recurrent-type breeding were evaluated. The donor parents were two-row barley cultivars `Harrington' from Canada, `Femina' and `Cheri' from Germany, and `Prisma' and `Cebeco 8858' from the Netherlands. Harrington, Cheri, and Prisma were widely grown malting cultivars. The recurrent parents in the three cycle program were Minnesota-developed cultivars Stander and `Excel' and candidate cultivars M66, M72, M81, and M83. At the time of this study they were collectively among the best performing cultivars and lines in the Minnesota barley program.

Selection was conducted differently among cycles. Cycle 1 and 2 lines tested in field trials were chosen at random to represent each population. Here the main interest was the effect of introgressed genes from the two-row donor parents on yield, kernel weight, and head number. Selection in Cycle 3 was conducted similar to a breeding program. The parents for Cycle 3 were selected based on grain yield in Cycle 2, and lines for evaluation were visually selected based on height, maturity, and lodging. Lines in Cycle 3 were evaluated for grain yield only.

Population Development
Cycle 1 Populations
Five populations were used in the initial cycle, including two single-cross, two backcross, and one three-way cross population (Table 1) . They were advanced from the F₂ through the F₄ generation by single-seed descent, followed by evaluation of F_4:6 lines. A stratified random sample (lines 4 d later maturing than Stander were excluded) of 28 six-row and 10 two-row lines were chosen from each population for evaluation.

Cycle 2 Populations
Eight populations were evaluated in Cycle 2 (Table 1). The donor parents were eight randomly chosen lines, two from each of four Cycle 1 populations. All were six-row, assuring that all future progeny would be six-row. Selection of Cycle 2 parents based on grain yield and for the prevailing phenotype of Minnesota barley was purposely avoided to maintain the genetic variability introduced from the Cycle 1 two-row parents. The recurrent parents of the Cycle 2 populations were Stander, M66, and M72 (Table 1). A stratified random sample of 20 F_2:4 lines per population was chosen for evaluation; as in Cycle 1, late-maturing lines were omitted.

The eight Cycle 2 populations were tested together in a sets-in-replications design consisting of four sets in each replicate with three replicates. Each of the four sets was randomly assigned five lines from each population for a total of 40 lines per set. Three check cultivars, `Morex', `Robust', and Stander, were included in each set. The eight Cycle 2 populations were analyzed separately and in a combined analysis of variance. The populations were evaluated in nurseries at Crookston and St. Paul, MN, in 1994 and 1995.

Cycle 3 Populations
The protocol for Cycle 3 was different from that for Cycles 1 and 2 and similar to a traditional breeding effort. Three populations were developed by crossing the line with the highest grain yield in Populations 1.2, 4.1, and 5.2 with Excel, M83, and M81, respectively (Table 1). The populations were advanced from the F₂ to the F₄ generation by single-seed descent. The F₅ generation had 190 lines in each population from which the most attractive lines were selected based on height, maturity, and lodging. Heads from selected F₅ lines provided seed for a winter nursery increase so that F_5:7 lines were evaluated in Minnesota in 1997. Following the 1997 evaluation, a subset of lines with high grain yield and favorable malting quality attributes were chosen for a second year of testing in 1998.

Trait Evaluation
Trait comparisons were made with the parent cultivars in Cycle 1 and with commercial cultivar checks in Cycles 2 and 3. Head number and kernel weight were evaluated in Cycles 1 and 2. Head number was determined on one linear meter within a plot row. Kernel weight was assessed on a 200-kernel sample from each plot. In Cycles 1 and 2, each plot consisted of two rows 3.05 m long with 30 cm between rows. The plots were separated by one row of winter wheat.

Cycle 3 evaluation began in 1997 where grain yield was measured on F_5:7 lines in two-row plots 3.05 m long, the same as in Cycles 1 and 2. The lines were replicated three times and grown at St. Paul and Crookston, MN. About one-half of the lines evaluated in 1997 were evaluated a second time in larger plots in 1998. Plot length was 3.05 m with seven rows spaced 18 cm apart. The 1998 trial included three replicates and was grown at St. Paul, Morris, and Crookston, MN.

An approximate F test (Steel and Torrie, 1980) was used to test significance of genotypic mean squares. Fisher's protected least significant differences (Carmer and Walker, 1982) were used to distinguish differences among Cycle 1 and Cycle 3 line means. In Cycle 2, planned comparisons were made between check and populations means using a single degree-of-freedom t test (Steel and Torrie, 1980).

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Line-within-population variation was significant for yield, head number, and kernel weight in nearly all Cycle 1 and Cycle 2 populations, reflecting the wide nature of the crosses and the corresponding genetic diversity. Significant line x environment interactions occurred. However, in all populations variation due to lines was larger than variation due to interaction (data not shown). Analysis of variance of the Cycle 2 checks from the sets-in-replication design revealed no significant differences among sets for the various traits; thus, comparison of line means within each population was possible. Check means were combined across sets for comparison with lines in each population. In Cycle 3, statistically significant differences for grain yield occurred among lines within each population in 1997 and 1998.

Cycle 1
Grain yield of the individual two-row parents ranged from 79 to 90% of the six-row parents in the five populations (Table 2) . The higher yield of the six-row parents was expected since they were developed locally. Across the five populations, yield of the six-row progeny subsets ranged from 86 to 97% of their respective six-row parent mean (Table 2). The two-row subset yields ranged from 73 to 92% of the six-row parent mean, and were below the yield of their respective two-row parent in four of the five populations. The individual two-row parents averaged 43 to 57% more heads and 5 to 25% higher kernel weight than the six-row parents, making them a promising source of genetic diversity for these traits. The highest yielding six-row lines in the five populations were similar in yield to their respective six-row parents (Table 2), but none were significantly higher.

Based on an additive genetic model, yield means for the six-row and two-row subsets should be intermediate to the parents. This was the case for the six-row subsets, but a marked departure from the additive model occurred for the two-row subsets in the two backcross and one three-way cross population, where the two-row subsets yielded 94, 86, and 84% of their respective two-row parent. Apparently, good yield performance is precluded when the two-row gene is in a largely six-row genetic background, illustrating the impact a single gene can have on yield when in an incompatible genetic background.

All six-row subsets had lower kernel weight than the two- and six-row parents, whereas the two-row subsets had higher kernel weight than either parent (Table 2). The low kernel weight of the six-row progeny was probably caused by the lateral kernels that are routinely small in crosses of six-row and two-row barley (Lambert and Liang, 1952). Mean kernel weight of lines in the two-row subsets exceeded the two-row parents in the five populations by 4 to 21%. The two-row progeny had fewer heads than the two-row parents, and were expected to have heavier kernels on a component compensation basis. Several researchers have found that kernel weight is higher when the other components are reduced (Grafius and Okoli, 1974; Knott and Talukdar, 1971).

The moderately high head number observed in Populations 3, 4, and 5 appeared to be more conducive to high yield of six-row progeny than the much higher head number observed in the six-row progeny of the single-cross Populations 1 and 2 (Table 2). In the single-cross populations, two lines with 49 and 44% more heads than the respective six-row parent were among the lowest yielding.

None of the lines in the five populations, including the two backcrosses and the one three-way cross, were at a level for yield, kernel weight, and maturity required in a midwestern U.S. six-row cultivar. While each of the five populations contained a few lines with acceptable yield, kernel weight was unacceptably low in these higher yielding lines. Relatively high head number in the six-row subsets indicated successful introgression of diversity for this trait in Cycle 1.

Cycle 2
In Cycle 2, mean yield of five of the eight populations was equal to or higher than the mean of the checks (Table 3) . Mean yield for the eight populations was 99% of the check mean, and several lines were numerically higher yielding than the highest yielding check. The Cycle 2 populations with the highest yield were those with a parent from a Cycle 1 backcross or three-way cross population (4.1, 4.2, 5.1). Assuming no selection, these populations had 12.5% two-row donor germplasm, while two of the three lowest yielding populations (1.1 and 1.2) had 25% two-row germplasm. This finding is consistent with other studies (Eaton et al., 1986; Vello et al., 1984) in which mean grain yield increased as the percentage of unadapted germplasm decreased.

High head number was observed in Populations 1.1, 3.1, and 4.1 (Table 3). In population 4.1, 30% of the lines were significantly higher in head number than the check mean. The two populations with the highest head number (3.1 and 4.1) were derived from backcrossing to the six-row parent, M66, which is intermediate in head number, suggesting that genes from the two-row donor parent were maintained through two backcrosses. High head number was recovered in combination with high yield, particularly in Populations 3.1 and 4.1.

Correlation data offered little help in understanding the yield component basis for high yield. Most correlations in the eight populations between yield and the three yield components and among the yield components were not significant (data not shown). Significant positive correlations were found between yield and kernel weight in Populations 1.1 and 4.2 and between yield and head number in Population 5.2. Nearly all correlations between kernel weight and head number were negative, but only three of the eight were significant and none exceeded 0.50.

Cycle 3
Mean yield of the lines in the three populations in Cycle 3 were 112, 119 and 114% of the check mean in 1997 and 116, 114 and 116% of the check mean in 1998 (Table 4) . The yield ranges showed that almost all lines exceeded Morex and Robust in both years. Stander, the highest yielding check, was near the low end of the yield range in 1997 and near the middle in 1998. In relation to the three checks that were common to the Cycle 2 and Cycle 3 trials, sizable yield gains appear to have been made by crossing high yielding Cycle 2 lines with three Minnesota parents (Excel, M83, and M81) (Tables 3 and 4). Using the highest yielding check, Stander, as a reference point for yield improvement, it is evident that the Cycle 3 population subsets afford good opportunity for achieving yield gains. The highest yielding lines from each Cycle 3 population tested in both 1997 and 1998 yielded 113 % of Stander (data not included).

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

Cycle 2 populations, which theoretically contained 12.5 to 25% of two-row germplasm, were improved for the two economically important traits, yield and kernel weight. Improvement in kernel weight was substantial in that all Cycle 1 six-row lines would be rated unsatisfactory, while 42% of all Cycle 2 lines significantly exceeded the check mean and would be rated satisfactory by malting barley standards. Finding high kernel weight in most populations and high head number in three populations indicated that potentially useful diversity for increasing yield was introgressed from the two-row donor sources.

The yield levels recovered in Cycle 3 demonstrate that high yielding lines could be derived via introgression of genes from two-row cultivars. We favor the hypothesis that desirable alleles from the two-row donors interact in a positive way with alleles from the six-row recipients, that is, that the introgression program led to positive yield gains. Furthermore, we hypothesize that relatively small amounts of donor germplasm can have a sizeable impact on progeny performance. The Cycle 3 parents were selected based on high yield, so Cycle 3 lines may contain, more or less than 6.25 to 12.5% donor germplasm, the appropriate estimates based on the absence of selection.

The hypothesis that valuable genes were introgressed is supported by data on kernel weight and head number in Cycle 2 and grain yield in Cycles 2 and 3. Counter to that hypothesis, Rasmusson and Phillips (1997) demonstrated incremental gains in grain yield from crosses between parents wholly within the Minnesota six-row germplasm. Accordingly, the yield gains we observed could be attributable to crossing and recombination within the six-row recipient parent germplasm in the Cycle 3 breeding effort. Accepting the favored hypothesis, we conclude that an introgression strategy that maintains favorable gene combinations while introgressing small amounts of improved germplasm can result in incremental yield gains. In this research, the introgressed germplasm is similar to the cases cited above for wheat and tomato (Bernacchi et al., 1998; Carver and Rayburn, 1994), in that small amounts of donor germplasm apparently had sizable effects. As suggested by Murphy and Frey (1984) and Vello et al. (1984), the value of introduced germplasm often must await completion of several cycles of breeding. In this experiment, Cycle 3 afforded the best opportunity to find improved lines for grain yield.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Contribution of the Minnesota Agric. Exp. Stn. Scientific Journal Series Paper no. 991130115.

Received for publication March 29, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 

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