R.M. Trethowan, R.J. Peña, and M. van Ginkel

Introduction

Wheat breeders use grain or flour protein (FP%), sedimentation (SDS), and high molecular weight (HMW) glutenin subunit information to truncate their breeding populations early in the breeding process. This allows them to better utilize resources by testing genotypes in the later generations that have release potential. However, it is not clear what intensity of selection for FP%, SDS, and or the HMW glutenin subunits will provide breeders with a reasonable probability of selecting genotypes in advanced generations with the required grain quality and high yield potential. This paper examines the optimal balance between selecting for grain quality using easy-to-measure characters and maintaining germplasm with high yield potential.

Materials and Methods

A total of 1,267 bread wheat genotypes were grown in replicated trials at CIMMYT's research station in northwestern Mexico (27°N 109°W, 40 masl) during 1994-95 and 1995-96. Grain yield was measured per plot and analyzed using SAS to produce means for each genotype. A 1-kg sample of seed of each genotype was taken from the first replicate of each trial for grain quality analysis. The following grain quality parameters were measured for each genotype: Grain protein (GP%), flour protein (FP%), SDS-sedimentation (SDS), alveogram strength (ALW), alveogram tenacity/extensibility ratio (ALP/L), bread loaf volume (LV), and mixing time (MIX). High molecular weight (HMW) glutenin subunit composition, which is controlled by the Glu-1 complex loci in chromosomes 1A, 1B and 1D, was determined by SDS-PAGE.

The effect of truncating populations for FP% and SDS, and the ramifications for grain quality and yield were examined by calculating their impact, at various selection intensities, on the top 10% of individuals for ALW, P/L, LV, and grain yield.

Flour protein as a predictor of grain quality and yield If an arbitrary selection intensity of 50% is utilized for FP%, then the likelihood of selecting lines among the top 10% for LV is high (90%) (Figure 1). This likelihood drops to 75% for ALW and 52% for ALP/L at the same selection intensity as for FP%. However, the most dramatic consequence is the small number of lines among the top 10% for grain yield selected. Less than 20% of the highest yielding group are retained at the 50% selection intensity.

SDS as a predictor of grain quality and yield When SDS is used to examine the outcomes on grain quality and grain yield at the same selection intensity of 50%, a much stronger association with ALW was noted (Figure 2). The likelihood of obtaining lines in the top 10% for this character is 90%. This relationship does not change significantly as the selection intensity is further relaxed. The probability of obtaining lines ranking in the highest group for ALP/L and LV at the 50% selection intensity are 72% and 50%, respectively. Unlike FP%, there appeared to be no association between SDS and yield as the selection intensity of 50% identified slightly less than 50% of the highest yielding genotypes.

SDS/FP% as a predictor of grain quality and yield In order to correct for possible associations between SDS and FP% among some genotypes, SDS was divided by FP% and the ratio was used to examine changes in selection intensity (Figure 3). At the 50% selection intensity for SDS/FP%, the probability of obtaining the best lines for ALW, ALP/L, grain yield, and LV were 75%, 72%, 60% and 56%, respectively. Interestingly, this ratio better predicted the high yielding genotypes as 60% of the best were identified at the 50% selection intensity.

Change in the frequency of some HMW glutenin sub-unit combinations with selection for improved ALW, ALP/L, and LV When Glu-A1 allelic variations were compared, the frequency of lines containing sub-unit 1 increased with increasing ALW (indicated by the higher frequency classes) (Figure 4). In contrast the 2* subunit was relatively evenly distributed across frequency classes. A similar pattern appeared when the subunit combinations 2*, 7+9, 5+10 and 2*, 17+18, 5+10 were compared for ALW (Figure 5). The frequency of genotypes containing 17+18 increased significantly with increasing ALW. At the Glu-D1 locus there was a significant decrease in the frequency of lines carrying subunit 2+12 with increasing ALW (Figure 6). The 5+10 subunit was evenly distributed across the frequency classes. The effects of varying HMW-glutenin subunit composition in this way upon ALP/L were considerably smaller (data not shown). The Glu-A1 subunit 1 was more frequent with lower ALP/L (more extensible doughs) as was Glu-B1 subunit 17+18. There was no significant change in the frequency of Glu-D1 subunits with lower ALP/L. Similarly, subunits 1, 17+18, and 5+10 were the primary influences on LV differences.

Effects on grain quality and yield of removing sub-optimal HMW glutenins from populations already truncated for SDS/FP% Grain quality improved when genotypes containing either Glu-A1 subunits 2*or 0, Glu-B1 subunit 7+9, and Glu-D1 subunit 2+12 were removed. As the ratio SDS/FP% gave the best resolution for the selection of grain quality and grain yield, it was decided to study the removal of genotypes carrying these sub-optimal HMW-glutenin subunits prior to truncating the populations on the basis of SDS/FP%. The results indicated that the probability of selecting genotypes with high ALW and ALP/L remained high (Figure 7). However, removing genotypes containing the 2*/0, 7+9, 2+12 combination lowered the capture of lines yielding among the top 10% from 60% (Figure 3) to 40%.

The best resolution of selection for grain quality and yield was obtained from the ratio SDS/FP%. This ratio is weighted against genotypes producing high SDSs primarily on the basis of their high FP%. As FP% is influenced more by environmental factors than SDS, this ratio improved the heritability of selection. If the top 50% of lines are retained on the basis of this ratio, then estimates of the percentage of genotypes maintained with strong dough, good dough extensibility, and high grain yield are better than those estimated for SDS alone.

When lines carrying the sub-optimal band combinations 2*, 7+9, and 2+12 are removed prior to selection using SDS/FP%, there is a significant reduction in the number of high yielding genotypes maintained in the top 10% of lines for grain yield. This reflects the high yielding nature of many lines containing the 1B/1R translocation. Although generally poorer in grain quality, a significant proportion of 1B/1R carrying lines have good dough properties, as witnessed by the 20% reduction in yield once the 2*, 7+9, and 2+12 combinations were removed (Figures 3 and 7).

Published on April 2001

August, 2004