Principles of Plant Genetics and Breeding

478 CHAPTER 27
F 1 hybrid
Backcross 1
Backcross 2
Backcross 3
Backcross 4
Backcross 5
Backcross 6
Self-pollination
Adapted parent Donor parent Progeny % of adapted genotype
No selection ×
0%
No selection
Select
Select
Select
Select
Select
Select
Select
Figure 1 Six backcrosses with marker-assisted backcrossing and selection for heterozygote plus self-pollination with
selection for homozygote results in progeny with less that 1% donor parent plus the desired gene.
Current use of MAS in US wheat breeding
Several traits have been incorporated into advanced wheat breeding lines using MAS. We will focus on two examples that
improve end-use quality and disease resistance. Additional details are available on-line (Dubcovsky & Soria 2005).
End-use quality improvements
Grain protein content is one of the major factors affecting bread-making and pasta quality. In spite of the importance of this
character, progress in breeding for high grain protein content has been slow and difficult for two reasons: (i) most variation in protein
content is due to environmental rather than genetic effects; and (ii) there is a strong negative relationship between grain protein
content and grain yield, so that cultivars selected for high grain protein content tend to have low grain yields. A promising source
of high grain protein content was detected in a survey of the wild tetraploid wheat relative, T. dicoccoides. It was crossed into the
durum cultivar “Langdon” and the responsible gene was mapped on the short arm of chromosome 6B (6BS) (Joppa et al. 1997). This
segment accounted for 66% of the variation in grain protein content observed in a cross between durum wheat and T. dicoccoides.
The same chromosome segment was transferred to hexaploid wheat by R. Frohberg. The chromosome segment carrying
the HPGC gene for high seed protein content from T. dicoccoides can be efficiently manipulated in tetraploid and hexaploid wheat
with microsatellite markers (also known as simple sequence repeats or SSRs). The more useful markers include Xgwm193 and a
cleaved amplified polymorphic marker for the NOR locus (Khan et al. 2000). Results from field trials have been mixed, but generally
indicate an increase in protein dependent upon genetic background and environment (K. Kidwell, personal communication).
Wheat grain end-use properties are affected markedly by endosperm texture. Hard wheat requires more grinding energy
to reduce endosperm into flour and during this milling process a considerable number of starch granules become physically
damaged. Soft wheats, by contrast, produce flours with smaller particles and lower levels of damaged starch. Damaged starch is
of value in yeast-leavened products because in addition to absorbing water, it acts as a substrate for α-amylase and creates a
favorable environment for yeast growth. In contrast, chemically leavened soft wheat products have better texture if they are made
from flour with small particle size, and low water retention capacity. Therefore, hard wheat lines have been selected for high
damaged starch and higher hardness values and soft wheat flour for the opposite parameters. Differences in endosperm texture
are associated with the complementary action of proteins puroindoline A (pinA) and puroindoline B (pinB), which are coded by
genes located on the distal part of chromosome arm 5DS. Most hard wheats possess a glycine to serine mutation in puroindoline
B (allele pinB-D1b) or they are devoid of puroindoline A (allele pinA-D1b) (Giroux et al. 1998). Cultivars carrying these different
mutations differ in their grain hardness and in their milling and baking characteristics. Hard red spring wheats with the pinB-D1b
allele have improved flour yield, milling quality, and loaf volume relative to sister lines with the pinA-D1b allele. Allelic variants
at the Ha locus are available to modulate grain texture in soft wheats. The replacement of the distal part of 5AS by the distal 5A m S
segment from T. monococcum (active pinB-A m 1a and pinA-A m 1a) results in a reduction of the hardness of soft cultivars
(G. Tranquilli and J. Dubcovsky, unpublished). These different alleles have been manipulated to confer particular levels of hardness.
50%
75%
87%
94%
97%
98%
99%
99%