Principles of Plant Genetics and Breeding

Introduction
BREEDING WHEAT 477
Industry highlights
Bringing genomics to the wheat fields
K. A. Garland-Campbell, 1 J. Dubcovsky, 2 J. A. Anderson, 3 P. S. Baenziger, 4 G. Brown-Guedira, 5
X. Chen, 1 E. Elias, 6 A. Fritz, 7 B. S. Gill, 8 K. S. Gill, 9 S. Haley, 10 K. K. Kidwell, 9 S. F. Kianian, 6
N. Lapitan, 10 H. Ohm, 11 D. Santra, 9 M. Sorrells, 12 M. Soria, 2 E. Souza, 13 and L. Talbert 14
1 USDA-ARS Wheat Genetics, Quality, Physiology, and Disease Research Unit, Washington State University, Pullman, WA
99164, USA; 2 Department of Agronomy and Range Science, University of California at Davis, Davis, CA 95616, USA;
3 Department of Agronomy and Plant Genetics, University of Minnesota, Twin Cities, St Paul, MN 55108, USA; 4 Department
of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, NE 68583, USA; 5 USDA-ARS Plant Science
Research Unit, North Carolina State University, Raleigh, NC 27606, USA; 6 Department of Plant Sciences, North Dakota
State University, Fargo, ND 58105, USA; 7 Department of Agronomy, Kansas State University, Manhattan, KS 66506, USA;
8 Wheat Genetics Resource Center, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506, USA;
9 Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164, USA; 10 Department of Soil and
Crop Sciences, Colorado State University, Fort Collins, CO 80526, USA; 11 Department of Agronomy, Purdue University,
West Lafayette, IN 47907, USA; 12 Department of Plant Breeding, Cornell University, Ithaca, NY 14853, USA; 13 Aberdeen
Research and Extension Center, University of Idaho, Aberdeen, ID 83210, USA; 14 Department of Plant Sciences and Plant
Pathology, Montana State University, Bozeman, MT 59717, USA
Wheat (Triticum aestivum (L.) em Thell) is well adapted to diverse climatic conditions around the world, and is grown in all
regions of the United States. Wheat genetic improvement was the foundation of the Green Revolution and recent progress in the
area of wheat genomics has been referred to as the beginning of a new Green Revolution. While wheat improvement through
plant breeding still requires a great deal of phenotypic or trait assessment, new knowledge about the wheat genome enables plant
breeders to target their breeding efforts with more precision than ever before.
Triticum aestivum (bread wheat) is hexaploid while T. durum L. (durum or macaroni wheat) is tetraploid. Hexaploid wheat has
three genomes made up of seven chromosomes each labeled 1–7. The genomes are named A, B, and D and closely related homeologous
chromosomes are named with the genome name, e.g., 1A, 1B, and 1D. Durum wheat has the A and B genomes. Genes
are frequently found in multiples, as gene orthologues located on each genome, that are closely related in their structure and
function. Orthologous sets of wheat genes are named with the gene name, the genome name, and the orthologous set designation.
Long and short arms of chromosomes are designated with L and S, respectively. For example, the genes for reduced height
that were fundamental to the performance of wheat cultivars developed during the Green Revolution are named Rht-B1b and
Rht1-D1b and are located on wheat chromosomes 4B and 4D.
Two growth habits are agriculturally important in wheat production, fall-seeded winter wheat and spring-seeded spring wheat.
In warm climates, spring wheat is also seeded in the fall. Winter wheat requires a period of cold temperatures, or vernalization, to
induce flowering. Wheat is classified in world markets according to its growth habit, grain texture (soft or hard), grain color (red or
white) and gluten properties (strong or weak). These characteristics result in specific end-use properties of each class of wheat,
and have led to targeted breeding efforts (e.g., bread versus confectionary uses).
Marker-assisted selection (MAS) is a method of rapidly incorporating valuable traits into new cultivars. Molecular markers, or
DNA tags, that have been shown to be linked to traits of interest are particularly useful for incorporating genes that are highly
affected by the environment, genes for resistance to diseases and pests, and to accumulate multiple genes for resistance to specific
diseases and pests within the same cultivar – a process called gene pyramiding. One of the first wheat cultivars to be developed
using MAS was the soft winter wheat cultivar “Madsen”, released in 1986 by the USDA-Agricultural Research Service (ARS) and
Washington State University. “Madsen” was developed using the isozyme marker from the endopeptidase protein, EpD1b, to
incorporate a gene for resistance to eyespot (Tapesia yallunde) (Allan et al. 1989). Since 1990, detailed molecular maps of wheat
have been constructed that include more than 3,000 molecular markers and several important traits have been associated with
DNA markers. Additional markers can be developed from the 8,000 expressed sequence tags (ESTs) that have been mapped in
wheat. (Maps and references are available on-line (USDA-ARS 2005).)
If selected genes or chromosome segments are incorporated from donor parents into adapted wheat breeding lines for six
backcross (BC) generations using markers to select for the targeted gene from the BC 2 generation, more than 99% recovery of
the adapted recurrent parent is expected (Figure 1) (Hospital et al. 1992). Seven BC plants per generation are adequate to have a
probability higher than 0.99 of recovering a single BC plant with the desired genotype. Linkage drag, which results when undesirable
chromosome regions from the donor parent are carried along during backcrossing, can be reduced by advancing more than
10 plants per cross and selecting for those with higher percentages of the adapted recurrent parent genome.