Principles of Plant Genetics and Breeding

512 CHAPTER 30
A-line
(male-sterile)
[A] rfrf
William Rooney
Texas A&M University, College Station, TX 77843, USA
Table 1 Genotypes and corresponding phenotypes for A-,
B-, and R-lines in the A1 cytoplasmic-genetic male sterility
system in sorghum.
Line Cytoplasm* Genotype† Phenotype
A-line [A] rfrf Male-sterile
B-line [N] rfrf Male-fertile
R-line [A] or [N] RFRF Male-fertile
Hybrid [A] RFrf Male-fertile
* Cytoplasm types: [A], sterility-inducing cytoplasm type; [N],
normal cytoplasm.
† RF is the dominant allele for fertility restoration, and rf is the
recessive allele for fertility restoration.
Producers buy
this hybrid seed
Producers grow
this crop
B-line
(male-fertile)
[N] rfrf
F 1 hybrid
A-line × R-line
R-line
(male-fertile)
[A or N] RFRF
Figure 1 The sorghum seed production process utilizing
CMS. The genetics for each line are described in Table 1.
The A-line parent is increased using pollen from the Bline.
The F 1 hybrid is produced by pollinating the A-line
with an R-line pollinator. Both the B-line and R-line are
maintained through self-pollination. [A], sterilityinducing
cytoplasm; [N], normal cytoplasm.
Industry highlights
Sorghum breeding
Introduction
Sorghum (Sorghum bicolor (L.) Moench) is one of the more
important cereal grain crops in the world. In 2001, sorghum
was produced on approximately 50 million hectares with an
average yield of 1,280 kg/ha worldwide (FAO 2001). It is
commonly grown in semiarid tropical, subtropical, and temperate
regions of the world. The crop is used for many different
purposes. The grain is used as a food grain, feed grain,
and for industrial purposes. In many production systems,
the vegetation is used as forage. The location of production
often defines the ultimate end use and the specific types of
sorghum that will be grown.
Sorghum is grown as a hybrid crop in the USA. Because it
is a predominately self-pollinated crop, hybrid seed production
requires the use of cytoplasmic genetic male sterility
(CMS). Without this system, hybrid seed production would
not be economically feasible. The CMS system is based on
male-sterility-inducing cytoplasm that is complemented by
alleles in the nuclear genome that either restore fertility or
maintain sterility. In the CMS system, lines that have [A]
cytoplasm must have a dominant allele present in the
nuclear genome to restore male fertility (Table 1). If the line
lacks the dominant allele for fertility restoration, the plant
will be male-sterile.
Hybrid seed production requires maintenance of A-, B-,
and R-lines (Figure 1). Seed of a male-sterile A-line is
increased by pollination using the complementary B-line.
The sole purpose of the B-line (also known as a maintainer)
is to perpetuate or maintain the A. The A-line and B-line are
genetically identical except that the A-line has a sterilityinducing
cytoplasm while the B-line has normal fertile cytoplasm.
Thus, A-line plants that are male-sterile can be
pollinated with pollen from B-line plants to regenerate
seed of the A-line. To produce hybrid seed, the male-sterile
A-line is pollinated with pollen from the male-fertile R-line
plants. The R-line (also known as a restorer line) is genetically
very different to the A-line and carries the dominant
fertility-restoration alleles needed to restore fertility in the
progeny of the A-line. The seed that is produced on the
A-line from this pollination is the seed that is planted by
the producer for commercial grain production.
Private and public research
Because sorghum is grown as a hybrid, the hybrids that producers
grow are produced and sold by private industry.
Private industry also maintains a limited number of breeding
programs for the production of new A/B- and R-lines for
new hybrids. This work is supplemented and enhanced by
researchers in public breeding programs, such as those at