Agriculture Reference
In-Depth Information
showing hybrid vigour, hence the importance of these genes for plant breeding.
In the absence of male sterility, controlled cross-pollination without any self-
pollination can be achieved only by laboriously removing ripening anthers from
a flower head before they shed pollen, and by then transferring the desired
crossed pollen to the stigmas by hand. Such a procedure is too labour intensive
for anything but a few controlled crosses in breeding or experimental work.
In onions, male sterility was first exploited by Jones and Clarke using a
male-sterile specimen of cv. 'Italian Red', which they found in breeding plots at
Davis, California in 1925. Fortunately, when this plant was prevented from
being cross-pollinated, bulbils were produced in the flower head (see Fig. 4.41)
and it could be propagated. Jones and Clarke (1943) published a classic paper
describing the genetics of male sterility and indicating how it could be used to
produce hybrid cultivars. On the basis of these techniques, originally developed
in onions, male sterility has since been exploited in hybrid breeding in several
other important crops.
Male sterility in onion was shown to depend on the combined effects of a
nuclear (i.e. chromosomal) gene and a cytoplasmic factor. The nuclear gene
has two forms, the dominant
Ms
form which, when present, always results in
fertile pollen, and the recessive
ms
form which, if homozygous, may result in
sterile pollen. Homozygous
ms
genotypes cause male sterility only if they are
combined with the cytoplasmic sterility factor S. The cytoplasm of the egg cell
can carry the factor S, allowing
msms
genotypes to express as sterile or it can
carry the factor N, which always results in a plant with fertile pollen,
irrespective of the
ms
and
Ms
allele content of its nuclear genes.
Three genotypes for the male-sterility gene can exist in a diploid onion:
MsMs
,
Msms
and
msms
. Each of these genotypes can occur in cytoplasm carry-
ing either the N or the S factor. The only combination that results in male
sterility is
Smsms
. The cytoplasmic factor (S) is transmitted only via the female
(pistillate) parent in a cross - it cannot be transmitted by pollen.
If a male-sterile mother plant is pollinated by the various possible fertile
pollen donor genotypes the results will be as shown in Table 3.1.
A second source of cytopasmic male sterility (CMS), called T cytoplasm,
was discovered in France and is now used for producing some European and
Japanese hybrid onions. Plants with T cytoplasm are fertile if they have the
dominant fertility-restoring allele A or both of two dominant alleles C and D for
two complementary genes that act together to restore fertility (Havey, 2002).
More recently, cytoplasmic male sterility has been introduced into onion by
crossing with the wild-species
A. galanthum
. Fertility can be restored by a
dominant allele at one gene locus and it has been shown that this is not the
same restorer gene as for the S type CMS detailed above (Havey, 2002; Kik,
2002). These alternatives should avoid dependence on a single genetic source
of CMS in hybrid onion breeding. This reduces the risk of disease susceptibility
linked to CMS in all hybrids, such as occurred for southern corn blight in
hybrid maize in the 1960s.
