THE GENETICS AND PLANT BREEDING OF ALLIUM CROPS - Onions and other Vegetable Alliums - page 67

Table 3.1. The outcomes when a male-sterile onion is pollinated by the various
possible male-fertile genotypes.

Mother plant Pollen donor F 1 Offspring (only the third combination

(male-sterile) (male-fertile) results in all male-sterile offspring)

Smsms NMsMs All SMsms, male-fertile

Smsms NMsms SMsms, male-fertile; Smsms, male-sterile

Smsms Nmsms All Smsms, male-sterile

Smsms SMsMs All SMsms, male-fertile

Smsms SMsms SMsms, male-fertile; Smsms, male-sterile

Plants with male sterility-inducing S cytoplasm and fertility-restoring N
cytoplasm can be distinguished by various molecular markers from the DNA in
their chloroplasts (Havey, 2002). One of these - the ability to amplify a section
of chloroplast DNA specific to N cytoplasm using the polymerase chain
reaction (PCR) supplied with appropriate oligonucleotide primers - has been
developed into a rapid test to distinguish N- from S-type cytoplasm. To establish
a plant's cytoplasm type by conventional cross-breeding would take 4-8 years.
These oligonucleotide primers are now used quickly and cheaply to classify
cytoplasms for maintainer and male-sterile breeding lines. They are also used
for quality control of hybrid onion seed-lots, all of which should have S-type
cytoplasm (Havey, 2002).
Analogous systems of male sterility, conditioned by the combination of a
cytoplasmic factor and a major gene factor, have been found in Japanese
bunching onion and chives (Havey, 2002); in both these crops the system has
been exploited to develop hybrid cultivars. In chives, the cytoplasmic sterility
factor has been shown to be carried by the mitochondria. In leeks, genes for male
sterility have been found, but a cytoplasmic male sterility factor has not been
identified. The T-type CMS of onion has been transferred to leek in experimental
hybrids between the two species and normal, vigorous growth of offspring with
the onion T cytoplasm and leek chromosomes was observed, provided chromo-
some 1C from onion was also present (Peterka et al ., 2005). A vigorous male
sterile line that was easily propagated from bulbils formed in the umbel was
crossed with several cultivars of leek to produce high-yielding hybrids (Smith and
Crowther, 1995). Variability in plant weight in these hybrids was about half that
in the parent varieties, mainly because weak, selfed seedlings cannot occur. F 1
hybrid cvs of this type are growing in popularity because of their improved
uniformity and higher yields (de Clercq and van Bockstaele, 2002).

TECHNIQUES OF BREEDING AND CULTIVAR IMPROVEMENT

Programmes of crop improvement currently used for edible alliums include:
(i) the simple selection of desirable individual plants from a population for use in