Agriculture Reference
In-Depth Information
studied 'type' plant for genetic studies. The nuclear DNA content of onion is
nearly four times more than that of barley, the nearest non-allium of a group of
11 major crop species noted by Havey (2002). The DNA content of onion is
equivalent to about 15
10
9
nucleotide base pairs. Eighty three per cent of
this DNA has been shown to be made up of just three repetitive fractions of
DNA, the most common of which is repeated about 21,600 times. The onion
genome consists of the active genes that code for proteins interspersed along
the chromosomes amid much repetitive-sequence, non-coding DNA. The
repetitive DNA is thought to have arisen by duplications of DNA sequences
within chromosomes (Havey, 2002). Large amounts of repetitive DNA and the
consequent large genome size have been correlated with slow growth and a
long generation time (Meagher and Vassiliadis, 2005), features typical of most
alliums, but a study comparing 75 allium species found no general correlation
between genome size and life strategy within the genus (Ohri and Pistrick,
2001).
As mentioned briefly above, several of the edible alliums are reproduced by
vegetative means. Rakkyo, great-headed garlic and, in most instances, garlic are
propagated from bulbs (cloves). All three species can produce inflorescences, but
seed is not set and the inflorescences revert to bulbil or bulb production at
various stages of development, except in the case of a few, recently discovered
fertile garlic clones. Chinese chives flower and produce viable seed but normally,
during egg cell formation, the chromosomes double and a complete set of 32
mother plant chromosomes enters the egg cell. There is, therefore, no
fertilization and recombination, and the outcome is genetically equivalent to
vegetative reproduction - this is termed apomixis. Hybridization occurs in about
10% of the offspring of crosses between cultivars of Chinese chives (Kojima
et al.
,
1991), so a low percentage of normal fertilization and gene exchange can occur.
Pollination, although not usually resulting in fertilization, is necessary for seed
production, since the nutritive seed endosperm tissue fails to form without
pollination (Kojima and Kawaguchi, 1989).
Clones of garlic that can produce viable seed following normal meiosis, pollen
formation and fertilization were discovered in the centre of origin of the crop in
central Asia. Selection from these clones has resulted in improvements in fertility,
seed yield and germination rate, and a reduction in seedling defects (Etoh and
Simon, 2002). Using these clones, genetic studies on garlic are now possible
(Zewdie
et al.
, 2005). Many strains of shallot are also maintained vegetatively, but
in recent years seed-propagated cultivars developed in Israel and The Netherlands
have been of growing importance (Rabinowitch and Kamenetsky, 2002).
Despite the lack of genetic recombination in the vegetatively propagated
crops, they still show great diversity between clones. For example, clones of garlic
