Agriculture Reference
In-Depth Information
sets of possible variants or 'alleles' for each gene, carries four such alleles. This
makes the study of genetics, including the identification of which variants or
alleles occur for each gene, more complicated than for diploids. Chromosome
numbers of 32 (tetraploids) and sometimes 24 (triploids) also occur in chives
and rakkyo, and Chinese chives are commonly tetraploid (Havey, 2002).
In onion, chromosome segments throughout the length of the chromosome
arms can cross over during meiosis whereas, in Japanese bunching onion,
crossover points are restricted to a region close to the centromere on each
chromosome (Havey, 2002). The same restriction of crossing over occurs in leeks
and gives rise to the characteristic cross-shaped patterns in the paired chromo-
somes seen in meiosis (see Fig. 3.2). It is thought that the localization of crossing
over to this short section of the chromosomes in leeks prevents the association of
more than two chromosomes during leek meiosis (Jones, 1990). Otherwise,
association and crossing over between groups of three or even four similar
chromosomes might occur, and thus prevent the normal separation of equivalent
pairs of chromosomes to each daughter nucleus in the later stages of meiosis.
Such irregularities usually result in infertility. Therefore, this localization of
crossing over in leek probably preserves its capacity for fertile pollen and ovule
formation. Irregularities of pairing in meiosis resulting in unbalanced, non-
viable chromosome complements in the germ cells are the cause of infertility in
hybrids between allium species, as has frequently been observed in onion
Japanese bunching onion crosses.
Onion is often used in laboratory cytogenetics teaching because it has a
few, very large chromosomes. These directly reflect its massive DNA content,
which is 107 times more than
Arabidopsis thaliana
, the small plant with a
relatively small quantity of nuclear DNA per cell and a short seed-to-seed
generation time, which has, for these reasons, become the most intensively
Fig. 3.2.
The 32 chromosomes of leek pairing into 16 'bivalents' during meiosis. Note
that crossing over (chiasmata) between bivalents occurs only near the centromere and
not at the ends of the chromosomes, hence the cross-shaped appearance of the
bivalents. Scale bar = 10
m (photograph courtesy of Dr R.N. Jones).
