Agriculture Reference
In-Depth Information
Chromosome appearance and behaviour (cytogenetics) in alliums was
described by Jones (1990). The basic chromosome number for this group of
plants is usually eight and therefore, except in unusual cases, the number of
chromosomes that can be seen during cell division is a multiple of eight (see
Table 1.2). In leeks and chives one to three additional small 'supernumerary'
chromosomes sometimes occur. The eight basic chromosomes in onion can be
distinguished by their lengths, the ratios of the two arms either side of the
centromere and dark-staining bands, and on this basis they have been
numbered one to eight (see Fig 3.1; de Vries, 1990). This is important in studies
that map the location in the chromosome where particular genes are located
(see Genetic Linkage Maps, this chapter).
The 32 chromosomes of leek are thought to have originated as a doubling-
up of an original set of 16 in an ancestral plant. Such a plant is termed an
'autotetraploid'. Because such a plant has double the diploid number of
chromosomes each set of chromosomes, instead of carrying the diploid's two
Fig. 3.1.
(a) The chromosomes of shallot,
Allium cepa
, during cell division (mitosis)
at the c metaphase stage, showing the 16 chromosomes each dividing into two
replicates. Scale bar = 10
m (photograph courtesy of Dr R.N. Jones). (b)
Diagramatic representation of onion chromosomes numbered 1C to 8C according
to the agreed naming system (de Vries
et al
., 1990). The numbers at the top are the
relative lengths; the numbers by the break in each chromosome, which represents
the centromere position, are the percentages of the total length forming the short
arms of the chromosomes. The 95% confidence limits for each length or percentage
length are also shown. The dark lines at the ends, and also within some
chromosomes, are the positions of bands shown up by Giemsa staining (from de
Vries and Jongerius, 1988).
