Agriculture Reference
In-Depth Information
This illustrates the important point that resistance, particularly the sort
conferred by a single gene, may break down in the face of new strains of the
pathogen. Disease and pest organisms usually have short generation times and
rapid rates of evolution, factors that favour the early breakdown of varietal
resistance, particularly if introduced on a widespread and continuous basis.
Various degrees of resistance, indicating the influence of a number of genes of
small effect, have also been observed for pink root disease. Such polygenic
resistance may not be as complete as single-gene resistance, but it is less liable
to catastrophic breakdown in the face of a new, virulent strain of the pathogen
than is single-gene resistance.
Allium fistulosum
is highly resistant to pink root as well as to the other
important onion diseases such as neck rot, anthracnose, smut and leaf rot due
to
Botrytis squamosa
(see Table 5.5). In view of this, many interspecific hybrids
between
A. fistulosum
and common onion have been produced and shown to be
resistant to pink root. One or two of these hybrids are grown as bunching
onions, but backcrossing the hybrids to onion to incorporate resistance into an
acceptable bulb onion variety has proved difficult because the hybrids are
largely sterile. This sterility results from various abnormalities in meiosis in the
hybrids, and these have been analysed in considerable detail (Kik, 2002).
In an effort to overcome sterility and therefore to be able to transfer some
of the disease-resistance genes found in
A. fistulosum
to
A. cepa
, some crosses
between the two species have been followed by the deliberate induction of a
doubling of the number of chromosomes. The resulting pairs of homologous
chromosomes from
A. fistulosum
and from
A. cepa
can pair with each other
during meiosis and normal crossing over, leading to viable pollen and egg cells,
can occur (Jones and Mann, 1963). Such plants are termed amphidiploids, an
example of which is the cv. 'Beltsville Bunching' that is grown from seed as a
minor crop in the USA. An amphidiploid from a cross between a shallot and
A.
fistulosum
, which was backcrossed to the shallot, gave rise to the vigorous, pink
root-resistant, shallot-like 'Delta Giant', which is grown for bulbs in Louisiana.
This is a triploid with 24 chromosomes, two sets of eight derived from shallot
and one set from
A. fistulosum
. The oriental Wakegi onion is a diploid hybrid
between shallot and
A. fistulosum
, but it is propagated vegetatively.
Several groups have demonstrated some exchange of genetic material
between homologous chromosomes of the two species in backcrosses of the hybrid
to
A. cepa
(Kik, 2002). More recently, it has been shown that both
A. cepa
and
A.
fistulosum
will cross with the wild species
A. roylei
to produce fertile hybrids.
Allium
roylei
is a diploid with an amount of DNA in the chromosomes that is intermediate
between the two crop species. In a cross between
A. cepa
and an
A. fistulosum
Alium roylei
hybrid to form the three-way hybrid, it was possible to show that
crossing over, and therefore genetic exchange, occurs between homologous
chromosomes from all three species. Moreover, recombination points were
randomly distributed over the chromosomes and not confined to a limited region
near the centromere, as occurs in
A. fistulosum
(Kik, 2002). Thus
A. roylei
can act
