Agriculture Reference
In-Depth Information
next step is to screen other lines that are not as adapted,
in an attempt to identify expression of the desired trait
in them. If the character cannot be identified within
this wider germplasm source then breeders spread their
search wider and will screen related species in an attempt
to find a natural genetic source.
Successful interspecific or intergeneric hybridization
should therefore be considered when:
Other traits that have been transferred through inter-
specific or intergeneric hybridization include abiotic
stresses, drought tolerance, heat tolerance and salinity
tolerance. Examples of enhanced yield or quality charac-
ters from wild relatives into crop plants, not surprisingly,
are very rare.
Factors involved in interspecific or
intergeneric hybridization
The desired expression of a character of interest is not
available within the gene pool of adapted genotypes,
or their unadapted counterparts from the same species
•
In order to make a successful hybrid involving two
different species, there needs to be some degree of
compatibility between the parents used. A number of
factors need to be addressed to ensure successful gene
introgression.
The first stage, which must be overcome, is that the
male and female gametes from the different genotypes
must unite to form a zygote. Failure at this stage can
result from:
•
Acceptable expression for this trait has been shown to
exist within a related species or genera
•
It is possible to introgress alleles from the related
species into the cultivated species
Successful interspecific crossing depends on two
factors: obtaining viable seeds from plants in the F
1
(and
later generations) and eliminating undesirable charac-
ters from the donor species. One, or both, of these
factors may be the major determining factor in the
actual success in gene transfer between species by this
approach (see later for the possibilities using genetic
transformation).
Inability of pollen grains to germinate on the receptive
stigma of the female parent
•
Failure of pollen tubes to develop successfully and
grow down the style or non-attraction of pollen tube
towards the ovary
•
Characters introduced to crops from wild
related species
•
Inability of male gametes that do reach the embryo
sac to actually fuse with the egg cell
•
Inability of the nuclei from pollen and egg to fuse
A high proportion (over 80%) of genes introduced to
our crop species through interspecific or intergeneric
hybridization relate to pest and disease resistance. This
trend continues today whereby wild related species to
our crop species are continually being screened and
evaluated to identify new genes for resistance to crop dis-
eases. Resistance to grassy stunt virus was introgressed
from
Oryza nivara
to cultivated rice. A number of
late blight resistance genes have been transferred from
Solanum demissum
into potato cultivars. In addition
most new potato cultivars released in the EU contain
the H
1
gene conferring resistance to potato cyst nema-
tode (
Globodera rostochiensis
) transferred from
S. verni.
Cabbage seedpod weevil resistance has been transferred
to rapeseed through intergeneric hybridization between
Brassica napus
and
Sinapis alba.
More recently genes
conferring resistance to Hessian fly, a major insect pest
of wheat in the US, have been transferred from
Aegilops
tauschii.
All of these aspects are related to fertilization bar-
riers, and a number of techniques can be used to
overcome incompatibility at each stage.
In vitro
fertiliza-
tion (i.e. using excised organs) can sometimes be used
to overcome some incompatibility factors involved in
the first two barriers listed above.
Success in interspecific hybridization may be unidi-
rectional (i.e. style length differences that cause pollen
tubes to fail to reach ovary) and these can be overcome
by attempting the reciprocal cross. Therefore successful
hybrids might be possible from the mating A
×
B but
difficult, or unsuccessful, when tried as B
A. A good
example of this is seen in the cross
Brassica napus
×
×
B. oleracea
that will produce viable hybrid seed if the
cross is carried out in this direction (i.e.
B. napus
as
female). If, however,
B. oleracea
is the female, then very
few or no seed is produced (without using tissue culture
techniques).
