Agriculture Reference
In-Depth Information
mature into pollen grains under normal conditions
(i.e.
in vivo
). The production of haploid plants from
anther culture has been reported for over 200 species of
higher plants. However, although the technique offers
great potential for use in plant breeding programmes the
current examples of its application on a large, practical,
scale are restricted, but some are provided by commer-
cial programmes in: rice; wheat; barley; rye; rapeseed;
tobacco; potato; pepper and maize.
of the parents showed a much greater propensity for
regeneration in culture and this would result in com-
binations with the genes that determined its response,
being represented more frequently in the population
of gametes. In experimental studies it has been shown
that non-randomness of the possible gametic combina-
tions can occur and can be influenced by the culture
protocols used.
Practical applications of haploids
Some potential problems
Progress in evaluating gametic-derived plants under
field conditions has been increasing dramatically;
reports using numerous crops have indicated the impor-
tance of continued research in this area.
However, it has been suggested that developing hap-
loids in a practical breeding scheme will not be as
effective as might be expected. In particular concerns
have been raised regarding:
Genotype dependence
One factor, which has limited the use of anther cul-
ture in practical plant breeding programmes, is that
even the different variants of the protocol often show
strong genotypic dependence. Therefore, if a protocol
is identified which is effective for one genotype that
protocol often needs to be modified (sometimes to a
large extent) to obtain success with another genotype
or, more appropriately, a range of genotypes.
•
The cost of producing haploids
•
The inability to easily produce large numbers of
homozygous lines through haploidy
Somaclonal variation
The techniques noted above involve producing plants
that have been regenerated following
in vitro
culture.
Variation can often be detected among such plants
that are regenerated and this variation has been termed
somaclonal variation. The frequency of such variation
has been suggested as reflecting the occurrence and
length of the callus phase. In a haploid production
scheme it is therefore essential that callus stages are kept
to a minimum so that any somatic variation is kept to
a minimum.
The deleterious variation that is sometimes exposed
as a result of deleterious recessive alleles in the original
material or mutational/somaclonal variation induced
as a result of the
in vitro
techniques
•
The dependence on the genotype of the parental
material used in influencing the frequency of haploids
produced - which often means that the very material
the breeder most wants to use is non-responsive and
haploids are not easily obtained
•
However, as refinements are made in methods and
protocols, it is likely that it will become easier and
cheaper to produce haploids on a routine basis. This
will then mean that their impact on plant breeding
programmes will be larger in the future. To date very
few cultivars have been introduced as a direct result of
haploidy (perhaps China being the exception). How-
ever there is little doubt that these techniques have
added valuable information for plant breeders with
regard to a number of aspects of genetics and tissue
culture.
As noted earlier, one limitation to the widespread use
of doubled haploids among many crops is the inabil-
ity to produce large enough numbers of plants from
Non-random recovery of haploid lines
An important feature underlying the application of
haploids in a plant breeding context is that the pop-
ulation of homozygous lines, derived from the chro-
mosome doubling of the haploids produced, are a
random representative of the gametic array possible.
In other words, the possibility of unconscious selec-
tion occurring (effectively gametic selection) must be
avoided. The genetic combinations recovered from hap-
loid systems may be disproportionately composed of
combinations from one of the original parents that
were used to make the hybrid crosses from which the
anthers were taken. An obvious possibility is that one
