Agriculture Reference
In-Depth Information
Cross incompatibility resulting from failure of pollen
grains to germinate and develop pollen tubes is
associated with proteins on the pistil that interact
unfavourably with proteins in the pollen. In some cases
this reaction has been overcome by mixing pollen from
the donor species with compatible pollen from the
female species.
Pollen tubes often fail to reach the ovary (or 'miss' the
ovary) due to the physical differences in style lengths
between the different species. This can sometimes be
overcome by mechanically reducing the style length of
the longer style parent. Although this will only be suc-
cessful if the shortened pistils remain receptive (i.e. as
in maize). In the extreme case the complete pistil can
be removed and pollen applied directly into the ovary,
usually requiring
in vitro
techniques.
Once fertilization is achieved the problems are not
necessarily over. When two species differ in ploidy level
it may be necessary to reduce or increase the ploidy of
parents prior to crossing. In potato, potato cyst nema-
tode resistance was identified in
Solanum verni
(a close
relative to cultivated potato,
S. tuberosum
).
S. verni
is a
diploid while
S. tuberosum
is tetraploid.
Two methods of successful hybridization have been
achieved:
between two species may be feasible using protoplast
fusion, followed by regeneration of plants from isolated
wall-less cells (protoplasts). Resulting somatic hybrids
will have the combined chromosome number of both
parents (e.g. as in allotetraploids obtained by inter-
specific hybridization) so it may be necessary to first
reduce the ploidy of parental lines or reduce the ploidy
of hybrid combinations. However, the most difficult
aspects of this technology are:
•
Being able to regenerate plants from protoplasts, even
without fusion
Selecting fused heterokaryons from unfused or self-
fused parental protoplast
•
After fertilization, failure of seeds to develop and/or
to reach maturity can result from embryo and/or
endosperm abortion or failure in the stages of embryo,
or fruit development to complete their necessary stages
to give mature seeds.
Successful fruit and flower retention after fertiliza-
tion can be a simple function of a dependency on
having a sufficient number of developing embryos. In
some interspecific or intergeneric hybrids the number
of fertilized ovules is too low to stimulate mature fruit
development. Growth regulators (e.g. gibberellic acid)
have been used as a means to encourage fruit retention.
It has also been suggested that increasing the frequency
of developing seeds in fruits (by applying a mixture of
compatible and incompatible (mentor) pollen) can be
used to avoid flower or fruit abscission.
Many interspecific or intergeneric hybridizations fail
as a result of post-fertilization factors, which cause
embryo or endosperm abortion. It may be possible to
obtain hybrid plants despite abortion by using
in vitro
techniques such as:
•
Doubling the ploidy level of
S. verni
, using colchicine
and then carrying out interspecific hybridization at
the tetraploid level
•
Producing dihaploids from
S. tuberosum
(using
parthenogenesis) and crossing the two species at the
diploid level. Progeny from the hybrid cross are then
doubled to the tetraploid level using colchicine or by
spontaneous doubling resulting from callus growth
in vitro
A similar manipulation of ploidy in interspecific
crosses in potato was used to introgress late blight
resistance (
Phytophthora infestans
) into cultivated potato
cultivars. The source of blight resistance was found in a
wild relative of cultivated potato (
S. demissum
), which
is a hexaploid. A small proportion of tetraploid progeny
can be obtained by crossing dihaploid
S. tuberosum
(see above and in haploid section) with the hexaploid
S. demissum
.
If attempts to obtain hybrid seed by means of sexual
crossing fail, then somatic fusion (fusion of proto-
plast) may seem a realistic possibility. Genetic transfer
Ovule culture, where the complete ovary is removed
from the plant and aseptically transferred to a growth
media chosen where the nutrients are therefore pro-
vided and thus seed development proceeds
•
•
Embryo rescue, where immature embryos are excised
from the ovary and transferred to growth media;
the chosen media therefore replaces the natural
endosperm (which may have aborted or have been
about to abort)
Sometimes a combination of both techniques is nec-
essary to achieve interspecific hybrid seed. Early in the
