2839 Barriers to Interspecific Hybridization
The hybridization procedure starts with pollination of the flower of one species with
the pollen of the other. After gametes fusion/fertilization and then cell division,
seeds of the hybrid are formed containing the embryo and the endosperm for male
and female parents. In the process of producing interspecific hybrids in bread wheat
several problems are confronted; these problems are known to be the barriers. The
nature of a barrier determines the procedure to be used to overcome the barriers.
Producing F 1 hybrids even within the tritceace requires special tools for successful
hybrid production.
9.1 Pre-fertilization Barriers in Wide Crosses
In a natural cross the pollen of the donor male plant has to germinate on the stigma
of the recipient female plant. The pollen and stigma interaction is very specialized
in identifying certain genotypic combinations. Protein in the pollen coat, which is
originally formed by fraternal parent, plays a crucial role in the recognition events
of pollen reception by the stigma on the recipient maternal plant (Baum et al. 1992 ).
The pollen adhesion to the stigma is possible with a cell to cell recognition system
and a compatible combination (Heslop-Harrison 1982 ). Lein in 1943 and later oth-
ers reported that success of wheat and rye hybridization depends highly on the
wheat variety (Lein 1943 ). He demonstrated that the dominant allele of the genes
(Kr1 on the 5B and Kr2 on the 5A) were responsible for problems of compatibility
of cross between wheat and rye. Snape et al. ( 1979 ) reported that the Kr1 and Kr2
are also responsible controlling the compatibility of cross between wheat and bar-
ley. Majority of the wheat varieties possess the dominant Kr1 and Kr2 alleles, the
famous variety Chinese spring possess the rare recessive Kr1 and Kr2 alleles, as a
result this variety can be easily crossed with rye (Baum et al. 1992 ). Other barriers
could be slow pollen tube growth, pollen unable to reach the style, arresting of pol-
len tube in the style and of course differences in ploidy levels.
9.2 Post-fertilization Barriers
Once the pre-fertilization barriers have been taken care of the fertilization can be
restricted by post-fertilization barriers. Post-fertilization barriers are more common
in wheat compared to pre-fertilization barriers (Ishii et al. 2012 ). Usually the more
distant the two species are more abnormalities are seen. The most common post
fertilization barriers are; (i) sterility of the hybrid, (ii) death of the hybrid embryo as
result of endosperm development, (iii) chromosome elimination of one of the
parent.
Breeding and Genetic Enhancement of Dryland Crops