85these species cannot be achieved by homologous recombination (Friebe et al. 1996 ;
Molnár-Láng et al. 2014 ).
One of the major obstacles to interspecifi c breeding is the incompatibility
between species, as the male and female gametes from different species are unable
to unite to form a zygote. There may be many reasons for this, for instance: the
inability of pollen grains to germinate on the receptive stigma; the failure of pollen
tubes to grow successfully in the style; non-attraction of the pollen tube to the ovary;
or the inability of the male gamete to fuse with the egg cell. In many cases embryo
or endosperm abortion occurs after successful fertilisation. To avoid this problem,
in vitro techniques such as ovule culture or embryo rescue are applied (Brown and
Caligari 2008 ). It often happens that the F 1 plants from interspecifi c crosses are
sterile. Many methods have been introduced to overcome incompatibility between
the species and achieve successful hybridisation. These include doubling the ploidy
level, protoplast fusion, etc. Colchicine treatment is usually applied to F 1 hybrids to
induce chromosome doubling in order to produce fertile amphidiploids (triticale,
synthetic hexaploid wheat, etc.).
The use of a genome homozygous for the crossability alleles ( kr1kr1kr2kr2 ) may
contribute to higher seed set when wheat is crossed with rye , barley , etc. The reces-
sive crossability allele kr1 was transferred from the spring wheat cultivar Chinese
Spring (CS) into the winter wheat cultivar Martonvásár 9 (Mv9) by backcrossing
Mv9 × CS hybrids with Mv9. As a result of fi ve backcrosses with Mv9 and two
selfi ngs after each backcross, the selected progenies had over 50 % seed set with rye
when tested on a large number of individual plants (Molnár-Láng et al. 1996 ).
In the case of wild relative × wheat crosses the pairing homoeologous gene, Ph1 ,
suppresses the pairing and recombination of wheat and alien chromosomes , so no
alien genetic transfer can occur. However, in plants nullisomic for the Ph1 gene, and
in ph1b mutant stocks having a large deletion at the Ph1 locus, homoeologous wheat
and alien chromosomes can pair and recombine. An original ph1b mutant stock in
the Chinese Spring background, which has poor agronomic characteristics, was
transferred into an adapted Kansas winter wheat, which was released as
KS12WGGRC55. This new germplasm will accelerate the evaluation and utiliza-
tion of wheat alien recombinants in cultivar improvement (Friebe et al. 2012 ).
3.4 New Approaches to Widen Genetic Variation in Wheat
Breeding
Wheat improvement in the twentieth century was carried out using traditional
breeding methods. However, genetic, pathological, physiological and chemical
knowledge has signifi cantly contributed to a better understanding of this crop.
Selection for better adaptability has become particularly important in the light of
climate change during the last two decades, especially with the increasing frequency
of extreme weather events (Veisz et al. 1996 ). Considerable fl uctuation is observed
not only between regions, but also between years. Price volatility in recent years has
3 Wheat Breeding: Current Status and Bottlenecks