105Crosses between whea t group 5 chromosome substitution lines with three differ-
ent cultivar backgrounds indicated that there may be multiple alleles for reduced
crossability with rye and H. bulbosum on both the 5A and 5B chromosomes (Falk
and Kasha 1983 ). No reduction in seed set was observed for any of the 5D substitu-
tion lines. The Kr1 locus on chromosome 5B was found to have a more pronounced
effect on both rye and H. bulbosum crossability than the Kr2 locus on chromosome
5A, and the effects of the two loci were shown to be cumulative. Tests with tetrasomic
and nullitetrasomic lines of Chinese Spring indicated that the kr allele is “null” or
ina ctive in promoting crossability, while the Kr allele is active in reducing cross-
ability with both rye and H. bulbosum. Thus, extra doses of the kr allele do not
increase rye or H. bulbosum crossability in the prese nce of the corresponding Kr
allele. Th e crossability of the various Kr / kr combinations with rye exhibited a sig-
nifi cant correlation with Lein’s ( 1943 ) rye crossability data. The linearity of the
crossability response with respect to the dose of Kr alleles suggested an additive
genetic system, rather than the complete dominance of the Kr alleles. Genotypes
which carried heterozygous Kr / kr alleles gave better seed set with rye than those
with homozygous dominant Kr alleles. Krolow ( 1970 ) suggested that a Kr3 gene
was located on the D genome , but the effect of this gene could not be confi rmed by
other authors. Substitution lines involving the cultivars Hope, Atlas 66, and
Cheyenne reduced the crossability with rye to various extents, so it was assumed
that these cultivars may have different alleles for reduced crossability at each of the
known Kr loci on chromosomes 5A and 5B, as compared to the allele in Chi nese
Spring 5B (Falk and Kasha 1983 ).
Snape et al. ( 1979 ) found a strong positive correlation between the crossability
of wheat varieties with Hordeum bulbosum and with rye. Crossability with H. bul-
bosum was investigated using chromosome substitution lines of t he non-crossable
variety, Hope, into the crossable variety Chinese Spring. Two chromosomes, namely
5A and 5B of Hope, were found to cause a marked reduction in the crossability of
Chinese Spring with Hordeum bulbosum. It was concluded that the crossability of
wheat with both species was governed by the same genetic system. Fifty-six wheat
( Triticum aestivum L.) genotypes representing wide genetic diversity were screened
for crossability with rye by Falk and Kasha ( 1981 ). The 56 wheats produced seed
set values ranging from 0.0 to 95.4 %. The distribution of percentage crossability
was similar to that published by Lange and Wojciechowska ( 1976 ) for 177 cultivars.
A high proportion had no or very low seed set, while the others were distributed
fairly evenly across the rem aining range. Twenty-eight of the wheat lines, represent-
ing a range of crossability with rye, were then pollinated with tetraploid Hordeum
bulbosum L. (Falk and Kasha 1981 ). The seed set ranged from 0.0 to 32.2 %. Wheats
that set seeds with H. bulbosum generally had medium-to-high crossability with
rye. A high correlation was obtained for the crossability of rye and H. bulbosum
with wheat, indicating that the main genetic system ( Kr1 and Kr2 ) governing cross-
ability with rye is also th e main system operating in crosses with H. bulbosum. Ten
F 1 hybrids were produced by crossing the highly crossable cv. Chinese Spring with
nine other wheat cultivars and the highly crossable cv. Zaragosa 75 with the medium
crossable cv. Songlen. Crossability with rye was generally intermediate between the
4 The Crossability of Wheat with Rye and Other Related Species