117backcrosses the substituted chromosome was mainta ine d in the monosomic condition.
The backcross process was continued up to BC 4 for chromosomes 5A and 5B, but
only up to BC 1 for chromosome 5D. After four backcrosses, monosomic plants were
selfed, and plants with 2 n = 42 were selected from among the progeny. The most
potent compatibility factor was detected on chromosome 5B ( kr 1 ); this allele was
suffi cient to achieve a seed set of about 50 %, depending on environmental factors.
Conversely, the effect of the other two chromosomes, whatever the donor parent,
was hardly detectable; if they have any crossability alleles, these have a low level of
expression, being inhibited by the Kr1 allele (Gay and Bernard 1994 ). The recovery
of the Courtot genetic background during subsequent backcross generations did not
alter the degree of crossability estimated in BC 1.
An intervarietal molecular-marker map was used by Tixier et al. ( 1998 ) for the
detection of genomic regions infl uencing crossability between wheat and rye. The
mapping population consisted of doubled haploid (DH) lines and was produced at
Clermont-Ferrand by anther culture from Courtot × Chinese Spring F 1 hybrids. The
two parents of the DH population, Courtot and Chinese Spring, differed greatly,
with a 95 % success rate for crosses with Chinese Spring, and only 10 % with
Courtot. Testing for the presence of QTLs in the whole genome was carried out
using 110 DH lines, while 187 DH lines were employed to explore the effect of
homoeologous group 5 chromosomes. Analysis of deviance and logistic marker-
regression methods were pe rformed on data from doubled haploid lines. A major
quantitative trait locus (QTL) involved in crossability and associated with the
marker Xfba367 - 5B was detected on the short arm of chromosome 5B (Tixier et al.
1998 ). An additional locus, Xwg583 - 5B , was indicated on the long arm of chromo-
some 5B. This minor QTL might correspond to Kr1 , which was presumed to be the
major gene controlling crossability. Another locus Xtam51 - 7A on chromosome
7A, was signifi cantly associated with this trait. The three-marker model explained
65 % of the difference in crossability between the two parents. The results achieved
by Tixier et al. ( 1998 ) were not consistent, however with those of Lange and Riley
( 1973 ) and Sitch et al. ( 1985 ), who mapped the Kr1 locus on the long arm of chro-
mosome 5B. Tixier et al. ( 1998 ) found a ge ne suppressing crossability, denoted
Skr , on chromosome arm 5BS. The QTL associated with the locus Xfba367-5B on
the long arm of 5B had only a low value. The results could have been infl uenced
by the large phenotypic variance and by the different genotypes used by the vari-
ous authors.
The Skr locus, on the short arm of 5B, was identifi ed as a major QTL (Tixier
et al. 1998 ; Lamoureux et al. 2002 ) inhibiting the crossability of wheat and rye.
Alfares et al. ( 2009 ) used a recombinant SSD population originating from a cross
between the poorly crossable cultivar Courtot (Ct) and the crossable lin e MP98 to
characterize the major dominant effect of SKr and to map the gene at the distal end
of the chromosome near the 5B homoeologous GSP locus. In Courtot SKr had a
stronger effect than Kr1 on crossability of wheat to rye. It is possible that Courtot
carries a weak Kr1 allele. The results confi rm that there is allelic variability among
the different crossability genes at the Kr1 locus. Colinearity with barley and rice
was used to saturate the SKr region with new markers and establish orthologous
4 The Crossability of Wheat with Rye and Other Related Species