167genome ). Needless to say, there are few volunteers for this job and for many years
the number of complete sets of wheat–rye addition lines was very low: Chinese
Spring-Imperial, Chinese Spring-King II, Holdfast-King II and that was about it.
Attempts were made to circumvent the gametic selection process. E.R. Sears
recommended avoiding pollen competition by pollination with single pollen grains
but never tried it himself and did not fi nd volunteers to try it for him. Pollen compe-
tition can be minimized by increasing the proportion of pollen with abnormal chro-
mosome constitution: rather than selecting disomic additions from monosomic
additions, progenies of multiple monosomic additions can be screened or the fi rst
backcross can be made using the heptaploid (here AABBDDR) as male in a back-
cross to the amphiploid. In each case the proportion of abnormal gametes (21 + 1
chromosomes) will be much higher, and the proportion of euploid gametes will
much lower, than among those produced by a monosomic addition, and they would
be at a lower competitive disadvantage. Progenies are screened and plants with pairs
of individual chromosomes are selected and grown. After that it is only a matter of
time (generations) before monosomic chromosomes are eliminated, leaving the
selected disomes. This system seriously reduces the required amount of labor but at
the same time demands precise chromosome monitoring as unpaired chromosomes
are susceptible to centric misdivision and may form translocation s. Several sets of
additions were produced in this manner by the author, including BH1146-Blanco
(BH1146 is a spring wheat ; from Brazil known for high resistance to soil alumi-
num), CS-Blanco and CS- Haynaldia villosa (Lukaszewski 1988 ).
A different way of mitigating the effects of gametic competition is via haploidi-
zation. If an assumption is made that the presence of an alien chromosome (here a
rye chromosome) does not affect gamete’s performance in gametogenesis, a quarter
of haploids recovered from a monosomic addition would have to carry the added
chromosome; upon chromosome doubling, disomic additions are expected. The
author is not aware of any serious attempts to generate complete sets of additions in
this fashion; an attempt to generate a specifi c one was unsuccessful, apparently
because the added chromosome did affect gamete’s behavior (B. Friebe pers.
comm.).
There is no general system of keeping track of wheat–rye addition sets available
around the world right now but the number must have grown to at least a dozen. The
author himself created six such sets (CS-Blanco, BH1146-Blanco, Pavon-Salvo,
Pavon-Presto, Henika-Salvo, Henika-ANOAS; the last four using hexaploid tritica-
les as the starting point); recently a set of additions from S. africanum was described
(Lei et al. 2013 ).
7.3 Chromosome Su bstitutions
Single chromosome substitutions are usually the fi rst step in introgressi ons of spe-
cifi c segments of chromatin into the wheat genome. The author is not aware of any
whole chromosome substitutions in commercial wheats, but this may only refl ect
7 Introgressions Between Wheat and Rye