vi
also opened the way for the targeted exploitation of the genetic diversity in the
Triticeae tribe for wheat improvement, for which Sears coined the term “chromo-
some engineering” in 1972.
The species related to wheat exhibit very great genetic diversity, which can be
exploited by breeders. A large proportion of wheat relatives from the Triticeae tribe
can be crossed with wheat, and chromosome segments carrying genes coding for
specifi c traits can be incorporated into wheat by means of backcrossing or the more
sophisticated cytogenetic methodologies of chromosome engineering, depending
on the degree of relatedness between the genomes of wheat and of the alien species
concerned. To date, numerous genes, particularly for resistance to diseases, have
been transferred into wheat from rye and from various species in the Triticum,
Aegilops and Thinopyrum genera and other perennial Triticeae, as well as from the
Dasypyrum species. However, considering the extant genetic diversity, only a frac-
tion has been used, and a huge reservoir of unexploited genes and alleles for a wide
range of traits remains to be tapped. Another poorly explored resource is that of
existing wheat cultivars known to benefi t from alien introgessions for various
favourable traits, in which, however, the nature of the introgressed chromatin and
the genes underlying the alien traits are still unknown.
Each new technique developed in the twentieth century gave renewed momen-
tum to work on interspecifi c and intergeneric crosses. From the 1920s onwards, the
elaboration of cytogenetic methods for chromosome staining enabled the chromo-
some number of hybrids to be determined and chromosome pairing to be traced.
Improvements in in vitro tissue culture methods, including embryo rescue, enabled
new, previously unsuccessful, hybrid combinations to be developed. Methods
became available to control pairing between only partially homologous (homoeolo-
gous) chromosomes of wheat and of the related species, and to induce chromosome
rearrangements. The Giemsa chromosome banding technique, developed in the
1970s, solved the problem of identifying individual chromosomes of wheat, rye
and, later, of an increasing number of Triticeae species, enabling the presence of
alien chromosome(s) in hybrids and in their progeny to be traced. The application
of molecular cytogenetics (fl uorescence in situ hybridization [FISH] and genomic
in situ hybridization [GISH]) allowed the more accurate identifi cation of chromo-
somes and chromosome segments from different species and the pinpointing of
wheat-alien translocation breakpoints. The recent progress in molecular biology
and genomics makes the development of impressive numbers of DNA markers pos-
sible, thus facilitating the detailed characterization of genetic diversity, the mapping
of genes of interest, the identifi cation and characterization of introgressed chroma-
tin, and its traceability in cross progenies.
Despite these advances, alien gene transfer and introgression breeding remain
empirical to a large extent, and more research is needed to allow the targeted trans-
fer of genes and alleles of interest and the effi cient production of improved wheat
lines, without compromising other traits. Poor knowledge of the genome structure
of wild relatives may result in unexpected and undesirable outcomes of hybridiza-
tion and recombination between wheat and alien chromosomes. The mechanisms
controlling meiotic recombination and the determination of the position of recom-
Preface