30
economically important genes that might be exploited for the creation of potentially
new variation in domesticated wheat. Many of these species were crossed with each
other and consequently, many inter-generic and inter-specifi c hybrids were pro-
duced in this group. These hybrids, most of which were viable, have been used for
a variety of purposes including genomic analysis, studies of speciation , phylogeny
and evolution, and as the starting point for efforts at the introduction of alien varia-
tion into domesticated wheat.
2.3 Diffi culties in the Classifi cation of the Wheats
The classifi cation of the species of Amblyopyrum , Aegilops and Triticum is compli-
cated because of the following reasons: (1) Allopolyploidy —several species contain
genome s from two different genera e.g., bread wheat contains one Triticum genome
and two Aegilops genomes; (2) Occurrence of domesticated and wild forms in one
biological species; (3) The group is relatively young with possibilities for inter-
specifi c and inter-generic hybridization leading to introgression , formation of
hybrid species (e.g., Ae. sharonensis ) or allopolyploidy. These complications raise
the following problems: (1) Defi nition of the genera (if to keep Amblyopyrum ,
Aegilops and Triticum as separate genera or to combine them into a single genus);
(2) How to regard the wild and domesticated forms of one biological species ( taxo-
nomic vs. biological species); (3) How to regard new genera or species of inter-
generic or inter-specifi c synthetic allopolyploids; (4) Nomenclature of synthetic
allopolyploids (inter-generic and inter-specifi c mixoploids).
Following Eig ( 1929a ), Hammer ( 1980 ), and Van Slageren ( 1994 ), who based
their taxonomical classifi cation on the obvious morphological differences between
these genera, Amblyopyrum , Aegilops and Triticum are regarded as three separate
genera in this review. Nonetheless, it is important to stress the fact that these three
genera are very closely related to each other.
The concept of biological species has a cytogenetic and evolutionary (bio-
systematic) meaning which has a great relevance when genetic resources are considered.
On the other hand, taxonomic species are relevant when classifi cation and nomencla-
ture are considered. On this basis, and since this chapter is written from a genetic and
evolutionary approaches, the biological species concept is adopted. Consequently, and
following Mac Kay ( 1966 ) and Van Slageren ( 1994 ), the wild and domesticated forms
are included as subspecies in one species, even though they are under different selec-
tion pressure and consequently, different evolutionary direction.
2.4 Evolution of the Diploid Species
The 13 diploid species of the wheat group (the genera Aegilops , Amblyopyrum , and
Triticum ) are presented in Table 2.5. These species are differentiated from each
other in morphology, having unique and specialized dispersal unit, namely, wedge,
M. Feldman and A.A. Levy