40
supported by the fact that only bivalents were formed at fi rst meiotic metaphase of the
polyploids (except that of the autoallohexaploid T. zhukovskyi that has the genomic
constitution AAA m A m GG and therefore produces few quadrivalents at the fi rst mei-
otic metaphase). The diploid-like meiotic pairing pattern, i.e., regular bivalent forma-
tion due to exclusive homologous pairing , is characteristic of allopolyploids.
Since the discovery that the polyploid species of wheat comprise an allopoly-
ploid series attempts have been made to identify the diploid donors of the B and D
genome s to allopolyploid wheat. In this endeavor studies were extended to the wild
relatives of wheat, particularly to the closely related genus Aegilops , which also
comprises an allopolyploid series with diploid, tetraploid, and hexaploid species
(Lilienfeld 1951 , and reference therein). The species of these two genera have been
subjected to extensive taxonomic , cytogenetic, genetic, biochemical, molecular, and
evolutionary study by numerous scientists (see reviews of Kihara 1954 ; Mac Key
1966 ; Morris and Sears 1967 ; Kimber and Sears 1987 ; Feldman et al. 1995 ; Feldman
2001 ; Gupta et al. 2005 ; Dvorak 2009 ). One of the most extensively used methods
in this search was the cytogenetic approach of genome analysis, developed by
Kihara ( 1919 , 1924 ) and that was based on the concept of genome stability, assum-
ing that the genomes of the allopolyploid species remain similar to those of their
diploid parents. Genome analysis by the “analyzer-method” (Lilienfeld 1951 ) has
provided the most consistent recognition of genomic similarities in the wheat group.
Genome analysis of wheat and its relatives also provided an insight into the evolu-
tionary past of these species. With this method, Kihara used the diploid species of
Aegilops and Amblyopyrum as analyzers of the genomes in the allopolyploids of the
genera Aegilops and Triticum (Lilienfeld 1951 ). Based upon all possible cross com-
binations among the Aegilops and Amblyopyrum species, nine diploid analyzers
were established: one from Amblyopyrum ( muticum ), and eight from Aegilops ( cau-
data , umbellulata , comosa , uniaristata , tauschii ( squarrosa ), bicornis , l ongissima
(including sharonensis ) and speltoides ). All allopolyploid Aegilops and Triticum
species are made up from contributions of the genomes of these diploid analyzers
except A. muticum , Ae. bicornis , Ae. Sharonensis, and T. monococcum that did not
contribute a genome to the allopolyploid species (Kihara 1954 ). [ Ae. searsii was
discovered later on (Feldman and Kislev 1977 ) and was not included in Kihara’s
analysis]. Ae. bicornis and Ae. sharonensis are geographically isolated from non-
Sitopsis diploids but the lack of participation in the allopolyploid formation of A.
muticum , Ae. Searsii, and T. monococcum requires explanation).
Due to these extensive studies, the polyploid species of Triticum have served as
a classical example for evolution through allopolyploidy (Kihara 1924 ; Sax 1921 ,
1927 ; Sears 1948 , 1969 ; Kihara et al. 1959 ; Morris and Sears 1967 ; Van Slageren,
and reference therein). Von Tschermak and Bleier ( 1926 ) were the fi rst to identify a
spontaneous chromosome doubling in the cross of wild emmer ( T. turgidum ssp.
dicoccoides ) with Ae. geniculata, thus demonstrating the possibility of species for-
mation via allopolyploidy in the wheat group. Subsequent studies showed that the
frequency of unreduced gametes in inter-generic F 1 hybrids of wheat could be in
some hybrids as high as 50 % (Kihara and Lilienfeld 1949 ). Matsuoka et al. ( 2013 )
studied the genetic mechanism that causes spontaneous genome doubling in the F 1
M. Feldman and A.A. Levy