52
tetraploid level. Thus, the ability to exchange genetic material through spontaneous
inter-specifi c hybridization promotes further the convergent evolution of these
species. These reticulate patterns of evolution of course tend produce an increased
range of variation and consequent diffi culties in the identifi cation of species. Natural
hybrids also occur between the wild species and domesticated allopolyploid wheat.
Hybrids have been recorded recurrently in T. turgidum between subsp. durum and
its progenitor, wild emmer (subsp. dicoccoides ) , in Israel (Percival 1921 ; Feldman
2001 ; Huang et al. 1999 ; Dvorak et al. 2006 ; Luo et al. 2007 ) and between common
wheat and wild emmer (Zohary and Brick 1961 ).
Considerable evidence was obtained for spontaneous hybridization also between
allotetraploids and diploid species in mixed natural populations of tetraploids and
diploids (Vardi and Zohary 1967 ; Vardi 1973 ; Zohary and Feldman unpublished).
The occurrence of hybrid derivatives in such populations, particularly as a result of
backcrossing to the allotetraploid parents, indicates the possibility of gene fl ow
from diploid to tetraploid species.
In contrast to the wide distribution of the allotetraploid species, the distribution
area of the natural allohexaploid species is, in all cases, smaller than that of their
tetraploid and diploid parents. Also their ecological amplitudes are much more
restricted than those of the related tetraploids and even the diploid parents. They grow
in a smaller range of habitats and often are distributed sporadically. The morphologi-
cal variation of the allohexaploids is also relatively limited. All these indicate a rela-
tively recent origin of the allohexaploids ,
On the basis of plant habitus, spike morphology, and cytogenetic data, Zohary
and Feldman ( 1962 ) classifi ed the allopolyploid species of Triticum and Aegilops
into three natural clusters (Table 2.8 ). Genome analysis of the allopolyploids within
each cluster showed that they share one unaltered genome (the pivotal genome) and
a genome or genomes that is/are modifi ed [the differential genome(s)]. In laboratory
hybridization studies of allopolyploids, it was found that the common genome acts
as a buffer ensuring some seed fertility following pollination by the female parent,
while the chromosomes of the differential genomes, brought together from different
parents, may pair to some extent and exchange genes (Feldman 1965b , c ).
Consequently, the dissimilar genomes of these allopolyploids contain chromosomal
segments that originated from two or more diploid genomes. Such genomic consti-
tution reveals different evolutionary rates for each of the two or three genomes of
every allopolyploid.
Thus, all seven tetraploids and one hexaploid of the U-genome cluster share
a genome homologous to that of diploid Aegilops umbellulata (Kihara 1954 ), all the
three tetraploids and three hexaploids of the D-genome cluster share a genome
homologous to that of diploid Ae. tauschii (Kihara 1954 ; Kihara et al. 1959 ), and all
two tetraploids and two hexaploids of the A-genome cluster, including all the wild
and domesticated forms, share a genome homologous to that of diploid Triticum
urartu (Dvorak 1976 ; Chapman et al. 1976 ). The allopolyploids of each cluster
resemble the diploid donor of the shared genome in their basic morphology (stature,
leaf shape, and spike and spikelet morphology) and in the structure of the seed dis-
persal unit. They differ in features of the differential genome(s) that are primarily
M. Feldman and A.A. Levy