Agriculture Reference
In-Depth Information
hexaploid genome designations should be stated
as BBAADD as noted by Feldman (2001).
Research has shown that hexaploid wheat is
less variable than its diploid progenitors, suggest-
ing the possibility of a genetic bottleneck caused
by a very limited number of initial hybridizations
(Appels and Lagudah 1990). However, given the
obviously large distribution area of primitive tet-
raploid wheat and
Ae. tauschii
populations within
the cradle of agriculture (for an excellent review
of cultivation regions, see Feldman 2001), the
natural occurrence of multiple tetraploid wheat
and
Ae. tauschii
hybrids could be a more common
occurrence than originally thought. The presence
of several sets of alleles and microsatellites estab-
lished that hexaploid wheat resulted from several
hybridizations (Dvoˇák et al., 1998; Talbert et al.,
1998; Lelley et al., 2000; Caldwell et al., 2004;
Zhang et al., 2008). Zohary and Hopf (1993) sug-
gested that these hybridizations are still occurring
today.
Clearly, under certain environmental condi-
tions, some degree of outcrossing in wheat does
occur (Griffi n 1987; Martin 1990) and hybrids of
various ploidy levels can be formed (McFadden
and Sears 1947; Ohtsuka 1998), both of which
would indicate less of an evolutionary bottleneck
in the development of hexaploid wheat than pre-
viously suggested. In addition, hexaploid wheat
originated and still originates in a region where
all of the progenitors reside, thus allowing for a
continuous intercrossing and backcrossing with
diploid progenitors. Even with the presence of
ploidy and genome differences, the various types
of primitive wheat species are capable of wide-
spread intercrossing, culminating in intraspecifi c
hybrid swarms which would signifi cantly increase
the potential for gene fl ow over time. This is
possible because all polyploid wheat progenitors
share at least one common genome, which can
serve as a buffer or a pivot around which unpaired
homoeologous chromosomes can pair. Any
homoeologous chromosome pairing, within the
genomes of the Triticeae tribe, can and does allow
for the occurrence of additional gene recombina-
tion and exchange. Since tetraploid and hexaploid
wheat are predominantly self-pollinated, homo-
zygosity for any gene exchanges favored by natural
selection would be rapidly achieved and available
for artifi cial selection.
This presence of modifi ed genomes along
with unmodifi ed or pivotal genomes, widespread
throughout the Triticeae tribe, was originally
suggested by Zohary and Feldman (1962) and
later in wheat-rye hybrids by Gustafson (1976),
and it has been shown to occur more often than
expected. The presence of the pivotal (buffering)
genomes in primitive polyploid wheat crosses
made possible the rapid and very successful
expansion of wheat in a very short time. This
manner of polyploid speciation allowed for a
greater degree of gene fl ow and genome modifi ca-
tion than that which has been observed in any
diploid system of speciation. This process of
wheat polyploid speciation needs to be kept clearly
in mind when attempting to make wide crosses
(interspecifi c and intergeneric) to introduce genes
from other species and genera into wheat. The
presence of pivotal genomes in polyploid wheat
complexes makes it easier for breeders to under-
stand the processes involved in manipulating gene
complexes from related grass species into wheat.
It also makes very clear the importance of main-
taining and expanding all existing diploid and
polyploid germplasm collections of wheat and
wheat relatives as vast primary, secondary,
and tertiary gene pools for future use in wheat
improvement.
The polyploid wheat species represent a con-
verging form of evolution where several genomes
have been combined. This form of species hybrid-
ization coupled with inbreeding has resulted in a
very successful polyploid that is highly adaptable
to a wide range of environmental growing condi-
tions. The evolution of polyploid wheat and its
intimate connection with the transition of human
societies from hunting-and-gathering to an agri-
cultural culture occurred over a long time and
involved vast mixtures of wild and increasingly
domesticated populations, and of hulled and free-
threshing forms, ultimately resulting in a diverse
and dynamic wheat gene pool.
The genetic composition of polyploid wheat
species fully accounts for their successful estab-
lishment. The evolutionary development of a
genetic system conferring diploidization (
Ph
