Agriculture Reference
In-Depth Information
local differentiation over short geographic dis-
tances was the rule, and the frequency of some
common alleles (>10%) was localized and high.
The population genetic structure of wild T.
dicoccoides is obviously a mosaic and refl ects the
underlying ecological heterogeneity, which has
been derived from local and regional geological,
edaphic, climatic, and biotic differentiations. The
genetic landscape is defi nitely not random between
loci, populations, and habitats, and it most likely
displays adapted patterns predictable on the basis
of environmental factors. Could these polymor-
phisms represent adaptation to fl uctuating envi-
ronments? It has been possible to decide if
selection is responsible for the occurrence of
many DNA variants across the coding and non-
coding regions of a genome, and it is clear that
major DNA changes can and do occur within
and between T. dicoccoides populations over a
relatively short time frame, paralleling that of
allozymes.
Nevo and Beiles (1989) predicted that neither
migration nor genetic drift could have generated
the patterns observed between loci and alleles of
wild T. dicoccoides and that selection remained a
vital explanatory model. Environmental selection
also partly affected loci differentially, but differ-
ently from migration. This was supported by data
from Nevo and Beiles (1989) for three reasons: (i)
variation was found among loci; (ii) in an autocor-
relation analysis, positive correlations were found
in different distant T. dicoccoides groups, and not
necessarily in the fi rst one as would be expected
if migration determined the interpopulation
genetic structure; and (iii) the predominance of
negative correlations in the larger distant groups
was found to be due to decreasing ecological simi-
larity often observed with increasing distance.
The maintenance of polymorphisms in wild T.
dicoccoides may be explicable by both spatial and
temporal variation in selection. Theory indicates
that selection, acting differentially in space,
coupled with limited migration, which is typical
of wild T. dicoccoides , will maintain a substantial
amount of polymorphism (Karlin and McGregor
1972; Hedrick 1986; Nevo et al., 2000). Thus,
different polymorphisms will be favored in differ-
ent climatic and edaphic niches, from regional to
local, and at miniscule levels within a locality.
Microniche ecological selection (e.g., climatic
factors related to temperature, available water,
and biotic and abiotic stresses) could be a major
cause of genetic differentiation rather than sto-
chastic processes.
Origin of hexaploid wheat
There are two main forms of hexaploid wheat,
including T. zhukovskyi Men. & Er., which was
the result of a recent hybridization involving T.
timopheevi and T. monococcum , the only example
of hexaploid wheat to have the GGAAA m A m con-
stitution (Upadhya and Swaminathan 1963). The
most important hexaploid wheat group comprises
T. aestivum (BBAADD) and its several subspecies
containing 21 pairs of chromosomes with seven
pairs belonging to each of the A, B, and D genomes
(Sears 1954; Okamoto 1962) and containing
17.33 pg DNA (http://data.kew.org/cvalues/
introduction.html).
Triticum aestivum originated approximately
10,000 years ago after the domestication of tetra-
ploid wheat and was derived from the hybridiza-
tion of a primitive tetraploid (BBAA), as the
female, and T. tauschii ssp. strangulata [ Ae. taus-
chii (Coss.) Schmal, also known as Ae. squarrosa ,
DD, 2 n = 2 x = 14, 5.10 pg DNA], as the male
(Kihara 1944; McFadden and Sears 1944, 1946a,b;
Kimber and Sears 1987; Kimber and Feldman
1987; Dvoˇák et al., 1998). The fi rst primitive
hexaploid wheat was probably a hulled-type like
T. aestivum var. spelta , macha , or vavilovii . The
current free-threshing types, T. aestivum var. aes-
tivum , sphaerococcum , or compactum , were the
result of a mutation at the Q gene locus (Mura-
matsu 1986) followed by selection. All polyploid
wheat species are disomic in inheritance due to
complete diploidlike chromosome pairing, which
is controlled by two main homoeologous pairing
genes Ph1 (Riley and Chapman 1967) and Ph2
(Mello-Sampayo 1971) and several minor genes
(for a complete review, see Sears 1977). As previ-
ously stated, since the cytoplasm donor of hexa-
ploid wheat was the female in the original cross
creating the polyploid, it should be listed fi rst in
any pedigree or genome designation; therefore,
Search WWH ::




Custom Search