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Materials and methods
The Canadian cultivar Columbus, PI 355512, PI 348636, PI 428343
and semi-wild wheat (SWW), representing the groups common wheat,
macha, spelta, vavilovii and SWW, respectively, were used in
the cytological study. Crosses for a half-diallel among these wheats
were made in the greenhouse. F1's and their parents were
planted in the field and young spikes were collected between 10:00 am
and 1:00 pm and fixed in Carnoy's solution (6:3:1, alcohol:
chloroform: acetic acid) for meiotic studies. The number of chiasmata
was estimated for each F1 and its parents by observing
approximately 100 pollen mother cells and calculated by the following
formula: estimated number of chiasmata = 2(number of ring bivalents)
+ 1(number of rod bivalents) + 2(number of trivalents) + 4(number of
ring quadrivalents). The C-value (Driscoll 1979) is the observed
number of paired chromosome arms per cell as a proportion of the
theoretical maximum (42). Thus, the C-value was calculated by
dividing the estimated number of chiasmata by 42. The C-value of an
F1 is a measure of the chromosome homology between two
parents. The greater the C-Value of the F1, the more
homologous the chromosomes of the two parents. Thus, the C-values
were used as genetic similarity coefficients in this study.
Results
Chromosome pairing data are presented in Table
1. All parents had over 20 ring bivalents per pollen mother cell
(PMC) except SWW with 19.81. Trivalents and quadrivalents were not
found and the univalent frequency was very low in the parents. The
number of ring bivalents ranged from 13.22 to 19.27. Most
F1's involving macha had poor chromosome pairing. In these
F1's the number of univalents ranged from 0.75 to 4.64 per
PMC. In addition, F1's involving SWW also had a relatively
high number of univalents, with the exception of the
vavilovii/SWW F1. The F1 of common wheat
with vavilovii, however, had good chromosome pairing as did
the parents, exhibiting only 0.04 univalents. The F1's of
spelta with common wheat and vavilovii had higher chiasma
frequency compared to the F1's of spelta with
macha and SWW. The C-values of F1's in Table
1 were used as genetic similarity coefficients. Based on these
genetic similarity coefficients (Table 2),
a dendrogram (Fig. 1) was constructed for
the five groups of hexaploid wheat. Common wheat was clustered with
vavilovii. The genetic similarity coefficient for the two
wheats was 0.957. Spelta was not included in the common wheat
and vavilovii cluster but diverged only slightly as evidenced
by a genetic similarity coefficient of 0.921, followed by SWW (0.875)
and macha (0.843).
Discussion
Chromosome pairing information can be used to infer phylogenetic
relationships between species (Jauhar 1988). A good measure of
chromosome homology and the effectiveness of pairing is chiasma
frequency. In general, a higher chiasma frequency indicates better
pairing and greater homology among the parental chromosomes (Jauhar
and Joppa 1996). In the current study, chiasma frequency was
investigated in the hybrids from a half-diallel of five accessions
representing the five groups of wheat: common, spelta, vavilovii,
macha and semi-wild wheat. Chiasma frequency varied from 31.91 to
40.12 among the 10 hybrids. Theoretically, two chiasmata can occur in
a very long chromosome arm, however, the actual frequency of two
chiasmata on the same chromosome arm, is extremely low because of the
interference effect of chiasmata (Sybenga 1972). Instances of more
than one chiasma in a pair of chromosome arms were not considered in
this study.
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