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Miller et al (1980)
recognized four pairs of sites of rDNA in T. spelta. But, no
identification of their loci was carried out except that a third site
in size should be that on chromosome 1A. The presence of major rDNA
on chromosomes 1A was investigated by in situ hybridization
using plants in which chromosomes 1A from T. spelta have been
substituted into Chinese Spring wheat. On the contrary, the present
result clearly shows that the third site of rDNA is that of
chromosome 5D. In many strains of T. aestivum, the greater
most part of rDNA on chromosome 5D were deleted. The signal intensity
of rDNA was 6B >1B> 5D >1A. The rDNA site on chromosome 5D
of T. spelta is almost the same as that of Ae.
squarrosa, a diploid D-genome species, in the amount of
hybridization signal.
There are two hypotheses on the origin of T. spelta. One
hypothesis is that T. spelta is primitive species and
T. aestivum was produced from T. spelta. The other
hypothesis is that T. spelta is a relatively newly generated
species and generated from T. aestivum or from a hybrid
between T. dicoccum and T. compactum or T.
aestivum (MacKey 1966, Tsunewaki 1969). Our ISH
experiments fo rDNA of two genera, Triticum and
Aegilops demonstrate that the size of rDNA has a tendency to
diminish by terminal deletions of Sat-chromosomes in the course of
evolution after amphidiploidization (Mukai et al. in preparation).
The present result supports the first hypothesis, though many studies
support the second one (Liu et al 1990).
References
Liu YG, Mori N and Tsunewaki K (1990) Restriction fragment length
polymorphism (RFLP) analysis in wheat. I. Genomic DNA library
construction and RFLP analysis in common wheat. Jpn J Genet 65:
367-380.
MacKey J (1966) Species relationship in Triticum. Proc 2nd Int
Wheat Genet Symp 1963 Hereditas Suppl Lund 237-276.
Miller TE, Gerlach WL and Flavell RB (1980) Nucleolus organizer
variation in wheat and rye revealed by in situ hybridization.
Heredity 45: 377-382.
Mukai Y, Endo TR and Gill BS (1991) Physical mapping of the 18S-26S
rRNA multigene family in common wheat: Identification of a new locus.
Chromosoma 100: 71-78.
Rayburn AL and Gill BS (1986) Molecular identification of the
D-genome chromosomes of wheat. J Hered 77: 253-255.
Tunewaki K (1969) Orgin and phylogenetic differentiation of common
wheat revealed by comparative gene analysis. Proc 3rd Int Wheat Genet
Symp Canberra 71-85.
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