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3. Genome analysis in
Triticum and Aegilops
Guided by the principles of genome homology, he and his
colleagues developed the methodology for phylogenic classification of
plant species, called as genome analysis. The genome analysis is
mainly based on the degree of meiotic chromosome pairing in
interspecific hybrids, in addition to karyotyping and comparative
analysis of morphological characters between the concerned species.
Kihara's studies were very systematic ones; he included always
positive and negative controls when he conducted the experiments,
that is, he used three diploid species as the analyzers whose genome
had been known to be different each other to determine the genome
constitution of the given material.
After systematic observations of chromosome pairing in interspecific
hybrids, he concluded that a common wheat consists of three different
kinds of genome, namely A, B, and D, and that emmer wheat had two
common genomes, A and B, with common wheat. The genome formula of
einkorn, T. monococcum and its wild species T.
boeoticum were commonly designated as A. D was designated for
Ae. squarrosa and the third genome of common wheat.
The genome analysis was conducted in the related species of wheat
covering all Triticum and Aegilops species by him and
his colleagues, revealing that these genera consisted of eight basic
genomes and their modifiers; A, B, C, Cu(U), D, M, Mt, and
S. Polyploid species were proved to be constructed from the
combinations of these genomes. This study had been initiated in early
1930's and concluded in 1972. His conclusions and the designated
genome formulas in Triticum and Aegilops were
essentially accepted by further detail analyses with cytological,
cytoplasmic and molecular examinations by recent investigators (see
"Nuclear and organelar genomes of wheat species" edited by Sasakuma
1992 for the details).
These discoveries led one of the important concepts of plant
evolution; some plant species contain multiple differentiated genomes
in their nucleus, that is, allo-polyploidy. This indicated that these
species had generated through interspecific hybridization and
successive amphiploidization in plant evolution.
4. Disconvery of ancestral progenitors of common wheat
It was in 1945 just after the world war II, that Dr. Kihara
become a world-wide famous geneticist as a discoverer of wheat
ancestral progenitors, when American military mission visited his
laboratory to search for scientific development during the war. He
explained experimental and theoretical results that Ae. squarrosa
should be a progenitor to D genome of hexaploid common wheat by
means of genome analysis, as well as morphological analysis. He was
noticed that the same conclusion had been independently achieved by
Dr. Earnest R. Sears in University of Mioussouri, Colombia, USA at
almost the same time (McFadden and Sears 1944). This coincidence is
symbolic for the two big scientists in the wheat research.
Not only these analytical examinations, he employed two further
methodologies to reach the conclusion. He made several hybridization
experiment between emmer wheat and squarrosa accessions to
produce ABD amphiploids called as synthetic wheats. These synthetic
wheats were then crossed with conventional common wheats, and
F1 and F2 progenies were cytologically and
genetically examined. Since these synthetic wheats exhibited
genomically identical to common wheat, it was sure that squarrosa
is the D-genome progenitor to hexaploid common wheat. The thrid
and final method was the ecological one. In 1954, he organized a
plant expedition team (Kyoto University Scientific Expedition to
Karakram Hindukush) to examine ecological distribution and habitat of
squarrosa in Middle East. They found that squarrosa
grew in the cultivated field of emmer wheat as a field weed,
suggesting a possibility of natural hybridization to generate
hexaploid wheat in nature. (See the picture in this page which shows
the moment that Kihara found natural habitat of squarrosa in
the suburbs of Gorgan, Iran in 1954.)
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