Genome Analysis and a Re-evaluation on Genome Homology
Determination of the genome homology is based on the meiotic chromosome pairing between two genomes in comparison. Kihara (1930) illustrated the schemes for genome analysis of an autoand allotetraploid. Before their application, we need to
Fig. 1.1 Male and female transmission rates of the D-genome univalents in the crosses between (T. polonicum x T. spelta) F1 and T. polonicum. (a) 5x F1 hybrid x T. polonicum (Kihara and Wakakuwa 1935; Matsumura 1940), (b) T. polonicum x 5x F1 hybrid (Matsumura 1936)
have a set of diploids, each having a genome different from the others, which he called genome analyzers. When the tetraploid is an autotetraploid with A genome, its hybrid with the A genome analyzer forms x”', whereas the F1's with other analyzers form x” + x', from which results genome constitution of the tetraploid is determined as AAAA. If the tetraploid is an allotetraploid with AABB genomes, its F1 hybirds with the A and B genome analyzers form x” + x', whereas the F1's with other analyzers form 3x', thus genome constitution of the tetraploid is confirmed to be AABB. Kihara and his collaborators determined the genome constitutions of all Triticum and Aegilops species (Kihara 1924, 1945; Kihara and Tanaka 1970; Lilienfeld 1951).
After Kihara's genome analytical works, several genetic factors became known to influence the meiotic chromosome pairing: They are a suppressor, Ph1, of the homoeologous chromosome pairing, an enhancer of the homoeologous pairing in Ae. speltoides and a suppressor of the homoeologous pairing in B-chromosomes. To distinguish between the homologous and homoeologous pairing, using B-chromosomes of Ae. mutica, Ohta (1995) produced hybrids between eight diploid species and an Ae. mutica strain having the B-chromosomes, selecting hybrids with zero, one or two B-chromosomes. The presence of two B-chromosomes did not affect the number of bivalents in Ae. mutica itself. The T genome of Ae. mutica was highly homologous to S genome of Ae. speltoides and D genome of Ae. squarrosa, forming five bivalents in the presence of two B's, whereas it was non-homologous to A, C, M, Sb and Sl genomes of the respective species, forming no or one bivalent with two B's. This type of research should be extended to polyploid species for reevaluation of their genome relationships to the diploid species.
Plasmon Analysis as the Counter Part of Genome Analysis
Later, Kihara's interest shifted to the genome-plasmon interaction. He produced an alloplasmic line of a common wheat, T. aestivum var. erythrospermum (abbrev. 'Tve') by repeated backcrosses of the F1 hybrid, Ae. caudata var. polyathera x Tve, with Tve as the recurrent pollen parent. This alloplasmic line, designated by (caudata)-Tve, expressed male sterility in its SB3 and later backcross generations, leading Kihara to discover the cytoplasmic male sterility in wheat (Kihara 1959).
The pioneering works of Kihara and others suggested the presence of plasmon diversity in the Triticum-Aegilops complex. There were three research groups actively working on the plasmon diversity in wheat and its related genera: Maan and Lucken in the North Dakota State Univ., USA, Panayotov and Gotsov in the Wheat and Sunflower Institute, Bulgaria, and Suemoto and Tsunewaki in Kyoto University, Japan. In an international cooperative work, we compared plasmons that were independently introduced by these groups into their own wheat stocks (Mukai et al. 1978). This work showed the wide scope of plasmon diversity in this complex. I obtained 7 plasmons from Maan, 8 plasmons from Panayotov and 15 plasmons from other researchers to enrich our plasmon collection, totaling 46 plasmons, including 16 of our own.
In 1963, I initiated a program to produce alloplasmic lines using a set of 12 common wheat genotypes as the alloplasmon recipients, whose list and reasons of selection are given elsewhere (Tsunewaki et al. 1996). The aim was 10 backcross generations of each alloplasmic line to recover the tester's genotype in 99.9 % purity. The total number of the alloplasmic lines in all combinations between the 12 wheat genotypes and 46 plasmons amounts to 552 NC hybrids, all of which reached SB10 or later backcross generation by 1997. A field test of all alloplasmic and 12 euplasmic lines was carried out in the crop season of 1992–1993. By that time, 87
% of the alloplasmics reached at the SB10 or later backcross generation. With all lines, 14 vegetative and 8 reproductive characters were observed. The genetic relationships between the 47 plasmons were analyzed (Tsunewaki et al. 2002). With the same plasmons, RFLP analyses of ct and mtDNAs were carried with 13 and 3 restriction enzymes, respectively (Ogihara and Tsunewaki 1988; Wang et al. 2000). Phylogenetic trees depicted from the data on the organellar DNA polymorphisms revealed molecular differentiation between the plasmons in Triticum-Aegilops complex. Combining the phenotypic effects on wheat characters and organellar DNA differences revealed by the RFLP analyses, 47 plasmons of this complex were classified into 18 major types and five subtypes (Tsunewaki et al. 2002). The genome
Fig. 1.2 Phylogenetic relationships between the diploid, tetraploid and hexaploid species based on their genome-plasmon constitutions (After Tsunewaki 2009). Inner and outer circle: genome and plasmon symbol, respectively. Modified genome: underlined
and plasmon analyses together clarified the maternal and paternal lineages of all
Triticum and Aegilops species (Fig. 1.2; Tsunewaki 2009).