Kihara's Genome Concept and Supporting Evidence
Winkler (1920) proposed the term 'genome' for the haploid set of chromosomes. Due to the discovery of polyploidy, this definition required modification, because their gametes contain two or more chromosome sets. Kihara and Lilienfeld (1932) and Kihara (1982) defined the genome concept as follows: (1) Homologous chromosomes have homologous loci identical in sequence as well as in distance. Therefore, when two genomes are homologous, an exchange of homologous partners causes no physiological damage to either the gametes or the zygotes. Nonhomologous chromosomes have different loci or the same loci different in sequence or in distance. As a consequence, the homologous chromosomes can synapse in the meiotic prophase, forming bivalents in the metaphase I (MI) by exchanging their homologous parts. On the contrary, non-homologous chromosomes fail to pair, becoming univalents in MI. (2) Genome has no homologous chromosomes within it. Consequently, a zygote having two homologous genomes forms x pairs of bivalents with no univalents, whereas that with two non-homologous genomes forms 2x univalents with no bivalents in meiosis. The homologous vs. non-homologous relationship between two genomes can be determined by the number of bivalents formed in meiosis. (3) Genome is a functional unit of life. The deletion of a chromosome or a part of it from a genome causes the loss of life or, at least, a significant loss of functions of the gamete and zygote. In essence, Kihara was first to define the homology of chromosomes by their meiotic behavior, based on which he defined genome homology, and proposed its functional role in life.
Supportive evidence of the functional role of genome in gametic and zygotic development was obtained from the fertility of gametes and the viability of progenies of the 5x hybrids, respectively. Fertility of the female and male gametes of the 5x hybrids was studied by Kihara and Wakakuwa (1935) and Matsumura (1936, 1940). The transmission rate of a D-genome chromosome was 0.440 when the 5x hybrid was backcrossed as female to the 4x parent, and 0.673 when the 5x hybrid was backcrossed as pollen parent to the 4x wheat. Based on these univalent transmission rates, the fertility rates of female and male gametes having zero to seven D-genome chromosomes of the 5x hybrid when backcrossed as female or male parent to the 4x wheat were estimated, and compared to the observed frequencies. The female gametes of the 5x hybrid having no D-genome chromosomes or the complete set of D genome chromosomes took part in fertilization in 3.8 or 5.6 times higher frequencies than the expected ones, whereas those with one to five D genome chromosomes showed fertility rates nearly equal to or lower than the expected frequencies (Fig. 1.1a). Similarly, the male gametes of the 5x hybrid having no D genome chromosomes or the complete set of D genome chromosomes took part in fertilization in 391 or 5.7 times higher frequencies, respectively, than expected, whereas those with two to six D genome chromosomes showed fertility rates nearly equal to or lower than the expected frequencies (Fig. 1.1b). These results showed that complete missing or presence of the complete D genome guaranteed both sexes of the gametes for high ability of fertilization. The effect of the genome completeness on the viability of sporophytes was traced for four generations of selfpollination of the 5x hybrid (Kihara 1924). The pedigree was converged at two extremes, 2n = 28 with 14” and 2n = 42 with 21”, one exception being the stable 2n = 40 (20”) progenies called D-dwarfs. This convergence of the pedigrees to 2n = 28 or 2n = 42 plants demonstrated importance of the genome completeness for the continuation of life even in the hexaploid wheat.