Prof. H. Kihara’s Genome Concept and Advancements in Wheat Cytogenetics in His School

Abstract

This article introduces Kihara's main achievements in wheat cytogenetics and the succeeding developments in a few fi elds of wheat cytogenetics, which were founded by Kihara. Following the discovery of polyploidy in wheat by Sakamura (Bot Mag (Tokyo) 32:150–153, 1918), Kihara established the cytogenetics of inter-ploid hybrids, clarifying the meiotic chromosome behavior as well as the chromo-some number and genome constitution of their progeny, based on which Kihara formulated the concept of genome. Here, evidence supporting his recognition of the genome as a functional unit is presented. Kihara proposed the methodology for genome analysis and determined the genome constitution of all Triticum and Aegilops species. Ohta re-evaluated the genome relationships among the diploid species, using the B-chromosomes of Ae. mutica . After completing the genome analyses, Kihara's interest was shifted to the genome-plasmon interaction that led to the discovery of cytoplasmic male sterility in wheat. Using the nucleus substitution method elaborated by Kihara, we carried out plasmon analysis of Triticum and Aegilops species. We classifi ed their plasmons into 17 major types and 5 subtypes and determined the maternal and paternal lineages of all polyploid species. An allo-plasmic line, (caudata)-Tve, retained male sterility induction and germless grain production for 60 generations of backcrosses with wheat pollen. We are trying reconstruction of the Ae. caudata plant from the genome of its native strain and the caudata plasmon in the alloplasmic wheat. Two groups of Kihara's school reported paternal transmission of the mtDNA sequences in alloplasmic wheats. Their fi nd-ings are incompatible with the genetic autonomy of the plasmon, casting a new challenge to the genome-plasmon interaction. Sakamura (1918) who was a graduate student at the Faculty of Agriculture, Hokkaido University, studied both root-tip mitosis and meiosis in PMC's of the fol-lowing eight Triticum species; T. aestivum, T. compactum, T. spelta, T. turgidum, T. durum, T. polonicum, T. dicoccum , and T. monococcum , fi nding 2n = 14 for T. mono-coccum , 2n = 28 for T. turgidum, T. durum, T. polonicum and T. dicoccum , and 2n = 42 for T. aestivum, T. compactum and T. spelta . This result led him to discover a polyploid series of the diploid, tetraploid, and hexaploid in wheat, with the basic chromosome number of x = 7. Sakamura planned a further study on chromosome numbers of the offspring of the hybrids between different ploidies. In 1917 he made crosses between 4 x and 6 x wheat in three combinations. At this point, Sakamura was informed from the Ministry of Education, Japan, to go abroad for advanced study. He asked Kihara who had just enrolled in the graduate school to succeed his wheat research and handed the 5 x hybrids to Kihara (1951). Because of some delay in departure, Sakamura was able to see Kihara's fi rst slide of PMC's of the 5 x hybrid, and gave a few minutes advice that determined Kihara's later career as the wheat researcher. Kihara (1924) analyzed the meiotic chromosome behavior of the three 5 x hybrids of Sakamura and two 3 x hybrids that he produced. The modal meiotic chromosome confi gurations of the 3 x and 5 x hybrids were 7 " + 7' and 14 " + 7', respectively (Kihara 1924 , 1930), based on which he assigned genome formulae AA to the dip-loid, AABB to the tetraploid, and AABBDD to the hexaploid wheat. Later Kihara obtained a new tetraploid wheat, T. timopheevi , and analyzed the meiotic chromo-some behaviors of its hybrids with one diploid and two tetraploid Triticum species. From the results, he designated genome formula AAGG to this wheat (Lilienfeld and Kihara 1934).