I Wheat Genetics: Past, Present, and Future
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 fields 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 interploid hybrids, clarifying the meiotic chromosome behavior as well as the chromosome 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 classified their plasmons into 17 major types and 5 subtypes and determined the maternal and paternal lineages of all polyploid species. An alloplasmic 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 findings are incompatible with the genetic autonomy of the plasmon, casting a new challenge to the genome-plasmon interaction.
Keywords Aegilops • B-chromosomes • Genome analysis • Genome concept • Maternal lineage • Plasmon analysis • Plasmon autonomy • Polyploids • Wheat
Discovery of Polyploidy and Cytogenetics of Interploid Hybrids in Wheat
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 following eight Triticum species; T. aestivum, T. compactum, T. spelta, T. turgidum, T. durum, T. polonicum, T. dicoccum, and T. monococcum, finding 2n = 14 for T. monococcum, 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 4x and 6x 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 5x hybrids to Kihara (1951). Because of some delay in departure, Sakamura was able to see Kihara's first slide of PMC's of the 5x 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 5x hybrids of Sakamura and two 3x hybrids that he produced. The modal meiotic chromosome configurations of the 3x and 5x hybrids were 7” + 7' and 14” + 7', respectively (Kihara 1924, 1930), based on which he assigned genome formulae AA to the diploid, AABB to the tetraploid, and AABBDD to the hexaploid wheat. Later Kihara obtained a new tetraploid wheat, T. timopheevi, and analyzed the meiotic chromosome 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).