III Cytogenetics and Allopolyploid Evolution
Wheat Chromosome Analysis
Abstract The 13th International Wheat Genetics Symposium (IWGS) is being held in the year I begin my phased retirement, marking a career of 40 years in wheat genetics, beginning with a postdoctoral fellowship in 1973 with Ernie Sears and Gordon Kimber at Columbia, Missouri, then the premier center for wheat chromosome research. I was fortunate to have won a DF Jones fellowship for my research proposal, “Exploration and application of the Quinacrine and Giemsa staining technique in the genus Triticum” that led to the cytogenetic identification of wheat and rye chromosomes (Gill and Kimber 1974a, b). In 1973, I also attended, for the first time, the meetings of the 4th IWGS in Columbia, Missouri, and was in awe of the research presentations and heated discussions on wheat evolution. In 1979, I established my own research group and laboratory at Kansas State University focusing on wheat chromosome mapping and manipulation for crop improvement under the auspices of Wheat Genetics Resource Center (reviewed in Raupp and Friebe, Plant Breed Rev 37:1–34, 2013). Among the first visitors to my laboratory were Takashi Endo, then at Nara University, Japan, and Chen Peidu, from Nanjing Agricultural University, and presented this research at the 6th IWGS in Kyoto, Japan. Therefore, it is a special feeling to be returning to Japan for a farewell presentation. My intent is to briefly review the history of wheat chromosome research and how our laboratory played a role in advancing wheat chromosome analysis leading to the chromosome survey sequencing paper utilizing telosomic stocks (IWGSC, Science 345:285–287, 2014).
Keywords Aneuploid stocks • C-banding • Evolution • In situ hybridization • Wheat
Laying the Foundation of Wheat Chromosome Research: Genome Analyzer Method
By 1915, botanists had described three classes of cultivated wheats, the one-seeded monococcum (Triticum monococcum L.), the two-seeded emmer (T. turgidum L.), and dinkel (T. aestivum L.). The one-seeded wild relative of monococcum was reported in Greece and Anatolia between 1834 and 1884. Aaronsohn discovered the two-seeded wild relative of emmer in 1910 in Lebanon, Syria, Jordan, and Israel. Therefore, it was well accepted, as Candolle had suggested in 1886, that since wild wheats grow in the Euphrates basin then wheat cultivation must have originated there. Between 1918 and 1924, Sakamura (1918) and his colleague Hitoshi Kihara (1919), at Hokkaido Imperial University in Japan, and Karl Sax (1922), at Harvard University, reported their classic studies on the genetic architecture of the three groups of wheats (Fig. 7.1). Sax (1922) and Kihara (1924) analyzed meiosis in wheat species and hybrids and were the first to establish the basic chromosome number of seven and document polyploidy in the wheat group. This method of delineating species evolutionary relationships based on chromosome pairing affinities in interspecific hybrids came to be called as the genome analyzer method (Kihara 1954; see also Fig. 7.1 in Gill et al. 2006). These were exciting observations and established polyploidy as a major macrospeciation process and wheat as a great polyploidy genetic model. These interploidy wheat hybrids of course could also be
Fig. 7.1 Meiotic metaphase I chiasmate pairing in wheat parents and F1 hybrids. The F1 hybrid between einkorn and dicoccum showed 7″ and 7′, indicating that they share one set of chromosomes in common, and it was called a genome. The second genome in dicoccum was called the B genome. The F1 hybrids between dicoccum and dinkel showed 14″ and 7′ indicating that they share the AB genomes in common, and dinkel wheat had a third genome that was later identified as D genome
exploited in plant breeding for interspecific gene transfers (McFadden 1930; Gill and Raupp 1987).
The crowning achievement of the genome analyzer method was the identification of the D-genome donor of wheat (Kihara 1944; McFadden and Sears 1946) and the production of synthetic wheat (McFadden and Sears 1944). These discoveries are fueling a second green revolution (reviewed in Gill et al. 2006). Kimber practiced the genome analyzer method with passion and developed some quantitative models for measuring genomic affinities. Application of Giemsa staining methods to meiotic preparations allowed the measurement of pairing potential of specific chromosomes, however, perfectly homologous chromosomes may suffer structural aberrations and lose the ability to pair (Gill and Chen 1987; Naranjo et al. 1987). Obviously, the genome analyzer method had reached its limitations, but meiotic pairing analysis remains an important method for monitoring chiasmate pairing and the potential of genetic transfers in interspecific hybrids.