Wheat cytogenetics and cytogenomics: the present status

Abstract

The chromosome number of hexaploid wheat (2n = 6x = 42) was first worked out in 1918 and was followed by research on intergeneric and interspecific hybridization, mainly in Japan under the leadership of Hitoshi Kihara. Starting in 1935, Ernie Sears in USA produced a large collection of aneuploids including nullisomics, monosomics, trisomics, tetrasomics, ditelocentrics and nullisomic–tetrasomic (NT) lines. In parallel, through a study of meiotic chromosome pairing in hybrids produced through interspecific and intergeneric crosses, progenitors of the A and D genomes of 6x wheat were determined with certainty; the progenitor of B genome was difficult and still not known with certainty. The diploidizing system and the Ph1 locus restricting meiotic chromosome pairing to only between homologous (but not between homoeologous) chromosomes was also initially discovered by Okamoto (Wheat Inf Serv 5:6, 1957), and was subsequently studied in more detail by Ralph Riley and his group at Cambridge, UK during late 1950s. With the establishment of International Triticeae Mapping Initiative (ITMI) and the development of ITMI mapping population in 1990, first genetic maps were produced for all the seven homoeologous groups involving 21 wheat chromosomes by 1995. High-density genetic maps were developed later through large-scale development of SNPs through next generation sequencing (NGS) technology. During 1990s, TR Endo and BS Gill at Kansas State University, USA also produced > 400 chromosome deletion stocks that were extensively used for the development of physical maps for all the 21 wheat chromosomes. More recently, during the last 12 years (2005–2017), cytogenomics research was conducted, which included separation of longest chromosome 3B and the 40 chromosome arms for the remaining 20 chromosomes through flow sorting and their utilization in the development of BAC libraries that were used for developing BAC-based physical maps. These maps were used in generating chromosome survey sequences, and assembly of the whole genome sequence of 6x wheat Chinese Spring. During 2017–2018, whole genome sequences for a wild emmer tetraploid wheat (T. turgidum) and those for the two diploid species (T. urartu, Ae. tauschii) representing the donors of A and D sub-genomes of 6x wheat, were also published. For hexaploid wheat, estimates of a pangenome with ~ 140,000 genes and that of the core genome with ~ 80,000 genes also became available. These resources are already being extensively utilized and will continue to be utilized throughout the world for several decades not only for basic research, but also for the improvement of wheat crop to feed the growing world population.