Supernumerary chromosomes in crested wheatgrass : their cytological behavior and breeding significance
Variations in chromosome number occur between and within individuals of many plant and animal species. In many cases differences are due to varying numbers of supernumerary, accessory, or B-type chromosomes. Supernumerary chromosomes are usually smaller than chromosomes of the basic set. They do not pair with chromosomes of the basic set and in many species they are reported to be heterochromatic. Some species have supernumeraries in somatic and reproductive tissues while others exhibit them only in reproductive tissues. The mode of transmission of supernumerary chromosomes varies in different species. Usually there is directed non-disjunction of the supernumeraries towards the gamete in pollen mitosis. In rye, however, there is directed non-disjunction of supernumeraries in both embryos and pollen grains. In some species directed non-disjunction is not observed. The origin of supernumerary chromosomes as well as their function remain to be elucidated in most species. It is believed that supernumeraries are genetically modified fragments of normal chromosomes. Some indications of a detrimental effect from supernumeraries have been obtained. Generally, however, they appear genetically inert. An examination of meiosis in crested wheatgrass (42) indicated a fairly high frequency of supernumeraries in plants of the tetraploid type. Summit and Nordan, varieties of the tetraploid type, and Fairway, a variety of diploid form, are used extensively as forage and seed crops in the drier parts of Western Canada. Extensive breeding programs, designed to improve both diploid and tetraploid forms of crested wheatgrass, are being conducted at the Forage Crops Section, Canada Department of Agriculture, Research Station, Saskatoon. Since supernumerary chromosomes occur with a high frequency in the Summit variety, and because this variety is characterized by low fertility, this study was initiated to determine the significance of supernumeraries in breeding of improved strains of crested wheatgrass. Investigations of the effect of supernumeraries on fertility were emphasized in particular. Combining ability of selected parents as shown by forage and seed yields of their progenies, was studied in relation to supernumeraries. In addition the present study was designed to investigate more fully the occurrence of supernumerary chromosomes in diploid and tetraploid species of crested wheatgrass and to determine their cytological behavior. Numbers in brackets refer to bibliography.