Using Mutations With Terminal Deficiencies

Deletion mapping was employed to determine the physical order of five morphological variants, pydl, yg2, wdl, u28 and u31, with respect to restriction fragment length polymorphism (RFLP) markers located at the distal end of chromosome 9s in maize. The genetic materials used were a series of terminaldeficiency mutants, newly derived with MCCLINTOCK’S original stocks developed in the 1940s, via breakage-fusion-bridge cycles. A combined physical map and genetic map has been constructed based on data gathered from both genetic complementation tests and RFLP analysis. The location of u3l in relation to RFLP markers was further determined by interval mapping. The physical distance between the healed telomeric end and the most distal RFLP marker in two terminal-deficiency lines was established by using pulsed field gel electrophoresis and verified by BaZ31 digestion. The results from this study set a foundation for studies on the mechanism of healing of broken chromosome ends in higher plants. C YTOGENETIC materials containing chromosomal alterations, such as translocations, inversions, and deficiencies, derived by various physical means, unquestionably have made a tremendous contribution to our understanding of chromosome behavior in plants (BURNHAM 1962). In maize, such materials often have been used for gene mapping and linkage group assignment dating back to the 1930s (BURNHAM 1930; MCCLINTOCK 1931). Terminal deficiencies generated by nondisjunction in B-A translocations have been employed widely to map genes to chromosome arms (BECKETI 1991) and, in one notable study, to derive segmental location of genes and of endosperm size factors on the long arm of chromosome 10 (LIN 1982). One process by which randomly generated terminal deficiencies can be derived is the breakage-fusionbridge cycle. During meiosis, plants heterozygous for a paracentric inversion will give rise to a dicentric bridge following a crossover within the inverted segment, deficient for the terminal portion. Random breakage at this bridge configuration leads to formation of chromosomes with further deficiencies or duplications. The fate of the chromosomes with broken ends has been examined, and it was found that when a chromatid is broken at meiotic anaphase, fusion will occur between two sister halves of this chromatid and a bridge will reform during the following mitotic anaphase (MCCLINCurresponding author; Edward H. Coe, Jr., USDA-ARS, Curtis Hall, University of Missouri, Columbia, MO 65211. E-mail: [email protected]