Use of monosomics to map cloned DNA fragments in maize

Abstract

Monosomic maize (Zea mays L.) plants were generated using the r-X1 deficiency system, and the monosomy was confirmed both genetically and cytologically. Genomic DNAs prepared from a group of plants, each monosomic for one chromosome, were digested with restriction enzymes, electrophoresed in agarose gels, and blotted onto nylon membranes. Hybridization of labeled cloned DNA fragments to these blots proved efficient in assigning each fragment to the chromosome from which it originated. Cloned DNA has previously contributed to loci detection through the use of the restriction fragment length polymorphisms (RFLPs), these loci subsequently being arranged into linkage groups by segregation analysis. In this study, these linkage groups were assigned to specific chromosomes, facilitating the construction of a linkage map for maize containing 112 RFLP loci. An additional 35 loci were also assigned to chromosomes by this method; however, the linkage relationships of these loci to other RFLP loci on each chromosome remains undetermined.When a cloned fragment of genomic DNA hybridizes to a Southern blot containing genomic DNAs (prepared from related but distinct organisms) and a hybridization pattern difference appears, this difference is termed a "restriction fragment length polymorphism" (RFLP) (1). The potential application of RFLPs to plant genetics (2-4) and the variability among species discovered with cloned DNA probes have been reported (5, 6). These RFLPs have been used to detect loci that have led to the preliminary construction of linkage groups in maize and tomato (7); this process, however, could not identify the chromosomal locations of most of these linkage groups. A major difficulty in developing genetic maps based upon RFLPs is incorporating these results onto existing genetic maps that are drawn from cytological, morphological (8), and isozyme (9) data. Although conventional linkage analysis could be used to place the RFLPidentified loci on an existing genetic map, the task would be time-consuming and require the inheritance analysis of many RFLPs in several different populations segregating for known morphological markers or isozymes.The r-XJ deficiency system in maize produces both monosomic (lacking one chromosome) and trisomic plants (10-12). These plants, germinated from kernels carrying the submicroscopic r-XJ deficiency on chromosome 10 that contains the R locus, typically include 10-18% monosomics-in instances in which the female parent does not supply the missing chromosome-and 10-18% trisomics. Most remaining plants are diploids, but some multiple aneuploids and partial chromosome deficiencies are also produced. Several thousand maize monosomic plants including monosomics for each of the 10 maize chromosomes, have been recovered using this system. It has also been used successfully to assign a biochemical marker [the benzoxazinless locus (13)], a histone electrophoretic variant (14), and an isozyme locus of peroxidase (15) to the respective chromosomes. Monosomics have been compared with...

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