Northeast Missouri State University
Iowa State University

Cytogenetic localization of the y1, l10, l12, ms1 and si1 loci
--Brent Buckner and Donald S. Robertson

We have produced plants heterozygous for reciprocal translocations involving different breakpoints in the same two chromosomes ("overlapping translocations"). When these plants undergo meiosis one-fourth of the gametes produced should be deficient for the chromosomal region between the interchange breakpoints. A megagametophyte receiving a large deletion will probably abort. However, megagametophytes with small deficiencies might possibly be viable and if they are fertilized by sperm carrying a recessive allele for a trait located within the deficient chromosomal segment, the resulting sporophytes should be hemizygous for the recessive trait.

There were several reasons why we initiated these studies. We were interested in generating plants that contain a small interstitial hemizygous region for the long arm of chromosome 6 including the y1 locus. During our attempt to isolate the Y1 gene, the DNA from these plants was analyzed by DNA blot hybridization analysis using maize genomic sequences that were putative clones of the Y1 gene as hybridization probes. If a clone was found to be in the hemizygous state in these plants it demonstrated that the sequence was closely linked to the y1 gene and, therefore, the clone was further characterized. We also produced these plants to test if a deletion in chromosome 6, including the chromosomal segment containing Y1, was viable in the megagametophyte. In addition, these studies have allowed us to begin to compare and correlate the cytogenetic, classical and molecular maps of maize for a small region of 6L.

We produced plants that are heterozygous for 8 different combinations of overlapping translocations having breakpoints in 6L (Table 1). When these plants were pollinated by plants that were homozygous or heterozygous for several different recessive y1 alleles, white kernels were observed in six different overlapping translocation combinations. In several crosses the allele uncovered was a recessive temperature-sensitive pastel allele of the y1 gene. Seedling tests confirmed that the plants possessed the pastel phenotype. In no case did the number of white kernels approach one-fourth of the kernels on the ear. Typically we observed from 1 to 12 kernels per ear. In addition, the white kernels were reduced in size when compared to sibling nonwhite kernels. Our observations suggest that megagametophytes that possess deficiencies for the segment of 6L containing the Y1 gene are viable, however, these deficiencies reduce their fitness.

Table 1. Combinations of translocations tested.
Translocation 6L Breakpoints Y1 Uncovered
T6-9e/T6-9(043-1) 0.18/0.36 yes
T6-9(6270)/T6-9(043-1) 0.19/0.36 yes
T6-9(6270)/T6-9 (6019) 0.19/0.27 no
T6-9(6019)/T6-9 (0431) 0.27/0.36 yes
T4-6(055-8)/T4-6(8428) 0.25/0.28 no
T4-6(055-8)/T4-6(6623) 0.25/0.31 yes
T4-6(8428)/T4-6 (6623) 0.28/0.31 yes
T6-10b/T6-10d 0.12/0.16 yes
Breakpoints are those listed by Longley, AE (USDA ARS 34:16, 1961).

The DNA from plants derived from white kernels of two different combinations of overlapping translocations were analyzed by DNA blot hybridization analysis using the Y1 gene as a hybridization probe. These plants were found to be hemizygous for the Y1 gene (data not shown).

Results of our studies using overlapping translocation combinations in which interchange points were between chromosome 6 and chromosome 4 or 9 place the y1 gene in the region between positions 0.28 and 0.31 on 6L [T4-6(055-8)/T4-6(6623)]. The largest interstitial hemizygous region that uncovered the y1 gene was delineated by positions 0.18 and 0.36 on 6L [T6-9e/T6-9(043-1)]. However, overlapping translocation combinations in which the interchange points were between chromosome 6 and 10 place the Y1 gene between positions 0.12 and 0.16 on chromosome 6L (T6-10b/T6-10d). Because the positions estimated for Y1 using the 4-6 and 6-9 overlapping translocations are consistent, it would suggest that the estimations of the T6-10b and T6-10d interchange points may be in error.

Several other loci on 6L were also uncovered. The l10, l12, ms1 and si1 loci were uncovered by overlapping translocation combinations T6-9e/T6-9(043-1). In addition, l12 was also uncovered by overlapping translocation combinations T6-9(6019)/T6-9(043-1) and T4-6(8428)/T4-6(6623). Since l12 is the most proximal of those analyzed, these data suggest that these loci, which span approximately 4 map units on 6L, all map to the region between positions 0.28 and 0.36 on 6L. 

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