We have used a recently isolated clone of the gene for sucrose synthetase-2 (Ss2) to place the locus on the maize genetic map. Ss2 is located in a region near the centromere on chromosome 9 approximately 32 map units from Sh1, the gene encoding the major endosperm form of sucrose synthetase.
Ss2 was assigned to chromosome 9 by Southern blot analysis of DNA isolated from a series of monosomic plants developed at NPI (Salt Lake City, Utah) using a cross involving the r-x1 deficiency The r-x1 deficiency (D.F. Weber; Maize for Biological Research, University of North Dakota Press, 1982) conditions a high frequency of chromosome loss when transmitted through the female parent. From a cross of a Mangelsdorf tester as male (R R) with a R-g r-x1 (in a W22 background) female, monosomic F1 plants generously supplied by Dave Weber have been obtained for all chromosomes except 1 and 5. Southern blot analysis of DNA from these plants and the parents provides a very direct means of determining the chromosomal location of molecular markers.
Based on the restriction map of p21.2 (the Ss2 clone), digestion of maize DNA with SstI is expected to yield four fragments: two internal 2.3 kb fragments which migrate as a single intense band, and two border fragments of indeterminant size. When p21.2 was used to probe DNA digested with SstI from the parents, a normal diploid F1 and the monosomics, we observed a pattern that was consistent with this. All plants exhibited an intense band at 2.3 kb and a fainter band at 6.8 kb representing one of the border fragments. The second border fragment exhibited a size polymorphism between the parents. In the male parent (R R) a 12.0 kb fragment is observed, whereas in the female (R-g r-x1) the 12.0 kb fragment is replaced by a 4.7 kb fragment. Both the 12.0 kb and 4.7 kb fragments were present in DNA from the normal F1 (as expected) and all of the monosomics except those plants that were monosomic for chromosome 9. The chromosome 9 monosomic plants lacked the 4.7 kb fragment characteristic of the Ss2 allele contributed by the female. This clearly demonstrated the loss of the female Ss2 allele concomitant with the loss of chromosome 9 induced in the female by r-x1.
The assigment of Ss2 to chromosome 9 is further supported by the observation that the intensity of the hybridizing bands (on equal DNA basis) of the chromosome 9 monosomic material was reduced relative to that of the parents and other monosomics. This was also evident when HindIII digests were probed. The reduced band intensity is consistent with the loss of one copy of Ss2 with the loss of chromosome 9.
The map position of Ss2 was further resolved by screening 50 plants from an F2 population segregating for the SstI restriction fragment length polymorphism and other markers. As expected Ss2 showed linkage to chromosome 9 markers. With these data Ss2 was placed relative to two randomly cloned molecular markers (locus 80 and locus 14) that had been previously mapped at NPI (see Figure 1). Ss2 maps 2 ± 2 map units from locus 80 and 21 ± 6 map units from locus 14. This places Ss2 some 32 map units from Sh1, in the vicinity of the centromere on chromosome 9.
D.R. McCarty, S. Wright*, I Helentjaris*, J. Shaw and L.C. Hannah
*Native Plants, Inc., Salt Lake City, Utah
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