Starch gel electrophoresis of maize coleoptile extracts in our laboratory has shown two zones of staining for isozymes of hexokinase. Genetic analyses 9 indicate that the slower migrating set of bands is encoded by the structural gene Hex2 and that the enzyme is monomeric. Recent linkage analyses and work with B-A translocation stocks (generously supplied by Jack Beckett) have allowed for the placement of Hex2 on the long arm of chromosome 6. Its location relative to other markers on 6L was determined from the analysis of two very large F2 populations generated for other reasons. One of these (the F2 of inbreds Tx303 and C0159) segregated for 6 loci on 6L (isozyme loci Pgdl, Enpl, Hex2, Idh2, Mdh2, and color factor Pl). The other F2 (from inbreds CM37 and T232) was polymorphic only for Hex2, Idh2 and Mdh2. Joint segregation data and maximum likelihood estimates of recombination are presented in Table 1.
The data of Table 1 allow for the following map to be constructed:
This map was generated with the assumptions that Pl is located at position 45. The linkage between Enp1 and Y (yellow) remains to be determined. As illustrated, the map has two particularly noteworthy features:
1) Idh2 and Mdh2, which were previously shown to be on 6L (Goodman et al., Genetics 96:697-710) perhaps 12 units from py (footnote in McMillin et al., Genetics 92:1241-1250) at approximately position 77, must be moved a minimum of 23 units distal to their previously assigned locations. As a consequence, the genetic map for 6L must be expanded by a third over its prior size.
2) The B-A translocation stock TB-6Lc, which uncovers Y, also uncovers all isozyme markers on 6L except Pgd1 (Table 2). Consequently, Pgd1 must be proximal to the TB-6Lc breakpoint.
The proper gene order of the two closely linked loci Idh2 and Mdh2 cannot unequivocally be decided from the F2 data at hand. The order illustrated must be considered tentative until testcross progenies are evaluated. Stocks have been obtained for the reexamination of the relationship of Hex2-py-Idh2-Mdh2 and for the relationships among Pgd1, Enp1 and Y.
Table 1. Two-locus segregation data and maximum likelihood estimates of recombination between loci on the long arm of chromosome 6.
Table 2. TB-6Lc uncovers all isozyme markers on 6L with the exception of Pgd1.
J. F. Wendel, C. W. Stuber, and M. M. Goodman
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