Peroxidase isozymes governed by the Px3 locus are among the most ubiquitous and easily studied in maize (Brewbaker and Johnson, MGCN 46:29-33, 1972). The chromosomal location of Px3 was studied using genetic markers and translocations on chromosome 7. Dr. David Weber (1975, unpublished) was able to localize Px3 to chromosome 7 using trisomics in crosses with our stocks. Four chromosome 7 mutants were used as genetic markers--opaque-2 (7-16), Teopod (7-46), slashed (7-50) and Papyrescent glumes (7-112). Waxy T7-9a, with breakpoint at 7L.63, was also used.
Testcrosses for the mutant linkage study were made between pertinent heterozygotes and homozygous testers of the U. Hawaii collection, based on the tropical flint inbred Hi27 (Brewbaker, MGCN 46:33-37, 1972). The unique allele Px3-5, derived from the Race Puya, was introduced to the crosses to rule out contamination during pollinations.
Highly significant linkage (P < .01) was obtained only between the Px3 locus and Pn (7-112), near the long arm end of chromosome 7 (Table 1). Our Pn stock shows excellent penetrance in heterozygotes. The recombination percentage between Px3 and Pn was 15.38%, thus suggesting the location of Px3 to be at or near 7-96.62.
The translocation stock, wx T7-9a (breakpoints 7L.63 and 9S.07), was also used in crosses, with the heterozygotes testcrossed to a stock made homozygous for wx and Px3-1 through repeated backcrosses. Linkage between Px3 and the translocation breakpoint (7L.63) was highly significant (Table 2), with a-recombination percentage of 41.84.
The suggested genetic location of the 7L.63 breakpoint is 7-58.8 (Phillips, 1969, Genetics 61:107), based on linkage with 7L markers including sl (7-50) and ij (7-52). Px3 showed no linkage with genes proximal to ij (Table 1), and must clearly be distal to the 7L.63 breakpoint, at or near 7-100.64 (58.8 + 41.84).
The locus PA may thus be localized between 96.62 and 100.64 on the long arm of chromosome 7. A working value of 7-99 is proposed, based simply on the average of these two values. This region of chromosome 7 is poorly marked:
sl ij Bn Px3 Pn
50 52 71 99 112
We could not obtain a reliable stock of Bn, a gene which was together with slashed in the early studies of Hayes and H. E. Brewbaker (J. Amer. Soc. Agron. 18:761-767, 1926). The ubiquity of Px3 in maize tissues, its polymorphism and ease of identification (Brewbaker and Johnson, MGCN 46:29-33, 1972) thus make it a convenient marker for this region.
James L. Brewbaker
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