Location of indeterminate gametophyte (ig) on chromosome 3

A set of chromosome-9, waxy translocations furnished by the Maize Cooperation Stock Center was used to place ig to a chromosome arm. To score ig, advantage was taken of the fact that many female gametophytes carrying it yield kernels of reduced size (miniature kernel class). Because this trait is incompletely penetrant, full-sized kernels present in the backcross populations could have inherited ig or Ig. Kernels of the miniature class, however, can reasonably be assumed to carry ig. Linkage would cause a deficit below 50% of waxy kernels within the miniature class. All of the translocations tested gave an acceptable fit to 1 Wx:1 wx except for a moderate deficit in the waxy class for two chromosome 6 translocations and a marked deficit for two 3L stocks. The following data for these four translocations were obtained from crosses of the type (T wx Ig/(N Wx ig) x wx wx Ig Ig:
 
    Waxy among miniature
Translocation Breakpoints Proportion %
T6-9a 6S.79; 9L.40 89/223 39.9
T6-9b 6L.10; 9S.37 95/255 37.3
T3-9c 3L.09; 9L.12 19/153 12.4
T3-9(8562) 3L.65; 9L.22 42/148 28.4

Further tests focused on chromosome 3 because a moderate deficit of waxy kernels involving the chromosome 6 backcrosses could be explained by functioning of aneuploid gametes.

A two-point test of linkage involving lg2 in repulsion with ig yielded 226 miniature kernels. Of these, 24 or 10.6% gave liguleless seedlings, indicating linkage.

Four-point linkage data were obtained utilizing the Coop's gl6 lg2 a1 tester. Because size was highly variable in the backcross kernel population produced by pollinating heterozygotes with the tester, it was not possible in this case to identify an ig carrying class on the basis of this trait. Alternatively, the induction of multiple seedlings by ig was utilized. Among 4699 kernels sprouted, 365 yielded multiple seedlings of which 359 sets could be scored for both gl6 and lg2. Because multiple seedlings induced by ig derive from a single embryo sac, members of one set are expected to be identical with respect to the heterozygous markers. Each set therefore tests only one gamete. The 359 sets gave the following distribution:
 
  Gl Lg Gl lg gl Lg gl lg
A 144 7 57 3
a 103 29 13 3

The critical recombination values are given below in conjunction with the standard map locations of gl6 lg2 and a1.

Figure.

Placing ig proximal to lg2 gives good linear additivity, with only six cases classified as double crossovers in the gl6 - ig - lg2 interval. Combining the available two and four-point data gives 66/585 or 11.3% recombination between ig and lg2.

Jerry Kermicle and Jim Demopulos-Rodriguez


Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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