Map positions for Idh-1 and Nir-1.

Tanksley, S.D., C.E. Valleios and F. Loaiza-Figueroa.

Nitrite reductase (NIR) (E.C.1.6.6. 4.) catalyzes the reduction of nitrite to ammonia and is thus a key enzyme in nitrogen assimilation in higher plants. A histochemical assay is available for the detection of NIR on electrophoretic gels (Vallejos, 1983). NADP-dependent isocitrate dehydrogenase (IDH) (E.C.1.1.1.42) oxidizes isocitrate to ketoglutarate.

Using two interspecific crosses -- L. esculentum cv VF36 x L. pennellii LA 716 BC pennellii and L. pennellii LA 716 x L. hirsutum LA 407 selfed - we have detemined the map positions of the single genes encoding both of these enzymes. In both cases these genes have been mapped relative to other, previously-mapped enzyme-coding genes. Nir-1 maps 11 cM from Est-3 (Table 1). Est-3 is known to be near the end of the long arm of chromosome 1 and thus by association, Nir-1 is also on the long arm of chromosome 1. Nir-1 showed no linkage with other markers on chromosome 1, Prx-1 and Skdh-1, thus the orientation of Est-3 and Nir-1 relative to the centromere cannot be determined from this data.

Idh-1 showed linkage with both Prx-1 and Skdh-1 (Table 1). The maximum likelihood estimates for map distances among these three loci suggest the following gene order: Idh-I -- 7 cM -- Prx-1 -- 34 cM -- Skdh-1. Skdh-1 and Prx-1 have previously been mapped to chromosome 1. The proposed gene order above would place Idh-1 on the short arm of chromosome 1.

Table 1. Segregation of Nir-1 (backcross) and Idh-1 (F2) with previously mapped enzyme genes on chromosome 1.

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                       Nir-1
Locus        p/p  p/+        p/p    p/+  probability
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Prx-1   p/p   19   26    p/+  19    16     0.28
Skdh-1  p/p   15   22    p/+  23    20     0.25
Est-3   p/p   31    5    p/+  3     34     0.00

                       Idh-1
Locus        p/p  p/h    h/h
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Prx-1   p/p   21    5      0
        p/h    2   40      3   p =  0.00
        h/h    0    2     11  ldh-1 -- 7 cM Prx-1

Skdh-1  p/p    6    6      1
        p/h   10   14      8  p = NS
        h/h    3    4      2  Idh-1 -- 42 cM Prx-1

                       Prx-1
Locus        p/p  p/h    h/h
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Skdh-1  p/p    3    3      2
        p/h    7   14      5  p =  0.15
        h/h    1    7      8 Skdh-1 -- 34 cM -- Prx-1
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Literature Cited:

Vallejos, C.E. 1983. Enzyme activity staining. In: S.D. Tanksley and T.J. Orton (eds.) Isozymes in Plant Genetics and Breeding. Elsevier, Amsterdam.