A simplification of chromosome-mapping technic is possible by the use of haplo-viable deficiencies transmitted through female and not through male germ cells. These are fairly common among the variants induced by x-ray treatment. The most useful ones are those located in the middle region of the chromosome. This technic may be illustrated by an example using Df 51 (described in abstract in Records Genetics Society 3: 56-57). This deficiency includes the locus of V3 and is located on the longer arm of chromosome 5 near the spindle node. It is transmitted with little loss through female gametes but deficient pollen is defective and does not function.
In using the deficiency for chromosome mapping it is used with a dominant marker on the same chromosome. We use Pr, since the mutants to be treated are induced in a pr stock. (With mutants not known to be pr the same method could be used with Ch as the dominant marker, since all new mutants will presumably be ch.
The new mutant x is crossed on the Df 5 Pr stock and a Df 5 plant of the F1 (recognized by its partially defective pollen) is crossed on the x stock. The progeny of this cross shows the location of the new mutant with reference to the loci of V3 and Pr, and since it is virtually a backcross test a relatively small progeny is sufficient. Since the Df pollen is eliminated, the dominants Pr and X appear only in gametes resulting from crossing over between their loci and that of the Df. Thus the regional location of the new locus will be indicated in three point order in the second generation from the original cross, without the necessity of producing the double recessive in a large F2 and a third generation for the backcross ratio.
If Df 51 is representative in its effect on crossing over, these crosses will not serve to determine the normal crossover frequency. Df 51 greatly reduces crossing over in the region including it (Pr-V3 reduced 26-33% to 5-12%; V3-Bm reduced from 4-6% to 1%). Cytological observations indicate that this effect may be general for internal deficiencies. This means that backcrosses of non-deficient individuals will have to be used for final mapping, but the non-deficient sibs of the same crosses may be used for this. The reduction of crossing over in the deficient plants will be an advantage in reducing the genetic length of the chromosome so as to permit the detection of linkage over longer actual distances.
It might be worth while to construct haplo-viable Df stocks deliberately for this purpose, particularly in the case of the longer chromosomes. Probably one well placed Df would do for each chromosome. Preferably the Df should include a locus somewhere in the middle region, and the dominant marker used should be far enough away for fairly frequent crossing over. The dominant should be one not likely to occur in the mutant stocks, as P, B, Rg, Ch, Pl, etc. The recessive should be a seedling character so that a large number of plants may be examined in looking for the induced deficiencies. Such deficiencies may be obtained by irradiating the pollen of the dominant stock, pollinating on the recessive, growing to maturity the F1 plants showing the recessive character, and pollinating all which by their plant development and pollen development seem likely to be haplo-viable deficiencies. The best pollen to use on these plants will be pollen carrying two (or more) recessive markers widely separated in the chromosome. Then, when the Df plants are pollinated by the new mutant, the Df progeny may be used as outlined above and a few non-deficient sibs may be selfed to provide F2 material with widely separated markers, for accurate mapping if the Df test indicates linkage. Thus, for chromosome 3, a suitable technic would be as follows: Treat Rg and pollinate on lg2, save only lg2 seedlings, and pollinate suitable ones by a d1. The Rg (lg2)-/ a d1 plants thus secured are suitable for pollination by the new mutants, and the Df stock is maintained by pollinating in each generation by a d1 and using only the Rg Df plants of the progeny.
If any corn breeder not having x-ray equipment available wishes to make up such a stock for his chromosome, we should be glad to make the necessary treatments and pollinations for him here next season, using the stocks designated by him for the purpose.