Rick, C. M. and A. C. Sawant.
Another attempt was made to obtain crossovers between j and lf. To facilitate this effort, a stock has been derived in which j-lf has been combined with ms10 to be used as a parent in backcrosses with i-lf/+ +. Seed of the BC was obtained in 1953, part being sown directly in the field and part in the greenhouse for later transplanting to the field in 1954. In both lots plants were closely spaced in the field, the larger plants being removed as classified to leave more room for the development of the smaller ones. Survival of the direct seeded material was only about 30% whereas nearly, 100% of the seeds germinated and seedlings retained of the transplanted portion. Since the ratios were approximately equal in both lots, the totals are pooled with the following frequencies: 1080 + +, 0 j +, 0 + lf, 1028 j lf. When combined with our previous backcross segregations, we obtain the following totals: 1330 + +, 0 j +, 0 + lf, 1273 j lf. If these results are to be explained by separate, but tightly linke genes, the upper fiducial limit of the crossover value at the 1% level is 0.2%.
It is noteworthy in this connection that leafy inflorescences are also associated with other genes that eliminate or modify the joint. Thus inflorescences of mc either lack joints or have very poorly developed ones and under our conditions always continue growth as indeterminate or leafy structures. Similarly pi and two other recently encountered mutants are all characterized by the absence of joint and indeterminate inflorescences. The two effects are not invariably associated, however, for bu, which shifts the joint to a position very close to the fruit and interferes to some extent with abscission, scarcely ever shows leafy inflorescences. The tendency for these two effects to appear together in so many mutants hints very strongly that they are closely associated in development and leads us to the conclusion that leafiness and jointlessness are but pleiotropic effects of a single gene, j.
From a practical standpoint the undesirable leafiness encountered with j can be effectively reduced by interaction with sp. The possibilities in this respect were first called to our attention by a stock sent in reply to our request for crossover lines by Dr. P. A. Young, to whom we are greatly indebted for sharing his stock. This stock obviously had j and sp but very little leafiness. Appropriate crosses proved, however, that when the of this stock was recombined with sp+ it became as leafy as ever. To obtain some concise information on the effects of this gene interaction, the leafiness of inflorescences was measured in the four different phenotypes of the dihybrid F2, the results being summarized as follows:
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Percentage inflorescences with:
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Phenotype No leaves 1-3 leaves More than 3 leaves
(usually indeterminate)
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+ + 88 12 0
+ sp 100 0 0
j + 0 7.8 92.2
j sp 10.3 76.0 13.7
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The tendencies of sp to suppress vegetative growth, which condition
determinate growth, apparently also greatly subdue further growth of the
inflorescences, and sp therefore offers a solution to the problem of leafiness
associated with j.