By means of a series of deficiencies lacking variable amounts of the terminal segment of the K10-I chromosome, we were able to demonstrate that the K10-I chromosome differed from normal chromosome 10 (N10) by (1) insertion of foreign chromatin (the differential segment) between the R and W2 loci, (2) inversion of the W2 07 L13 region of N10 and its insertion distal to the differential segment, and (3) addition of a large heterochromatic knob plus a terminal euchromatic tail beyond the distal Sr2 locus. (See diagrams in Rhoades and Dempsey, Plant Genetics, Alan R. Liss, Inc., 1985).

We have recently produced a number of deficient K10-II chromosomes, selected through the high-loss system (Rhoades and Dempsey, Genetics, 1972) as having lost the Sr2 locus, and some information about the structure of K10-II is now available. The following chart is a summary of results with 6 deficient K10-II chromosomes. Each was tested with l13 and w2 testers to determine if the dominant alleles of those markers were still present. In 2 cases, data on male transmission of the r marker on the deficient chromosome are available.

Values in parentheses need further tests. The fact that the deficient chromosomes are marked by r instead of R makes their identification difficult and has allowed us to obtain transmission data only in plants where the ratio was obviously aberrant. Other male testcrosses giving 1R:1r ratios could have involved sib plants not carrying the deficient chromosome. Aside from this difficulty, the r allele serves as an efficient marker for the Df chromosome since crossing over between r and Sr2 in K10-II R Sr2/N10 r sr2 compounds is less than 4% and a low frequency of r Sr2 recombination should also occur in Df K10-II/N10 plants.

We can now draw some conclusions about the structure of the K10-II chromosome. Since loss of W2 can take place while L13 is still retained, the order of these 2 genes must be inverted in K10-II with respect to their order in N10, i.e., the order of the two genes is the same in K10-II and K10-I. All the deficient K10-II chromosomes are at least as long as N10; Df(N) was noticeably longer than N10. Therefore, the segment bearing the W2 and L13 loci in K10-II must lie distal to the differential segment, as is true for K10-I.

Some crossover studies with K10-I and K10-II also indicate that K10-II is more similar to K10-I in structure than to N10. The Df chromosomes shown below are all derived from K10-I.

Both K10-I and K10-II show the greatest reduction in recombination when opposed by a N10 chromosome. Obviously, the 2 abnormal chromosomes 10 show little homology with N10 in the distal part of the long arm. The increase in recombination when K10-I is combined with deficient chromosomes of increasing length [(Df(C)<Df(F)<Df(K)] is due to the increase in homologous segments where crossing over can occur. A similar but much less pronounced increase was found with the K10-II series, indicating a lesser degree of homology between K10-II and the Df chromosomes derived from K10-I. Unfortunately, a direct comparison of the 2 abnormal chromosomes 10 by recombination studies in K10-I/K10-II compounds cannot be made because of the lack of a distal marker gene.

One obvious difference in the 2 K10 chromosomes is evident from cytology; the differential segment of K10-I (and of the Df K10-I chromosomes) possesses 3 prominent chromomeres while the differential segment of K10-II has only I enlarged chromomere in a more distal position. The 2 chromosomes also differ in compounds with N10 in the amount of crossing over between R and Sr2. These crossovers occur in a region from the R locus to the proximal end of the differential segment. The higher crossing over in K10-II/N10 plants indicates that the differential segment is located more distally in K10-II than in K10-I. Because of this change, the inverted region containing the L 0 W loci must be longer in K10-I than in K10-II by the inclusion of chromatin adjacent to those loci. (See diagram below). Another point of difference between the 2 abnormal chromosomes 10 is in male transmission; in K10-I R/N10 r pollen parents, only 42% of the functioning pollen grains carry the R allele. In similar crosses involving K10-II R/N10 r plants, 50.3% of the progeny from male testcrosses (I = 3555) have the R allele.

Our present interpretation of the structure of the distal region of N10, K10-I and K10-II is shown below:

The differential segment is indicated by a wavy line and X marks the chromatin adjacent to R which is located distally in K10-I.

M.M. Rhoades and Ellen Dempsey

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