Effect of K10-II on preferential segregation of chromosome 9

K10-II was found by T.A. Kato (B. McClintock, TA. Kato, and A. Blumenschein, 1981) in a strain of Mexican teosinte. Unlike the K10-I chromosome, which occurs in both maize and teosinte, K10-II has not been found in maize. We obtained a sample of the teosinte K10-II strain from Kato and have now introduced this chromosome into maize. It resembles K10-I in having an extra partially heterochromatic segment attached to the long arm of chromosome 10 but differs morphologically in several ways. In the region comparable to the differential segment of K10-I, identified in that chromosome by 3 prominent chromomeres, K10-II shows only a single chromomere in a more distal position. Instead of a single large knob, K10-II possesses 2 smaller knobs separated by a stretch of euchromatin. In spite of the morphological differences, the 2 abnormal chromosomes 10 both induce neocentromeres and both undergo preferential segregation in K10/N10 heterozygotes. In testcrosses of K10-II R/N10 r female parents, the R allele was recovered with a frequency of 70.8%, comparable to the 70% found with K10-I. This past summer, we were able to show that K10-II also causes preferential segregation of chromosome 9 alleles. Female parents of K10-II/N10; K9L yg2/K9S Yg2 constitution, along with N10/N10 controls, were testcrossed by yg2 male parents and the following data were obtained:
  Yg2 yg2 Total  % yg2
K10-II/N10 602 1293 1895  68.2
N10/N10 366 333 699  47.6

It is evident that K10-II, like K10-I, induces preferential segregation of the marker carried by the chromosome 9 with the larger knob. K10-I possesses a third attribute, the ability to increase crossing over in sensitive regions susceptible to changes in crossover frequency. Studies are underway utilizing the a2 bt region of chromosome 5 to test whether or not K10-II also possesses this ability.

M.M. Rhoades and Ellen Dempsey

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