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:
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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