Further studies on K10-I and K10-II

--M. M. Rhoades and Ellen Dempsey

Abnormal chromosome 10, type I, in addition to its primary effect as an inducer of neocentromeres and preferential segregation, also influences crossing over in sensitive regions of the genome. For example, Nel (Theor. Appl. Genet., 1973) reported an increase from 6.1% to 16.9% for recombination in the A2-Bt interval of chromosome 5 when K10-I was present. An attempt to localize the enhancement effect to a particular region of the foreign chromatin of K10-I was made by J. Miles (Indiana University Ph.D. Thesis, 1970). She found that various derivative chromosomes 10, missing portions of the large knob, still increased recombination in chromosome 9 (sh-wx) and in chromosome 3 (gl6-lg2-a). One derivative, KoVII, which retained the differential segment possessing three prominent chromomeres but lost all of the large knob, was no longer able to affect crossing over.

We have recently confirmed the Miles finding, making use of a well-studied chromosome (Df(H)) derived from K10-I. This chromosome possesses the differential segment, as well as the inverted distal region homologous to a segment of normal 10, but is missing the Sr2 locus and the large knob. We chose the sensitive A2-Bt region including the centromere of chromosome 5 to study recombination. The data are given below:
 
Female parent A2 Bt A2 bt a2 Bt a2 bt Total A2-Bt%
R N10/R N10 847 42 58 814 1761 5.7
r Df(H)/R N10 1180 83 65 1190 2518 5.9

The control and experimental plants were identified by testing full siblings as the male parent in crosses with r-tester females. The closely linked r allele marks the Df(H) chromosome and those plants heterozygous for Df(H) segregated for R and r in testcrosses. The data indicate the crossover enhancement factors found in K10-I are not located in the differential segment since Df(H) shows no increase in A2-Bt recombination over the control.

The K10-II chromosome, transferred from teosinte to maize, was also tested for its ability to increase crossing over. K10-II differs structurally from K10-I, both in the differential segment and in the distal knobbed segment (see MNL 62:33, 1988). K10-II is preferentially segregated to the basal megaspore in chromosome 10 bivalents heterozygous for K10-II and N10. It also causes preferential segregation of the chromosome 9 with the large knob in bivalents of Ks9/Kl9 constitution. In these respects, K10-II closely resembles K10-I. The question remained whether K10-II also possesses the crossover enhancement factors. The data presented below address that question.
 
Female parent A2 Bt A2 bt a2 Bt a2 bt Total A2-Bt%
R N10/R N10 260 18 16 280 574 5.9
r K10-II/R N10 304 74 60 310 748 17.9

While the data are not as extensive as one might wish, the K10-II clearly causes a considerable increase in A2-Bt recombination in chromosome 5. The extent of the increase is very similar to that found by Nel for K10-I. Only in one functional aspect do the two chromosomes differ. The K10-II chromosome is pollen-transmitted more successfully than is K10-I when each is opposed by a normal chromosome 10 (MNL 62:33). No explanation is at hand to account for this difference.


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