Role of the knob and the differential segment of K10-I in preferential segregation

In the 1986 MNL, we suggested that the K10 knob near the terminus of the long arm of abnormal chromosome 10 type I was no different from any other knob and that the phenomena we had associated with this chromosome, namely preferential segregation and neocentromere formation, might be determined by the more proximally situated insert of foreign euchromatin designated the differential segment. The reason for this somewhat heretical view was the finding that plants containing deficient abnormal chromosomes 10, all lacking the knob and euchromatic segments of varying lengths but retaining the differential segment, displayed considerable neocentric activity in the two meiotic divisions. Tests demonstrating that preferential segregation also occurred in such plants were obviously required to support the hypothesis. A year ago, we conducted the first of such tests, using Df K10-I/N10; K9L yg2/K9S Yg2 female parents crossed by yg2 males. Earlier (G.Y. Kikudome, Genetics, 1959), female testcrosses of K10/N10 heterozygotes, where one chromosome 9 carried the Yg2 allele and a medium sized knob and the homolog possessed the yg2 allele and a smaller knob, produced progenies with 65-70 percent of Yg2 plants. However, no preferential segregation of the chromosome 9 with the larger knob was found in our data when a Df K10 chromosome, lacking the terminal knob, replaced K10. The data are presented below:
  Yg yg Total % yg
Df(C)/N10 ; K9L yg2/K9S/Yg2 1 157 1099 2256 48.7
N10/N10 ;K9Lyg2/K9S/Yg2 334 368 702 52.4
Df(K)/N10;K9Lyg2/K9S/Yg2 344 336 680 49.3

Since preferential segregation is dependent on crossing over and is known to vary in different environments, we repeated the test a second year. To maximize the degree of neocentromere formation, we used Df(K)/Df(K) homozygotes, heterozygous for the large knob on chromosome 9, as female parents and followed the yg2 marker as before. Again, no indication of preferential segregation was found (48% yg in a population of 1635). We are forced to conclude that the K10 knob is necessary for the occurrence of preferential segregation, although neocentromeres are formed in its absence. The question then arose as to whether the 2 phenomena of preferential segregation and neocentric activity are related at all. We have always believed that anaphase I and anaphase II neocentric activity at knobbed regions of bivalents heterozygous for dissimilar sized knobs in plants with the K10 chromosome was responsible for the orientation of chromatids with the larger knob toward the outer 2 spores in megasporogenesis and the preferential inclusion of the large knobbed chromatid in the basal megaspore. A second assumption was invoked, namely that the orientation established at anaphase I should be maintained into the second meiotic division, i.e., through the intervening interphase and prophase 11 stages. If the differential segment alone is able to induce neocentromeres but no preferential segregation follows, perhaps the K10 knob is responsible for the maintenance of orientation. If this be true, it is a unique property shared with no other knob. To further elucidate the activity of the knob region, we have undertaken the isolation of the K10 knob from the differential segment. This involves production of breaks in the short arm of chromosome 9 and the long arm of chromosome 10 and the transfer of the K10 knob from chromosome 10 to chromosome 9. We will use the high-loss system to engineer the desired 9-10 half translocation. If we are successful and the new 9-10 chromosome is transmissible, we will test the K10 knob for its ability to induce preferential segregation and neocentromeres in sporocytes having no differential segment in chromosome 10 or elsewhere in the complement.

M.M. Rhoades and Ellen Dempsey

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