The K10-I and K10-II chromosomes possess large heterochromatic knobs, and both chromosomes are responsible for the induction of neocentromeres and preferential segregation. We no longer are convinced that the K10 knobs differ in any way from the genetically inert knobs in other locations in the genome. Previously, we believed that the knobs in the K10-I and K10-II chromosomes not only possessed the highly repetitive 185 bp sequence characteristic of all knobs, but carried a specific DNA sequence, not present elsewhere in the chromosomal complement, which effected preferential segregation and neocentromere formation. We now predict that both phenomena are controlled by chromatin situated in the more proximal "differential segment" of both K10 chromosomes.
There is convincing evidence suggesting a causal relationship between neocentromere formation in the meiotic divisions at all knob locations and preferential segregation of knobbed chromosomes to the basal megaspore. We have no reason to question the validity of this conclusion, but the observation made this past summer of neocentric activity in Df(C)/Df(H) sporocytes has forced us to reconsider our position that the K10 knobs were the responsible agents. The K10 knobs in common with all knobs of the complement are involved in preferential segregation but in a passive manner. Neither the Df(C) or Df(H) chromosome has the large K10 knob formerly held responsible for neocentric activity and preferential segregation, but neocentromeres were present in the sporocytes of these plants. Clearly, the knobs possessed by the two types of abnormal 10 are not the inducers of neocentromeres. What region(s) of the K10 chromosome could be involved? The answer is not yet at hand but we consider it highly likely that the differential segment of K10, comprised of chromatin not found in normal 10 or elsewhere in the normal chromosomal complement, controls both phenomena. We have not at this writing demonstrated that the chromatin of the differential segment controls or effects preferential segregation of knobbed heterologues. To determine whether or not it has this ability we have initiated experiments where both neocentric activity and preferential segregation will be studied in plants with Df chromosomes 10, lacking the large heterochromatic knob, and having knobbed and knobless chroriiosomes 9 marked by Yg2 alleles. The data obtained from plants of Df(C)/N10, Df(H)/N10, and Df(C)/Df(H) constitution should clarify the role of the chromatin of the differential segment in neocentromere formation and preferential segregation.
M. M. Rhoades and Ellen Dempsey
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