Centromeres and knobs are the topographical references of pachytene chromosomes in maize and teosinte. Structurally and functionally both identities are distinct from the rest of the chromosome. It appears that DNA sequences among the centromeres of different bivalents, and the repetitive DNA within knob-heterochromatin in the genome, have considerable homology to pair and exchange the genetic material from nonhomologous counterparts.
The phenomenon of random centromeric fusion has been reported earlier by Gurgel (MNL 30:54-57, 1956). While studying knob constellations of the Northeastern Himalayan maize we came across centromeric fusion of nonhomologous bivalents at pachytene in Sikkim Primitives from Tripura (T-2 and T-26) and in other strains from Meghalaya (M-1, M-5 and M-12). In our case, it was observed that a particular chromosome with a subterminal knob on the short arm is frequently involved in centromeric fusion events (Fig. 1). Similarly, random homoeologous association and fusion of knobs between nonhomologous bivalents at pachytene in maize (M-5) and maize (CM-111) x Z. parviglumis crosses (Fig. 2) have been frequently observed. The consequences of centromeric fusion and knob fusion may lead to structural differences and thus have an evolutionary significance in maize.
Figure 1. Centromeric fusion in maize (M-5).
Figure 2. Knob fusion (__>) in maize (CM-111) x teosinte (Zea parviglumis) hybrid.
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