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Plant Cell Advance Online Publication
Published on February 23, 2007; 10.1105/tpc.106.049577

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Received December 9, 2006
Returned for revision January 4, 2007
Accepted January 28, 2007

Centromere Function and Nondisjunction Are Independent Components of the Maize B Chromosome Accumulation Mechanism

Fangpu Han 1, Jonathan C. Lamb 1, Weichang Yu 1, Zhi Gao 1, and James A. Birchler 1*

1 Division of Biological Sciences, University of Missouri, Columbia, Missouri, 65211-7400

* To whom correspondence should be addressed. E-mail: birchlerj{at}missouri.edu.

Supernumerary or B chromosomes are selfish entities that maintain themselves in populations by accumulation mechanisms. The accumulation mechanism of the B chromosome of maize (Zea mays) involves nondisjunction at the second pollen mitosis, placing two copies of the B chromosome into one of the two sperm. The B chromosome long arm must be present in the same nucleus for the centromere to undergo nondisjunction. A centromere, containing all of the normal DNA elements, translocated from the B chromosome to the short arm of chromosome 9 was recently found to be epigenetically silenced for centromeric function. When intact B chromosomes were added to this genotype, thus supplying the long arm, the inactive centromere regained the property of nondisjunction causing the translocation chromosome 9 to be differentially distributed to the two sperm or resulted in chromosome breaks in 9S, occasionally producing new translocations. Translocation of the inactive B centromere to chromosome 7 transferred the nondisjunction property to this chromosome. The results provide insight into the molecular and evolutionary basis of this B chromosome accumulation mechanism by demonstrating that nondisjunction is caused by a process that does not depend on normal centromere function but that the region of the chromosome required for nondisjunction resides in the centromeric region.






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