Abstract
The supernumerary B chromosome of maize is dispensable, containing no vital genes, and thus is variable in number and presence in lines of maize. In order to be maintained in populations, it has a drive mechanism consisting of nondisjunction at the pollen mitosis that produces the two sperm cells, and then the sperm with the two B chromosomes has a preference for fertilizing the egg as opposed to the central cell in the process of double fertilization. The sequence of the B chromosome coupled with B chromosomal aberrations has localized features involved with nondisjunction and preferential fertilization, which are present at the centromeric region. The predicted genes from the sequence have paralogues dispersed across all A chromosomes and have widely different divergence times suggesting that they have transposed to the B chromosome over evolutionary time followed by degradation or have been co-opted for the selfish functions of the supernumerary chromosome.
Highlights
Thousands of plants and animals have extra dispensable chromosomes called supernumerary or B chromosomes [1]
The B chromosome has a drive mechanism consisting of nondisjunction at the second pollen mitosis [5,6,7], which produces the two sperm cells, and the B containing sperm preferentially fertilizes the egg in the process of double fertilization [8]
The B chromosome has manipulated cellular processes to ensure proper segregation in male meiosis by increasing recombination in heterochromatin, by ensuring its transmission if it is in a univalent state, by delaying replication of the centromere at the one mitosis that produces the two sperm and by mediating fertilization of the egg by the B containing sperm
Summary
Thousands of plants and animals have extra dispensable chromosomes called supernumerary or B chromosomes [1] They typically do not associate with the normal set during meiosis and are of variable number between members of a population. The B chromosome has a drive mechanism consisting of nondisjunction at the second pollen mitosis [5,6,7], which produces the two sperm cells, and the B containing sperm preferentially fertilizes the egg in the process of double fertilization [8] (figure 2). The B chromosome has manipulated cellular processes to ensure proper segregation in male meiosis by increasing recombination in heterochromatin, by ensuring its transmission if it is in a univalent state, by delaying replication of the centromere at the one mitosis that produces the two sperm and by mediating fertilization of the egg by the B containing sperm
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