Molecular and Cytological Analyses of Large Tracks of Centromeric DNA Reveal the Structure and Evolutionary Dynamics of Maize Centromeres

We sequenced two maize bacterial artificial chromosome (BAC) clones anchored by the centromere-specific satellite repeat CentC. The two BACs, consisting of ~200 kb of cytologically defined centromeric DNA, are composed exclusively of satellite sequences and retrotransposons that can be classified as centromere specific or noncentromere specific on the basis of their distribution in the maize genome. Sequence analysis suggests that the original maize sequences were composed of CentC arrays that were expanded by retrotransposon invasions. Seven centromerespecific retrotransposons of maize (CRM) were found in BAC 16H10. The CRM elements inserted randomly into either CentC monomers or other retrotransposons. Sequence comparisons of the long terminal repeats (LTRs) of individual CRM elements indicated that these elements transposed within the last 1.22 million years. We observed that all of the previously reported centromere-specific retrotransposons in rice and barley, which belong to the same family as the CRM elements, also recently transposed with the oldest element having transposed ~3.8 million years ago. Highly conserved sequence motifs were found in the LTRs of the centromere-specific retrotransposons in the grass species, suggesting that the LTRs may be important for the centromere specificity of this retrotransposon family.