Rapid Birth or Death of Centromeres on Fragmented Chromosomes in Maize.

Comparative genomics has revealed common occurrences in karyotype evolution such as chromosomal end-to-end fusions and insertions of one chromosome into another near the centromere as well as many cases of de novo centromeres that generate position polymorphism. These findings raise the conundrum of how such rearrangements as dicentrics and acentrics can persist without the initial chromosome being destroyed or lost. Here we sought experimental evidence for the frequency and timeframe for centromere inactivation and de novo formation. The pollen from plants with supernumerary B chromosomes was gamma irradiated and then applied to normal maize silks of a line without B chromosomes. Among ~8000 first generation seedlings, many B-A translocations as well as centromere expansions and ring chromosomes were recognized. Many dicentric chromosomes were also found but a fraction of these show only a single primary constriction suggesting inactivation of one centromere. Also, chromosomal fragments were found without canonical centromere sequences, which revealed the presence of de novo centromere formation over unique sequences. Examples were validated by immunolocalization studies with H2ApThr133, a marker of active centromeres, and ChIPseq with CENH3 antibody to reveal the sites of de novo centromeres. These results illustrate the regular occurrence of both centromere birth and death following chromosomal rearrangement and that these events occur during a narrow developmental window spanning one to potentially only a few cell cycles in order for the rearranged chromosomes to be recognized in this experimental regime.