AbstractsBiology & Animal Science

Centromeres pose a special challenge to our understanding of the genome. Many basic functions of the cell are maintained by deoxyribonucleic acid (DNA) sequences that change very little over time. However centromere regions evolve rapidly; in fact no common sequence identifies centromeres of various species. Instead, repetitive DNA somehow propagates centromeres in one place over millions of years. The key to centromere stability is Centromere Histone H3 (CenH3). CenH3 is a variant type of histone that packages repetitive DNA into a functional centromere structure. How CenH3 is targeted to, and maintained at, ever-changing DNA sequences is a fundamental question of genome biology. Several approaches were used to understand how CenH3 is regulated in plant cells. We discovered that both strands of Zea mays (maize) centromeres are transcribed into ribonucleic acid (RNA), and that the transcripts are associated with CenH3. Usually, transcripts from regions of the genome that are transcribed from both strands hybridize and are cleaved into small interfering RNAs. In contrast, centromere RNAs remain single-stranded, suggesting a unique role in centromere maintenance and function. CenH3 maintenance appears to be very dynamic at newly initiated centromeres. We identified and characterized a ‘neocentromere’ that formed on a broken maize chromosome maintained in an oat genome. The maize chromosome was initially unstable during mitosis, suggesting a defect in centromere function. Although mitotic stability returned to the maize chromosome in progeny, its CenH3 domain was rapidly changing size among cells. These results point to CenH3-mediated instability in a newly established centromere. We suggest that the ultimate survival of a new centromere depends on its position and its mode of formation. CenH3 instability can conceivably be suppressed through experimental means. We overexpressed the Arabidopsis thaliana CenH3 gene, HTR12, in an effort to induce centromere formation. Transgenic plants developed abnormally, and we traced these defects to chromosome mis-segregation and cell death, consistent with the formation of multiple centromeres. The evidence presented here further defines the identity of centromeres, and processes that affect their establishment.