Seminars

Centromeres are essential for chromosome segregation during mitosis and meiosis. Therefore, it is aradoxical that their DNA sequences and the sequences of kinetochore proteins are very fast evolving. This observation has led to the suggestion that centromere differences can cause hybrid infertility when two different species are crossed. The centromere-specific histone CENH3 replaces H3 in centromeric nucleosomes, and is essential for kinetochore function. We have found that large-scale chromosome missegregation occurs when Arabidopsis thaliana plants expressed altered CENH3 proteins are crossed to wild type. Chromosomes that contain mutant CENH3 are completely eliminated in up to 50% of F1 progeny, yielding haploid plants with chromosomes from only their wild type parent. Either the maternal or paternal chromosomes may be eliminated depending on the direction of the cross. Chromosome competition after fertilization is a subtle assay for centromere defects, because some of the CENH3 mutants that cause missegregation in a cross are phenotypically indistinguishable from wild type. Haploid plants are easily converted back to diploids, allowing Arabidopsis geneticists to instantly produce true-breeding varieties from heterozygous parents. Our research may greatly accelerate plant breeding if genome elimination can be re-created in crops. We also show that changes in a single centromere protein can cause chromosome missegregation when two parental genomes are mixed after fertilization. This may indicate how rapid centromere evolution can create species barriers.