cenh3 mediated haploid induction in maize

In a process known as centromere-mediated haploid induction, crossing wild type with plants expressing specific variants of Centromeric Histone H3 (CENH3) can produce haploid progeny. We hypothesized that such CENH3 variants reduce the effective size of centromeres. In this model, centromere size or strength dimorphism between the wild-type and CENH3 variant genomes in the zygote leads to removal of the smaller centromeres and uniparental genome elimination. We created a maize cenh3 null mutant that is homozygous lethal but transmissible through both male and female gametophytes. The pool of CENH3 in gametophytes carrying the null allele must be derived from the sporophyte and necessarily diluted during the two cell cycles leading to sperm cell and three cell cycles leading to egg cells. CENH3 dilution should reduce effective centromere size. We found that 0.5% of the progeny from male cenh3 heterozygotes and 5.0% of the progeny from female heterozygotes were haploid, with a lower frequency of aneuploids. Endosperm genotyping demonstrated that haploids were fertilized by gametes carrying the cenh3 null mutation. These data show that centromere-mediated haploid induction can result from strictly quantitative differences in CENH3. This simplified method of haploid induction may be broadly applicable to diverse plant species.