Differential Genome Elimination in Embryo and Endosperm of Haploid-Producing Crosses

Production of haploid plants that inherit chromosomes from only one parent can greatly accelerate plant breeding. Haploids generated from a heterozygous individual and converted to diploid create instant true-breeding varieties, bypassing generations of inbreeding. Current methods for making haploid plants are often too inefficient for production breeding, and are usually restricted to particular genotypes. Centromeres are essential for chromosome segregation during mitosis and meiosis. Their function requires the centromere-specific histone CENH3. We have found that haploid Arabidopsis thaliana can be easily generated through seeds, by crossing plants that express altered CENH3 proteins to wild type. After fertilization, chromosomes with altered centromeres can be completely missegregated and lost, presumably because they compete poorly with chromosomes that have normal centromeres. Up to 50% of F1 progeny are haploids with chromosomes from only their wild type parent. Seed abortion in genome elimination crosses can reach 70-80%, although the reasons for this are unclear. We have investigated chromosome loss in embryo and endosperm using a seed-expressed GFP reporter gene. Non-fluorescent haploid embryo can be found within a chimeric endosperm that contains a mixture of GFP-containing and GFP-negative cells (bearing different karyotypes). Differential genome elimination in embryo and endosperm may explain how seeds bearing haploid embryos are able to survive. As CENH3 is universal in eukaryotes, our method may be extended to produce haploids in any plant species.