Seminars

Apomixis refers to the asexual reproduction through seed , which generates plants that are genetically identical to the mother plant. The engineering of apomixis has tremendous agricultural potential to maintain complex genotypes, e.g. those of F1 hybrids. Gametophytic apomixis deviates from sexual development in three major steps: (1) meiosis is circumvented or aborted, leading to the formation of unreduced, unrecombined embryo sacs (apomeiosis); (2) embryogenesis initiates without fertilization of the unreduced egg cell (parthenogenesis); and (3) developmental adaptations enable the formation of functional endosperm. The aim of our research is to identify mutations that mimic the major components of apomixis, and to combine them to engineer apomictically reproducing crops.
In a genetic screen in maize, we identified the non-reduction in female4 (nrf4) mutant, which mimics the first step of apomixis: apomeiosis. Homozygous nrf4 plants produce up to 95% unreduced embryo sacs. Using sequencing-based transposon display, the mutation was mapped to the long arm of chromosome 7, and the identity of the Nrf4 gene was confirmed by two additional mutant alleles. To identify whether nrf4 leads to first or second division restitution (FDR vs SDR), we analyzed the maintenance of heterozygosity in the progeny of nrf4 mutant plants, in comparison to mother plants, using a SNP array that enabled the analysis of 10-20 SNPs per chromosome. The effect of the nrf4 mutation turned out to be more complex than expected and leads to both FDR and SDR. Nonetheless, depending on the genetic background of the mother plant, up to 30% of the unreduced female gametes were diploid and generated by a mitotic division. Using nrf4 in combination with a tetraploid ‘haploid inducer” line we generated, we could recover up to 15% clonal progeny. To our knowledge this is first evidence that production of clonal individuals through seed is possible in maize.