A Conserved Apomict-Derived Babyboom Gene Promotes Parthenogenesis in Multiple Cereals

Apomixis, a seed generating, clonal reproductive process, is manifested in several grasses through aposporous embryo sac formation, fertilization-independent development of an unreduced egg, and pseudogamous endosperm formation. Egg cell parthenogenesis in Pennisetum squamulatum, an apomictic relative of pearl millet, requires cell autonomous expression of an Apospory-Specific Genomic Region-BabyBoom Like (ASGR-BBML) gene. ASGR-BBML genes are conserved across several apomicts including, Pennisetum, Cenchrus, and Brachiaria species, and are phylogenetically distinct from BBM genes in sexual species or in apomictic species where the genes are in genomic regions unlinked with apomixis. Expression of ASGR-BBML as a transgene in several cereals, including pearl millet, rice, and maize, results in similar frequencies of parthenogenesis. Since meiotic reduction proceeds normally in these sexually reproducing plants, expression of ASGR-BBML in reduced eggs leads to the formation of haploid embryos and recovery of haploid plants. Segregation distortion thus far has prevented the recovery of ASGR-BBML homozygous lines.