Apomixis Breeding and Discovery

Apomixis is a reproductive strategy that enables plants to form seeds asexually, producing offspring genetically identical to the mother plant. This phenomenon has enormous agricultural potential by stabilizing traits and preserving hybrid vigour. Currently, the production of hybrid seeds is accomplished through carefully controlled and labor-intensive crosses.

Historically, research on apomixis was focused on uncultivated species from the Poaceae (grasses) and Asteraceae families.  Despite the wide phylogenetic distance of apomictic species, common traits are shared by apomicts.  Apomictic plants are polyploids and the apomictic locus is genetically dominant, physically large and localized to hemizygous DNA regions with suppressed recombination. Polyploidy and the uncultivated nature of apomicts has complicated efforts to transfer this phenotype into agricultural crops while maintaining high apomictic penetrance and needed agricultural traits.

Another potential method to harness apomixis in agricultural crops is to create “synthetic apomixis” through the identification and bioengineering of apomictic genes. Since the early 1980s, combining breeding, molecular biology and genome sequencing efforts, led by Dr. Peggy-Ozias Akins (UGA), Dr. Wayne Hanna (USDA, UGA), and Dr. Joann Conner (UGA), has resulted in the in-depth analysis of apomixis in the genera Cenchrus, a member of the grass family.  

Fertile F₁ apomictic/sexual hybrids from a cross of apomictic C. squamulatus (syn. Pennisetum squamulatum) pollen to sexual pearl millet (C. americanus) were used to generate molecular markers tightly linked to the apomictic locus (designated as ASGR) in Cenchrus.  Chromosomal mapping of the ASGR across apomictic Cenchrus identified the ASGR as highly mobile, located at multiple chromosomal positions and residing on non-orthologous chromosomes when compared between species.  The evolutionary complexity of the ASGR within Cenchrus genomes can be further researched through whole genome assemblies.     

The ASGR gene responsible for parthenogenesis (the formation of an embryo without fertilization) in Cenchrus was characterized.  The parthenogenesis gene (ASGR-BBML) is a BabyBoom-Like transcription factor, whose altered expression in egg cells triggers parthenogenesis prior to fertilization.  Since this discovery, ASGR-BBML has been used to induce embryo formation in pearl millet, maize, sorghum and rice.  The ASGR-BBML discovery has led to the use of altered expression of species specific BBML genes to induce parthenogenesis in agricultural crops.