INDUCING ELEMENTS OF APOMIXIS THROUGH PLANT BREEDING

¹ Plants, Soils and Biometeorology Department, Utah State University, Logan UT 84322-4820

Gametophytic apomixis involves destabilizations of the megasporocyte or nucellar cells, the egg, and the central cell or polar nuclei. These destabilizations are followed by the classic heterochronic elements of apomixis, i.e. unreduced embryo sac formation, parthenogenesis and autonomous or pseudogamous endosperm formation. Starting with sexual diploids of Antennaria, Tripsacum and Sorghum, we bred plants that consistently express elements of apomixis. Success was achieved by manipulating genetic variation for initiation times and durations of ovule development sequence (ODS) steps, i.e. megasporogenesis, embryo sac formation, fertilization, endosperm formation and embryony. We found that initiation times and durations of ODS steps are influenced by different genes and that the heterochronicities that cause apomixis can be enhanced by uncoupling these steps through reticulation-based breeding. In modest implementations of the latter, we achieved a 3-fold increase in frequency aposporous embryo sac formation in Sorghum (from 9% of ovules to 27%) and a 6-fold increase in frequency diplosporous embryo sac formation in Tripsacum (15% to 90%). Apomixis may have evolved in nature in a similar manner. In well-adapted sexual ecotypes, gene systems responsible for ODS timing function harmoniously; but in certain hybrids, or their reticulation-derived derivatives, these gene systems compete causing intermediacy between sexuality and sterility with anomalies such as apomictic embryo sac formation, parthenogenesis, autonomous endosperm formation, bispory, tetraspory, and polarity reversals arising. For apomixis to persist, the causal allelic combinations, from multiple loci, must be genetically stabilized. This is accomplished through sexual sterility or karyotypic heterozygosity.