ASGR-BABY BOOM-like (ASGR-BBML) from Apomictic Pennisetum squamulatum Confers Parthenogenesis to Transgenic Pearl Millet

ASGR-BABY BOOM-like (ASGR-BBML) is a gene that was identified in bacterial artificial chromosome clones from Pennisetum squamulatum that were linked with the trait for aposporous embryo sac formation.  ASGR-BBML is predicted to encode an AP2-domain transcription factor, and similar genes have been shown to induce ectopic embryos when overexpressed in transgenic Brassica and Arabidopsis.  ASGR-BBML is an obvious candidate gene for parthenogenesis.  Parthenogenesis is the development of an egg into an embryo in the absence of fertilization and is a necessary component of apomixis.  The egg in an aposporous embryo sac is unreduced since the embryo sac develops from a somatic cell in the nucellus.  Occasional recombination between traits for aposporous embryo sac formation and parthenogenesis has been observed in Cenchrus ciliaris (P. ciliare) and non-parthenogenetic recombinants had lost ASGR-BBML.  In order to test for a role of ASGR-BBML in parthenogenesis, the site of expression was determined with a promoter:gus construct, the effect of knockdown in expression of ASGR-BBML was analyzed, and egg cell development was assessed upon expression of ASGR-BBML in sexual, tetraploid pearl millet.  The ASGR-BBML promoter expressed GUS only in the egg or egg apparatus of embryo sacs, the level of expression was positively correlated with the frequency of fertilization-independent embryo formation, and expression of the native gene in sexual tetraploid pearl millet led to the recovery of dihaploid progeny, i.e., meiotically derived eggs became parthenogenetic.  All lines of evidence support our hypothesis that ASGR-BBML plays a role in parthenogenesis in apomictic Pennisetum.