Apospory appears to accelerate onset of meiosis and sexual embryo sac formation in sorghum ovules

Genotype differences for onset timing of meiosis and sexual ES formation, relative to ovule development, were highly significant. A major source of variation in timing of sexual germline development was presence or absence of apomictic ES, which formed from nucellar cells (apospory) in some genotypes. Genotypes that produced these aposporous ES underwent meiosis and sexual ES formation precociously. Aposporous ES formation was most prevalent in subsp. verticilliflorum and in breeding lines of subsp. bicolor. It was uncommon in land races.The present study adds meiosis and sexual ES formation to floral induction, apomictic ES formation, and parthenogenesis as processes observed to occur precociously in apomictic plants. The temporally diverse nature of these events suggests that an epigenetic memory of the plants' apomixis status exists throughout its life cycle, which triggers, during multiple life cycle phases, temporally distinct processes that accelerate reproduction.For angiosperms, apomixis means asexual reproduction by seed [1]. It is strongly associated with hybridity and polyploidy, and molecular mechanisms responsible for it remain shrouded in complexity [2-4]. Apomixis involves the reprogramming of unreduced (2n) cells of the ovule, which thereafter follow a very different developmental trajectory than had the plant been sexual. Specifically, ovules of apomictic plants produce asexual totipotent cells. These form in the nucellus, chalaza or integuments, and embryos develop from them either directly (adventitious embryony) or after 2n embryo sac (ES) formation (gametophytic apomixis). Apomictic (2n) ES usually resemble sexual ES, but embryony in them occurs parthenogenetically and often precociously. Whether in sexual plants or apomicts, embryony is the result of epigenome modifications that begin as early as floral transition [5,6].Gametophytic apomixis is further divided into i) apospory, where the 2n aposporous ES (AES) forms from a cell of the nucellus,