Typically, a single AI cell undergoes mitotic divisions, forming a mature aposporous female gametophyte, while all four megaspores degenerate by unknown mechanisms and sexual development ceases ( Fig. AI cells enlarge in the ovule near cells undergoing meiosis. Conversely, in apomictic Hieracium, a somatic AI cell is the progenitor of the aposporous female gametophyte. Cellularization results in a mature sexual female gametophyte ( Koltunow et al., 2011b Hand and Koltunow, 2014). In sexual Hieracium species, the FM is the progenitor of the sexual female gametophyte, and it undergoes three rounds of mitosis. Three of these die during megaspore selection, while the megaspore closest to the chalazal end enlarges and matures into the FM ( Fig. The MMC undergoes meiosis, giving rise to a tetrad of haploid megaspores. In all of the aforementioned sexual and apomictic species, sexual reproduction initiates with the formation of a MMC in the ovule ( Fig. DM, Degenerating megaspores eFM, enlarging functional megaspore FAI, functional aposporous initial FM, functional megaspore LOA, LOSS OF APOSPORY locus required for AI cell specification and growth MMC, megaspore mother cell MS, megaspores NE, nucellar epidermis. The numbers of aposporous initial (AI) cells formed in apomicts, their locations, and steps leading to a functional AI cell were examined in this study. Both mutants form meiotically reduced embryo sacs. They undergo meiosis and functional megaspore formation as found in sexual P36, except that they form a linear tetrad of megaspores. Mutants m115 and m134 are defective in AI formation. B, Comparison of megasporogenesis in sexual H. piloselloides (D36) plant showing floral capitula, florets, and ovules. Specific candidates to further interrogate AI-FM interactions were identified here and include Hieracium arabinogalactan protein family genes.Įarly events of sexual and aposporous female gametophyte development in Hieracium spp. We propose that AI and FM cell contact provides cues required for AI mitosis and megaspore degeneration.
Further comparisons with AI-deficient mutants showed that AIs were enriched in transcripts encoding homologs of genes involved in, and potentially antagonistic to, known FM specification pathways. Transcriptome analyses indicated that the enlarged, laser-captured AIs were arrested in the S/G2 phase of the cell cycle and were metabolically active. The first expanding AI cell to contact the FM formed a functional AI that underwent mitosis soon after megaspore degeneration. At most, four AIs differentiated near developing megaspores. The steps leading to AI mitosis and sexual pathway termination were determined using antibodies against arabinogalactan protein epitopes found to mark both sexual and aposporous female gametophyte lineages at inception. Transcriptomes of laser-dissected AIs, ovule cells, and ovaries from apomicts and AI-deficient mutants were analyzed to understand the pathways involved. Then, the mitotically programmed FM dies along with the three other megaspores by unknown mechanisms. The sexual pathway terminates during functional megaspore (FM) differentiation, when an enlarged AI undergoes mitosis to form an aposporous female gametophyte. Aposporous initial (AI) cells form during meiosis in most ovules. In ovules of apomictic species of the Hieracium subgenus Pilosella, meiosis of the megaspore mother cell generates four megaspores. Apomixis results in asexual seed formation where progeny are identical to the maternal plant.
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