Asexual female gametogenesis involves contact with a sexually fated megaspore in apomictic hieracium

Abstract

Apomixis results in asexual seed formation where progeny are identical to the maternal plant. In ovules of apomictic, Hieracium subgenus Pilosella species, which primarily form seeds asexually, the meiotic events of sexual female gametophyte (sFG) development occur, generating four megaspores. Aposporous initial (AI) cells form during meiosis in most ovules. The sexual pathway terminates during the events of functional megaspore (FM) differentiation when an enlarged AI undergoes mitosis to form an aposporous FG (aFG). The mitotically programmed FM dies along with the three other megaspores at this time by unknown mechanisms. Transcriptomes of laser-dissected AIs, ovule cells and ovaries from apomicts and AI-deficient mutants were analyzed to understand the pathways involved. The steps leading to AI mitosis and sexual pathway termination were determined using antibodies against arabinogalactan protein (AGP) epitopes found to mark both sexual and aFG lineages at inception. At most, four AIs differentiated near developing megaspores. The first expanding AI cell to contact the FM formed a functional AI (FAI) that underwent mitosis soon after megaspore degeneration. In silico analyses indicted that the enlarged, laser-captured AIs were arrested in the S/G2 phase of the cell cycle and were metabolically active. Further comparisons with AI-deficient mutants showed AIs were enriched in transcripts encoding homologs of genes involved in, and potentially antagonistic to, known FM specification pathways. We propose that AI and FM cell contact provides cues required for AI mitosis and megaspore degeneration. Specific candidates to further interrogate AI-FM interactions were identified here and include Hieracium AGP family genes.