title: Sexual reproduction is the default mode in apomictic Hieracium subgenus Pilosella, in which two dominant loci function to enable apomixis.
abstract: Asexual seed formation, or apomixis, in the Hieracium subgenus Pilosella is controlled by two dominant independent genetic loci, LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP). We examined apomixis mutants that had lost function in one or both loci to establish their developmental roles during seed formation. In apomicts, sexual reproduction is initiated first. Somatic aposporous initial (AI) cells differentiate near meiotic cells, and the sexual pathway is terminated as AI cells undergo mitotic embryo sac formation. Seed initiation is fertilization-independent. Using a partially penetrant cytotoxic reporter to inhibit meioisis, we showed that developmental events leading to the completion of meiotic tetrad formation are required for AI cell formation. Sexual initiation may therefore stimulate activity of the LOA locus, which was found to be required for AI cell formation and subsequent suppression of the sexual pathway. AI cells undergo nuclear division to form embryo sacs, in which LOP functions gametophytically to stimulate fertilization-independent embryo and endosperm formation. Loss of function in either locus results in partial reversion to sexual reproduction, and loss of function in both loci results in total reversion to sexual reproduction. Therefore, in these apomicts, sexual reproduction is the default reproductive mode upon which apomixis is superimposed. These loci are unlikely to encode genes essential for sexual reproduction, but may function to recruit the sexual machinery at specific time points to enable apomixis.
authors.affiliation: Commonwealth Scientific and Industrial Research Organization Plant Industry, Glen Osmond, South Australia 5064, Australia. Anna.Koltunow@csiro.au
authors.forename: Anna M G
authors.initials: AM
authors.lastname: Koltunow
authors.forename: Susan D
authors.initials: SD
authors.lastname: Johnson
authors.forename: Julio C M
authors.initials: JC
authors.lastname: Rodrigues
authors.forename: Takashi
authors.initials: T
authors.lastname: Okada
authors.forename: Yingkao
authors.initials: Y
authors.lastname: Hu
authors.forename: Tohru
authors.initials: T
authors.lastname: Tsuchiya
authors.forename: Saira
authors.initials: S
authors.lastname: Wilson
authors.forename: Pam
authors.initials: P
authors.lastname: Fletcher
authors.forename: Kanae
authors.initials: K
authors.lastname: Ito
authors.forename: Go
authors.initials: G
authors.lastname: Suzuki
authors.forename: Yasuhiko
authors.initials: Y
authors.lastname: Mukai
authors.forename: Judith
authors.initials: J
authors.lastname: Fehrer
authors.forename: Ross A
authors.initials: RA
authors.lastname: Bicknell
mesh_terms: D019659:Asteraceae / Q000166:cytology / Q000235:genetics* / Q000254:growth & development / Q000528:radiation effects; D020090:Chromosome Segregation; D003433:Crosses, Genetic; D056130:Gametogenesis, Plant; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D056426:Genetic Loci; D018525:Germination; D008540:Meiosis; D056771:Ovule / Q000166:cytology* / Q000254:growth & development / Q000528:radiation effects; D030821:Plants, Genetically Modified / Q000235:genetics / Q000254:growth & development; D011058:Pollen / Q000254:growth & development; D054817:Pollination; D012100:Reproduction, Asexual; D012639:Seeds / Q000166:cytology* / Q000254:growth & development / Q000528:radiation effects; D057891:Tetraploidy