A MULTICOPY SUPPRESSOR OF IRA1 (MSI1) homologue is not associated with the switch to autonomous seed development in apomictic (asexual) Hieracium plants

In Hieracium, the formation of seeds through sexual and apomictic processes differ in two aspects: meiosis is avoided during apomictic female gametophyte formation (apomeiosis), which is controlled by the loss of apomeiosis (LOA) locus; and embryo and endosperm develop autonomously, a process controlled by an independent locus, loss of parthenogenesis (LOP). In sexual Arabidopsis, seed initiation is inhibited by the Fertilization Independent Seed (FIS) protein complex, which includes MULTICOPY SUPPRESSOR OF IRA1 (MSI1) and a distinct complex involving MSI1 and RETINOBLASTOMA-RELATED PROTEIN (RBR). msi1 mutants initiate autonomous egg cell proliferation forming a non-viable parthenogenetic embryo. Here we examined the association of HMSI1 orthologue with the initiation of seed development in Hieracium. HMSI1 is expressed in apomictic and sexual female gametes and in ovaries containing developing seeds. HMSI1 interacted with HRBR in yeast two-hybrid analyses as found in Arabidopsis. HMSI1 was not commonly altered in the genomes of gamma radiation lop mutants with lesions at the LOP locus and HMSI1 expression was down-regulated relative to the wild type in an unpollinated lop mutant that requires fertilization to develop functional embryos. The data suggest that HMSI1 is not linked to LOP and is thus unlikely to trigger the switch to autonomous seed development in apomictic Hieracium but more likely to be involved in seed formation events downstream of LOP activity.