Genetic analyses of the inheritance and expressivity of autonomous endosperm formation in Hieracium with different modes of embryo sac and seed formation.

Background and Aims: Apomixis, or asexual seed formation, in polyploid Hieracium subgenus Pilosella species results in clonal progeny with a maternal genotype. An aposporous embryo sac forms mitotically from a somatic cell, without prior meiosis, while embryo and endosperm formation is fertilization independent (autonomous). The latter two developmental components are tightly linked in Hieracium . Recently, two plants, AutE196 and AutE24, were identified from two different crosses. Both form embryo sacs via the sexual route by undergoing meiosis, and embryo development requires fertilization; however, 18 % of embryo sacs can undergo autonomous endosperm (AutE) formation. This study investigated the qualitative and quantitative inheritance of the AutE trait and factors influencing phenotype expressivity. An additional focus was to identify the linkage group bearing the AutE locus in AutE196.

Methods: Crosses and cytology were used to examine the inheritance of AutE from AutE24 and AutE196, and to reintroduce apomictic components into AutE plants, thereby changing the ploidy of developing embryo sacs and increasing the dosage of AutE loci. Markers from a Hieracium apomict linkage map were examined within a backcrossed AutE196 mapping population to identify the linkage group containing the AutE196 locus.

Key Results: Qualitative autonomous endosperm in the AutE24 line was conferred by a single dominant locus, and the trait was transmitted through male and female gametes in AutE196 and AutE24. Expressivity of the trait did not significantly increase when AutE loci from AutE196 and AutE24 were both present in the progeny, within embryo sacs formed via apospory, or sexually derived embryo sacs with increased ploidy. It remains unclear if these are identical loci.

Conclusions: The qualitative trait of autonomous endosperm formation is conferred by single dominant loci in AutE196 and AutE24. High expressivity of autonomous endosperm formation observed in apomicts requires additional genetic factors. Potential candidates may be signals arising from fertilization-independent embryo formation.