Genetic separation of autonomous endosperm formation (AutE) from the two other components of apomixis in Hieracium

Abstract

In apomictic Hieracium subgenus Pilosella species, embryo sacs develop in ovules without meiosis. Embryo and endosperm formation then occur without fertilization, producing seeds with a maternal genotype encased in a fruit (achene). Genetic analyses in H. praealtum indicate a dominant locus (LOA) controls meiotic avoidance, and another dominant locus (LOP) controls both fertilization-independent embryogenesis and endosperm formation. While cytologically examining developmental events in ovules of progeny from crosses between different wild-type and mutant Hieracium apomicts, and a sexual Hieracium species, we identified two plants, AutE196 and AutE24, which have lost the capacity for meiotic avoidance and fertilization-independent embryo formation. AutE196 and AutE24 exhibit autonomous endosperm formation and set parthenocarpic, seedless achenes at a penetrance of 18 %. Viable seed form after pollination. Cytological examination of 102 progeny from a backcross of AutE196 with sexual H. pilosella showed that autonomous endosperm formation is a heritable, dominant, qualitative trait, detected in 51 % of progeny. Variation in quantitative trait penetrance indicates other factors influence its expression. The correlation between autonomous endosperm development and mature parthenocarpic achene formation suggests the former is sufficient to trigger fruit maturation in Hieracium. The developmental component of autonomous endosperm formation is therefore genetically separable from those controlling meiotic avoidance and autonomous embryogenesis in Hieracium and has been denoted as AutE. We postulate that tight linkage of AutE and genes controlling autonomous embryogenesis at the LOP locus in H. praealtum may explain why inheritance of autonomous seed formation is typically observed as a single component.