Evolutionary shifts to gynomonoecy correlate with changes in capitulum architecture in South African Helichrysum (Asteraceae).

Gynomonoecy has evolved repeatedly in Asteraceae, suggesting selective advantages for species whose capitula combine pistillate and bisexual flowers. Theories invoking conspicuous peripheral flowers and achene heteromorphism do not account for gynomonoecy in the southern African-centred genus Helichrysum, which lacks both these features. We investigated whether gynomonoecy is associated with changes in capitulum architecture in South African Helichrysum, focusing on flower packing density and achene size. We compared flower packing density and achene size between hermaphroditic and gynomonoecious Helichrysum species using phylogenetic generalised least squares and reconstructed ancestral sexual systems using a well-sampled, time-calibrated phylogeny. Gynomonoecious species produced nearly twice as many flowers per unit receptacle area as hermaphroditic species and had subtly but significantly smaller achenes. Ancestral-state reconstruction strongly supported hermaphroditism as ancestral, with gynomonoecy originating independently 18 times. Our results provide the first phylogenetically informed evidence within a single Asteraceae genus that shifts to gynomonoecy correlate with shifts in capitulum architecture. The association between gynomonoecy, denser flower packing, and reduced achene size is consistent with the hypothesis that accentuated resource gradients within capitula and priority access to resources by first-maturing peripheral flowers may have favoured the evolution of gynomonoecy.