Early Stages of Development Shed Light on Fruit Evolution in Allopolyploid Species of Geum (Rosaceae)

Molecular phylogenetic studies have shown that the evolutionary history of Geum L. has been strongly influenced by allopolyploidy, and they have indicated widespread homoplasy in morphological traits within the group. This study of gynoecial development in Colurieae shows that fruits with jointed styles develop similarly. In early stages of development, the position of the joint is basal in all lineages where it is present. In the diploids, it remains at the base of the style, but in the tetraploid and the two hexaploid lineages, a beak forms from the distal part of the ovary, causing the joint to be brought upward as it elongates. In the two lineages of the hexaploid clade, a hook is subsequently developed in the distal part of the beak (the ‘‘fishhook fruit’’). One of these also undergoes a final stage when the beak is twisted. The similarity of development implies that the joint and the deciduous segment of the style are homologous structures inherited by the polyploids from one of their diploid ancestral lineages, although they are not expressed in all species. Genome-level processes that could affect the expression of morphological traits in groups of allopolyploid origin are discussed.