The big, the bad, and the beautiful: Biology of the world's largest flowers

Rafflesiaceae, crowned the greatest prodigy of the vegetable world, produce the largest flowers in angiosperms. They are also holoparasites residing inside their vine hosts, and emerge only during flowering. The floral gigantism and obligate parasitism of Rafflesiaceae have rendered their structure unrecognizable to most plant biologists. The vegetative body is composed of highly reduced strands of cells embedded in the host tissue, and does not differentiate into leaves, stems, or roots. The flowers look and smell like decaying animal flesh and exhibit numerous features unknown in the vast majority of flowering plants. This unusual combination of characters, alongside their generally elevated rates of molecular evolution among commonly used phylogenetic markers and propensity for host-to-parasite horizontal gene transfer, has obscured the phylogenetic affinities of Rafflesiaceae since their discovery two centuries ago. Here, we review the phylogenetic placement of Rafflesiaceae and how it has informed a deeper understanding of the pattern and magnitude of horizontal gene transfer in parasitic plants. We also examine the vegetative and reproductive morphology of Rafflesiaceae to provide insight into how these unusual plants are constructed, and offer clues on their evolution from tiny flowered ancestors to floral giants.