Rafflesiaceae

The Rafflesiaceae are a family of rare parasitic plants found in the tropical forests of east and southeast Asia, including Rafflesia arnoldii, which has the largest flowers of all plants. The plants are endoparasites of vines in the genus Tetrastigma (Vitaceae) and lack stems, leaves, roots, and any photosynthetic tissue. They rely entirely on their host plants for both water and nutrients, and only then emerge as flowers from the roots or lower stems of the host plants. Rafflesiaceae flowers mimic rotting carcasses in scent, color, and texture to attract their pollinators, carrion flies. For this reason, some flowers of the family Rafflesia are nicknamed 'corpse flowers'. Most members of Rafflesiaceae possess a large, bowl-shaped floral chamber formed by a perianth tube and a diaphragm. This diaphragm is the opening for carrion fly pollinators and is surrounded by attractive sterile organs. Flowers are generally unisexual, and can range from tens of cm to over a meter large. Past taxonomic works have varied as to the classification of Rafflesiaceae. The classification of Rafflesiaceae has been somewhat problematic due to their highly reduced vegetative parts, modified reproductive structures, and anomalous molecular evolution (Davis 2008). Rafflesiaceae lacks rbcL and other plastid genes commonly used for phylogenetic inference in green plants. In fact, Molina et al. (2014) found that a genus of Rafflesia is the first parasitic plant studied containing no recognizable remnants of the chloroplast genome. Most traditional classifications that were based entirely on morphological features considered Rafflesiaceae sensu lato (in the broad sense) to include nine genera, but the heterogeneity among these genera caused early workers, such as Harms (1935), to recognize four distinct groups that were then classified as tribes (still within Rafflesiaceae). This tribal system was followed by Takhtajan et al. (1985). The first molecular phylogenetic study (using DNA sequences) that showed two of these tribes were not related was by Barkman et al. (2004). This study showed three genera (corresponding to tribe Rafflesieae, that is, Rafflesia, Rhizanthes, and Sapria) were components of the eudicot order Malpighiales. The genus Mitrastema (tribe Mitrastemeae) was shown to be unrelated and a member of the order Ericales. Later that year, Nickrent et al. (2004), using additional molecular data, confirmed the placements by Barkman et al. (2004) and also examined the positions of the two other tribes, Cytineae (Bdallophyton and Cytinus) and Apodantheae (Apodanthes, Berlinianche,and Pilostyles). Nickrent et al. (2004) showed Cytineae was related to Malvales and Apodantheae to either Malvales or Cucurbitales. Apodantheae has since been confirmed to be in the Cucurbitales (Filipowicz and Renner 2010). Thus, the group traditionally classified as a single family, Rafflesiaceae, was actually composed of at least four distinct and very distantly related clades, with their similarities due to convergent evolution under their common parasitic lifestyle. A goal of taxonomy is to classify together only plants that all share a common ancestor, i.e., are monophyletic. Thus, the original Rafflesiaceae sensu lato is currently split into four families: