There are two types of heterotrophic plants: 1) mycoheterotrophs, plants that obtain nutrients from other vascular plants via a fungal intermediate, and 2) haustorial parasites, ones that connect directly to the host xylem and/or phloem. Here we will consider only the haustorial parasites. Some such as mistletoe and sandalwood are called hemiparasites because they can produce at least some of their own food via photosynthesis. But others such as dodder are holoparasites because they don’t photosynthesize and rely upon the host for sugars transported in the phloem. 

Holoparasites have evolved nine separate times among the 12 extant lineages of parasitic flowering plants. Classifications from a few decades ago recognized a single family Rafflesiaceae that contained distantly related holoparasites (as we know today). These four groups all have endophytes, but each differs in inflorescence and flower structure. The small-flowered holoparasite of legumes called Pilostyles is actually a member of Cucurbitales (containing cucumbers, squash). Moreover, Cytinaceae is a family in Malvales (mallows, cotton) and Mitrastemonaceae is within Ericales (heathers, blueberries). Finally, Rafflesiaceae (in the modern sense) is a member of Malpighiales which includes spurges (Euphorbiaceae), a remarkable finding because that family has members with small flowers. Compared to spurges, Rafflesia flowers are up to 79 times larger.

The endophytic part of the Rafflesia plant consists of strands of tissue that grow inside its host vine (Tetrastigma in the grape family Vitaceae). The presence of the parasite is revealed when the young flower bud emerges from the host bark. The flowers have the look and smell of rotting flesh, thus attracting pollinating flies. The size may be an adaptation to broadcasting the smell widely in a tropical forest with little wind and where the nearest other flowering Rafflesia may be kilometers away. Liquid pollen is attached to the fly. It later enters an open female flower where it navigates to the base of the column and pollinates the flower. Open flowers may be widely separated and flowers are usually either male or female, thus pollination is rare as are fruits that develop subsequently.

Rafflesia arnoldi has the largest single flower among flowering plants, over one meter in diameter. Sometimes people erroneously cite the corpse flower (Amorphophallus titanum, Araceae) as the largest flower, but that species has a large unbranched inflorescence (composed of many flowers). Although there has been much speculation about seed dispersal in Rafflesia, in the Philippines ants were observed carrying seeds underground into their nest. The presence of a fatty structure on the seed (elaiosome) supports the hypothesis of “myrmecochory” (ant dispersal). This dispersal mechanism needs to be verified in other Rafflesia species. Moreover, seedlings have never been observed parasitizing Tetrastigma roots, so this critical stage of its life cycle is still not known.

Plants typically have distinct genomes within their nucleus, mitochondria, and chloroplasts. Scientists interested in tracing the evolutionary history of plants have sequenced DNA from each of these subcellular compartments. By comparing DNA from a large sampling of species, a phylogenetic tree can be constructed. So for any plant, three different analyses could each produce a tree. Usually these three trees are congruent with each other. When all three compartments support the same branching pattern, this is considered strong evidence that the DNA is tracking the actual evolutionary relationships among the plants. But sometimes the trees are incongruent. There are many reasons why this can be the case, but one of these is a phenomenon happening quite frequently in parasitic plants called horizontal gene transfer (HGT).

HGT was unknown in plants until we had access to phylogenetic trees across a wide sampling of plants representing all three compartments. HGT contrasts with vertical transmission where genetic material is passed down from the parents, grandparents, etc. via sexual reproduction. Because parasitic plants are attached to another plant via their haustoria, the cells of the two unrelated plants are in close contact and this apparently facilitates HGT and contributes to parasite adaptation. The existence of HGT in Rafflesia was demonstrated by comparing nuclear and mitochondrial gene trees. Later, when entire mitochondrial genomes were sequenced, it was discovered that they were full of foreign DNA (from both hosts and non-hosts).

Originally it was thought that only two Rafflesia species existed in the Philippines: R. manillana and R. schadenbergiana (the latter thought to be extinct). With the discovery of R. speciosa in 2002, scientists and the general public became interested in Rafflesia and a wave of new discoveries ensued. Between 2005 and 2016, nine new species of Rafflesia were discovered and/or recognized in the Philippines, plus R. schadenbergiana was rediscovered at several locations on Mindanao. Despite the fact that the Philippines and other countries in Southeast Asia are experiencing loss of natural habitats, there is hope that Rafflesia species will persist. Rafflesia occurs in protected areas and forest fragments and these populations can be considered compound islands, islands of forest present on actual geographical islands. Local people in the Philippines are often aware of the presence of Rafflesia in their vicinity and this amazing flower is sometimes featured in artwork, public statues, and parades. National pride is demonstrated by the fact that Rafflesia is found on at least 17 stamps from Southeast Asia and R. arnoldii is one of three national flowers of Indonesia. In situ conservation could promote increased ecotourism and enhanced economic opportunity for local people.

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