|Rafflesia kerrii flower|
The Rafflesiaceae are a family of parasitic plants found in 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. Only the flowers emerge from the roots or lower stems of the host plants.
Past taxonomic works have varied as to the classification of Rafflesiaceae. 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. Thus, the group traditionally classified as a single family, Rafflesiaceae, was actually composed of at least three 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:
- Rafflesiaceae (sensu stricto): Rafflesia, Rhizanthes, Sapria — order Malpighiales
- Mitrastemonaceae: Mitrastema — order Ericales
- Cytinaceae: Bdallophyton, Cytinus — order Malvales
- Apodanthaceae: Apodanthes, Berlinianche, Pilostyles — order Cucurbitales
These four families can be easily distinguished by floral and inflorescence features:
- Rafflesiaceae: inferior ovary, large flowers occurring singly
- Mitrastemonaceae: superior ovary, flowers occurring singly
- Cytinaceae: inferior ovary, flowers in inflorescences
- Apodanthaceae: inferior ovary, small flowers occurring singly (but arising in clusters from host bark)
Early work on higher-level relationships was able to place Rafflesiaceae (in the strict sense) within the order Malpighiales, but was not able to resolve the closest ancestor within the order. A more recent phylogenetic analysis found strong support for Rafflesiaceae being derived from within Euphorbiaceae, which is surprising as members of that family typically have very small flowers. According to their analysis, the rate of flower size evolution was more or less constant throughout the family, except at the origin of Rafflesiaceae — a period of about 46 million years between when the group split from the higher Euphorbiaceae, and when the existing Rafflesiaceae split from each other — where the flowers rapidly evolved to become much larger before reverting to the slower rate of change. Given that this hypothesis is confirmed, in order to maintain monophyly of Euphorbiaceae, either the basal clade (represented by Pogonophora, Pera, and Clutia in the tree) has to be split off as the separate family Peraceae, as the Angiosperm Phylogeny Website does, or Rafflesiaceae would have to be included in the Euphorbiaceae.
Horizontal gene transfer
A number of mitochondrial genes in the Rafflesiaceae appear to have come from their hosts. Because the hosts are not closely related to the parasites (as shown by molecular phylogeny results for other parts of the genome), this is believed to be the result of horizontal gene transfer.
- Stevens, P.F. (2001), Angiosperm Phylogeny Website, retrieved 2012-02-02
- Barkman, T. J.; Seok-Hong Lim, Kamarudin Mat Salleh, Jamili Nais (January 20, 2004). "Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world's largest flower". PNAS 101 (3): 787–792. doi:10.1073/pnas.0305562101. PMC 321759. PMID 14715901.
- Davis C. C., et al. Science, doi:10.1126/science.1135260 (2007).
- Charles C. Davis and Kenneth J. Wurdack (30 July 2004). "Host-to-Parasite Gene Transfer in Flowering Plants: Phylogenetic Evidence from Malpighiales". Science 305 (5684): 676–678. doi:10.1126/science.1100671. PMID 15256617.
- Daniel L Nickrent, Albert Blarer, Yin-Long Qiu, Romina Vidal-Russell and Frank E Anderson (2004). "Phylogenetic inference in Rafflesiales: the influence of rate heterogeneity and horizontal gene transfer". BMC Evolutionary Biology 4: 40. doi:10.1186/1471-2148-4-40. PMC 528834. PMID 15496229.
- Barkman, T.J., S.-H. Lim, K. Mat Salleh and J. Nais. 2004. Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world's largest flower. Proceedings of the National Academy of Sciences of USA 101:787–792.
- Charles C. Davis, Maribeth Latvis, Daniel L. Nickrent, Kenneth J. Wurdack, David A. Baum. 2007. Floral gigantism in Rafflesiaceae. Science Express, published online January 11, 2007 (online abstract here).
- Meijer, W. 1997. Rafflesiaceae, in Flora Malesiana I, 13: 1–42.
- Nickrent, D.L., A. Blarer, Y.-L. Qiu, R. Vidal-Russell and F.E. Anderson. 2004. Phylogenetic inference in Rafflesiales: the influence of rate heterogeneity and horizontal gene transfer. BMC Evolutionary Biology 4:40 (HTML abstract PDF fulltext).
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