Rafflesiaceae Totally Explained

Everything about Rafflesiaceae totally explained

Rafflesiaceae is a family of parasitic plants found in east and southeast Asia, including Rafflesia arnoldii, the plant with the largest flower 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.

Relationships

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 that three genera (corresponding to tribe Rafflesieae, for example 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 that 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 distantly related clades. A goal of taxonomy is to classify together only plants that all share a common ancestor (for example are monophyletic). Thus, Rafflesiaceae sensu lato should be "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 Malvales or 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 wasn't 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. If this hypothesis is confirmed, in order to maintain monophyly of Euphorbiaceae, either the basal clade (represented by Pogonophora, Pera, and Clutia in the tree) will have to be split off as a separate family (and in fact Pera is recognized in the separate family Peraceae in some classifications), or Rafflesiaceae will 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.

Further Information

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