| Rafflesiaceae | |
|---|---|
.jpg) | |
| Rafflesia keithii flower | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Malpighiales |
| Family: | Rafflesiaceae Dumort. |
| Genera[1] | |
Rafflesia R.Br. ex Gray | |
TheRafflesiaceae are a family of rareparasiticplants comprising 36 species in 3 genera found in thetropical forests of east and southeast Asia, includingRafflesia arnoldii, which has the largest flowers of all plants. The plants areendoparasites of vines in the genusTetrastigma (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 familyRafflesia are nicknamed "corpse flowers". Most members of Rafflesiaceae possess a large, bowl-shaped floral chamber formed by aperianth tube and a diaphragm. This diaphragm is the opening for carrion fly pollinators and is surrounded by attractive sterile organs. Flowers are generallyunisexual, and can range from tens of cm to over a meter large.[2][3]
Past taxonomic works have varied as to the classification of Rafflesiaceae. Classifying has been somewhat problematic due to their highly reduced vegetative parts, modified reproductive structures, and anomalousmolecular evolution (Davis 2008). Rafflesiaceae lacksrbcL and otherplastid genes commonly used forphylogenetic inference about green plants. In fact, Molina et al. (2014) found that a genus ofRafflesia is the first studied parasitic plant that contains no recognizable remnants of thechloroplast genome.
Most traditional classifications that were based entirely on morphological features considered Rafflesiaceaesensu lato (in the broad sense) to include nine genera, but the heterogeneity among these genera caused early investigators, 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).
But that classification does not reflect evolutionary relations shown by DNA sequence studies. Three genera in tribe Rafflesieae (namely,Rafflesia,Rhizanthes andSapria) are in the eudicot order Malpighiales, while genusMitrastemon (tribe Mitrastemoneae) is in order Ericales [Barkman et al., 2004, Nickrent et al., 2004]. Also, tribe Cytineae (Bdallophyton andCytinus) is in order Malvales, and tribe Apodantheae (Apodanthes,Berlinianche, andPilostyles) is in order Cucurbitales [Nickrent et al., 2004; Filipowicz and Renner, 2010].
Thus, the group which was considered a single family, Rafflesiaceae, is composed of at least four distantly related clades, which have morphological similarities due toconvergent 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, currently the original Rafflesiaceaesensu lato is split into four families:[4]
These four families can be easily distinguished by floral and inflorescence features:
Early work on higher-level relationships was able to place Rafflesiaceae (in the strict sense) within the orderMalpighiales, but was not able to resolve the closest ancestor within the order.[5] A 2007 phylogenetic analysis found strong support for Rafflesiaceae being derived from withinEuphorbiaceae as traditionallycircumscribed, which was surprising as members of that family typically have very small flowers. According to this 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 Euphorbiaceaesensu stricto, 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.[6]
To maintainmonophyletic families, in 2016 theAPG IV system separated the familyPeraceae from the Euphorbiaceae.[7] A summary cladogram is shown below,[6] with family placements in theAPG IV system.[7]
| Euphorbiaceae sensu lato |
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A more recent study has been provided by Liming Caiet al. (2021)[8]
A number ofmitochondrial genes in the Rafflesiaceae appear to have come from their hosts (Tetrastigma). Because the hosts are not closely related to the parasites (as shown bymolecular phylogeny results for other parts of the genome), this is believed to be the result ofhorizontal gene transfer.[9][10] Especially high rates of HGT have been found to take place in Rafflesiaceae mitochondrial genes when compared tonuclear genes and to HGT inautotrophic plants.[11]
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