Phylogenetic Relationships of Parasitic Flowering Plants

For a number of parasitic flowering plants, their closest relatives have been known for some time. These include Cassytha with Lauraceae, Lennoaceae with Boraginaceae, Orobanchaceae with "Scrophulariaceae" in the traditional sense, and Cuscuta with Convolvulaceae. For others, particularly the holoparasites Hydnoraceae, Balanophoraceae, and Rafflesiaceae, placement among photosynthetic angiosperms has been difficult. For this reason, their traditional classifications often conflicted.

Through DNA sequencing and molecular phylogenetic analyses, relationships between parasitic and nonparasitic angiosperms have been greatly clarified. The topology of the phylogenetic tree shown below is from APG2 (2003) and is derived from the large-scale molecular phylogenetic analysis of angiosperms by Soltis et al. (2000). Parasitic plant families that were not included in that analysis were added (e.g. Mitrastemonaceae, Rafflesiaceae, etc.) based on more recent evidence (see below). Groups enclosed by a dashed border represent hemiparasites, that is, plants that obtain water and nutrients from a host yet still produce their own carbohydrates via photosynthesis. Groups enclosed by a black border are holoparasites that do not photosynthesize (or do so at very low levels), hence must obtain carbohydrates from their host plants. Two groups, Orobanchaceae and Convolvulaceae (Cuscuta) contain both hemi- and holoparasites. Overall, it appears that parasitism has arisen independently in angiosperms 11 or 12 times (red taxon names on tree).

To navigate to a specific family page, click the name in the colored box.

Notes on the various groups:

1. Laurales. Cassytha, the sole parasitic member of the large family Lauraceae, is uniquivocally assigned to this family based on morphological and molecular data. Its superficial resemblance to Cuscuta is remarkable and an excellent example of convergent evolution.

2. Piperales. Molecular data were used to place Hydnoraceae with Aristolochiaceae s. lat. (Nickrent et al. 2002). The exact topology of the component families of this order (Aristolochiaceae, Hydnoraceae, Lactoridaceae, Piperaceae and Saururaceae). More recent analyses (Nickrent 2005, IBC abstract) suggests Hydnoraceae is most closely related to Aristolochiaceae.

3. Santalales. The analyses conducted by Soltis et al. (2000) resolved the sandalwood order as monophyletic, but this clade was part of a large polytomy among the core eudicots. The number of taxa involved in this polytomy is quite high, involving caryophyllids, rosids and asterids. Further molecular work is clearly needed to break this large polytomy. The family Balanophoraceae, previously placed in its own order (Balanophorales, Takhtajan 1997), has recently been shown from molecular evidence (Nickrent et al. 2005) to be related to Santalales. This result is surprising but supported by both nuclear and mitochondrial genes. The exact relationship between these two groups remains to be determined.

4. Saxifragales. Although previous classified with Balanophoraceae, the family Cynomoriaceae has been shown to be a component of Saxifragales. See Nickrent et al. (2005) for a complete discussion of this finding.

5. Krameriaceae. The APG2 classification considered Krameriaceae as an acceptable, monophyletic alternative to Zygophyllaceae. This "family" is unresolved at the base of the eurosid I clade.

6. Rafflesiaceae. This family, considered here in the strict sense (including Rafflesia, Rhizanthes, and Sapria, i.e., the "large-flowered clade") was placed with Malpighiales by Barkman et al. (2004) using mitochondrial matR gene sequences. This position was confirmed by Nickrent et al. (2004) using both nuclear SSU rDNA as well as mitochondrial sequence data.

7. Cytinaceae. The "inflorescence clade" is composed of Cytinus and Bdallophyton. Mitochondrial matR and nuclear SSU rDNA both strongly support a position for this family in Malvales. See Nickrent et al. (2004).

8. Apodanthaceae. The "small-flowered clade" is composed of Apodanthes, Berlinianche, and Pilostyles. Mitochondrial matR and nuclear SSU rDNA data indicate either a relationship with Malvales or Cucurbitales (Nickrent et al. 2004). For now it is placed near Malvales and Cytinaceae, but in reality this remains as the last major "unsolved" relationship among parasitic flowering plants.

9. Mitrastemonaceae. This family, with but a single genus Mitrastema, was shown to be related to Ericales by Barkman et al. (2004) using mitochondrial matR gene sequences. This result is confirmed using nuclear SSU rDNA and mitochondrial sequence data (Nickrent et al. 2004).

10. Boraginaceae. These holoparasites have traditionally been placed in their own family, but APG2 lumps "Lennoaceae" with Boraginaceae, a family with which they are clearly related as shown by both morphological and molecular evidence. Boraginaceae s. lat. is currently unresolved at the base of the euasterid I clade.

11. Orobanchaceae. This family name formerly referred only to an assemblage of holoparasitic taxa that were recognized to be related to the hemiparasites of Scrophulariaceae. More recent circumscriptions of this group (see Young et al. 1999, Olmstead et al. 2001) place all parasitic "scrophs" in a monophyletic family Orobanchaceae. Morphological and molecular evidence clearly place this family in Lamiales.

12. Convolvulaceae. The sole parasitic genus of Convolvulaceae is Cuscuta that has sometimes been placed in its own family, Cuscutaceae. Analysis of sequence data from four chloroplast gene regions resulted in Cuscuta being nested within Convolvulaceae (Stefanovic and Olmstead 2000), thus the classification of APG2 is supported.