When the APG II system of plant classification was published in April 2003, fifteen genera and three families were placed incertae sedis in the angiosperms, and were listed in a section of the appendix entitled "Taxa of uncertain position".[1]

By the end of 2009, molecular phylogenetic analysis of DNA sequences had revealed the relationships of most of these taxa, and all but three of them had been placed in some group within the angiosperms. In October 2009, APG II was superseded by the APG III system.[2] In APG III, 11 of the genera listed above were placed in families, or else became families whose position within their orders was approximately or exactly known. The family Rafflesiaceae was placed in the order Malpighiales, close to Euphorbiaceae and possibly within it. Mitrastema became a monotypic family, Mitrastemonaceae. This family and Balanophoraceae were placed incertae sedis into orders, that is, their positions within these orders remained completely unknown. Metteniusa was found to belong to a supraordinal group known as the lamiids, which has not been satisfactorily divided into orders. Cynomorium was raised to familial status as Cynomoriaceae, and along with Apodanthaceae and Gumillea, remained unplaced in APG III. Five taxa were unplaced among the angiosperms in APG III because Nicobariodendron and Petenaea were added to the list.

Leptaulus

There is no apparent reason for the inclusion of Leptaulus in the list of unplaced taxa, other than the time lag between submission and publication. In 2001, in a phylogenetic study based on morphological and DNA data, Leptaulus was found to belong to a group of six genera that most authors now consider to be the family Cardiopteridaceae.[3] This was confirmed in a study of wood anatomy in 2008.[4] The genus is placed in the Cardiopteridaceae in the APG III system of 2009.[1] Before 2001, Leptaulus and the rest of Cardiopteridaceae had usually been placed in a broadly circumscribed Icacinaceae, which turned out to be polyphyletic.

Some botanists do not recognize Cardiopteridaceae as a family of six genera. Instead, they segregate Cardiopteris into a monogeneric Cardiopteridaceae sensu stricto and place the other five genera in the family Leptaulaceae.[5] The monophyly of Leptaulaceae has never been tested with molecular data.

Pottingeria

It had long been thought, at least by some, that the small Southeast Asian tree Pottingeria might belong in the order Celastrales.[6] In a phylogenetic study of that order in 2006, Pottingeria was found to be a member of the order, but not of any of its families. It was in an unresolved pentatomy consisting of Parnassiaceae, Pottingeria, Mortonia, the pair (Quetzalia + Zinowiewia), and the other genera of Celastraceae.[7] When the APG III system was published in October 2009, the Angiosperm Phylogeny Group expanded Celastraceae to include all members of the pentatomy mentioned above.[8]

Dipentodon

Dipentodon has one species Dipentodon sinicus.[9] It is native to southern China, Burma, and northern India.[10] In 2009, in a molecular phylogenetic study of the order Huerteales, it was shown that Dipentodon and Perrottetia belong together as the two genera of the family Dipentodontaceae.[11]

Medusandra and Soyauxia

In 2009, in a molecular phylogenetic study of Malpighiales, Kenneth Wurdack and Charles Davis sampled five genera and one family that had been unplaced in APG II. They placed some of these for the first time and confirmed the previous placement of others with strong statistical support.[12]

In their outgroup, they included four genera from Saxifragales. These were Daphniphyllum, Medusandra, Soyauxia, and Peridiscus. In their phylogeny, Medusandra and Soyauxia formed a strongly supported clade with Peridiscus, a member of the family Peridiscaceae, the most basal clade in Saxifragales. Wurdack and Davis recommended that Medusandra and Soyauxia both be transferred to Peridiscaceae. Thus the monogeneric family Medusandraceae is subsumed into Peridiscaceae. Soyauxia had been found to be close to Peridiscus in another study two years before.[13] Wurdack and Davis also found that the family Rafflesiaceae and the genera Aneulophus, Centroplacus, and Trichostephanus belong in the order Malpighiales.[12]

Aneulophus

Aneulophus consists of two species of woody plants from tropical West Africa.[14] Wurdack and Davis found the traditional placement of Aneulophus in Erythroxylaceae to be correct.[12] Its position within the family remains uncertain.

Erythroxylaceae is a family of four genera. Erythroxylum has about 230 species. Nectaropetalum has eight species and Pinacopodium has two. No one has yet produced a molecular phylogeny of the family.

Centroplacus

Centroplacus has a single species, Centroplacus glaucinus, a tree from West Africa. It was found to be close to Bhesa, a genus that had only recently been removed from Celastrales.[7] Bhesa was grouped with Centroplacus to become the second genus in Centroplacaceae.[12] Bhesa consists of five species of trees from India and Malesia.

Trichostephanus

Trichostephanus has two species, both in tropical West Africa. It had usually been assigned to Achariaceae, but it was found to be deeply embedded in Samydaceae.[12][15] Many taxonomists do not recognize Samydaceae as a separate family from Salicaceae.

Rafflesiaceae

Several genera have been removed from Rafflesiaceae, so that it now consists of only three genera: Sapria, Rhizanthes, and Rafflesia. All of these are holoparasites and, as discussed below, finding their relationships by molecular phylogenetics has presented special challenges. Rafflesia and its relatives were the subject of several papers from 2004 to 2009, and as the world's largest flower, Rafflesia has attracted special interest. In 2009, Wurdack and Davis confirmed earlier work in which it was found that Rafflesiaceae is nested within Euphorbiaceae sensu stricto, a circumscription of Euphorbiaceae that excludes Phyllanthaceae, Picrodendraceae, Putranjivaceae, Pandaceae, and a few other very small groups that had been included in it until the 1990s.[16] In order to preserve Rafflesiaceae, Wurdack and Davis split Euphorbiaceae sensu stricto into Euphorbiaceae sensu strictissimo and Peraceae, a new family comprising Pera and four other genera.[12]

Parasites

Four of the unplaced genera, and all three of the unplaced families of APG II consist of achlorophyllous holoparasites. In these, the chloroplast genes that are usually used in phylogenetic studies of angiosperms have become nonfunctional pseudogenes. If these evolve rapidly, they may be saturated with repeated mutations at the same site and consequently not be useful for phylogenetic reconstruction.

The relationships of some parasitic taxa have been elucidated in studies of nuclear and mitochondrial DNA sequences. But these sequences sometimes produce artifactual topologies in the phylogenetic tree, because horizontal gene transfer often occurs between parasites and their hosts.[17]

Bdallophyton and Cytinus

The parasitic genera Bdallophyton and Cytinus have been found to be closely related and have been placed together as the family Cytinaceae. On the basis of mitochondrial DNA, Cytinaceae has been placed in Malvales, as sister to Muntingiaceae.[18]

Mitrastemon

The parasitic family Mitrastemonaceae has one genus, known either as Mitrastemon or Mitrastema. The genus name and the corresponding family name have been a source of much confusion.[19] A phylogeny based on mitochondrial genes places Mitrastemon in the order Ericales, but this result had only 76% maximum likelihood bootstrap support.[20]

Hoplestigma

Hoplestigma consists of two species of African trees, notable for their large leaves, up to 55 cm long and 25 cm wide.[21] It is usually placed by itself in the family Hoplestigmataceae which is thought to be related to Boraginaceae.[22] In 2014, a phylogeny of Boraginaceae was published in a scientific journal called Cladistics.[23] By comparing the DNA sequences of selected genes, the authors of that study showed that Hoplestigma is related to members of Boraginaceae subfamily Cordioideae, and they recommended that Hoplestigma be placed in that subfamily. Other authors have suggested that, while Hoplestigma is the closest relative of Cordioideae, it should perhaps not be placed within it.[24]

Metteniusa

Metteniusa consists of seven species of trees in Central America and northwestern South America. Ever since Hermann Karsten proposed the name Metteniusaceae in 1859, some authors have placed Metteniusa by itself, in that family.[25] Most authors, however, placed it in Icacinaceae until that family was shown to be polyphyletic in 2001.[3]

In 2007, in a comparison of DNA sequences for three genes, it was found that Metteniusa is one of the basal clades of the lamiids. The authors recommended that the family Metteniusaceae be recognized.[26] Nothing is yet known about relationships among the groups of basal lamiids. The groups in this polytomy include the order Garryales, the families Icacinaceae, Oncothecaceae, and Metteniusaceae, as well as some unplaced genera, including Apodytes, Emmotum, and Cassinopsis.[24]

No phylogenetic study has focused on the lamiids, but phylogenies have been inferred for the asterids, a group composed of Cornales, Ericales, the lamiids, and the campanulids.[27][28]

Balanophoraceae

Balanophoraceae is a family of holoparasites with 44 species in 17 genera.[24] For a long time, Cynomorium was usually included in this family, but it is now known to be unrelated.[20] In 2005, Balanophoraceae was shown to be in the order Santalales, but its position within that order has not been determined.[29]

Two researchers in Taiwan announced on the internet in 2009 that they have results supporting the placement of Balanophoraceae in Santalales.[30] They have yet to publish anything in a scientific journal.

Cynomorium

Many names have been published in Cynomorium,[31] but there are probably only two species.[32] It is not closely related to anything else, so it is placed in the monogeneric family Cynomoriaceae.[20]

Attempts to find its closest relatives have demonstrated with special clarity that molecular phylogenetics is not a sure-fire, problem-free method of determining systematic relationships. One study placed it in Saxifragales, but not at any particular position within that order.[29] Doubts have been expressed about the results of this study. Another study placed Cynomorium in Rosales based on analysis of the two invert repeat regions of the chloroplast genome, which evolve at one fifth the rate of the two single copy regions.[33]

Gumillea

Gumillea has a single species, Gumillea auriculata,[34] and is known from only one specimen which was collected in the late 18th century in Peru.[35] It was named by Hipólito Ruiz López and José Antonio Pavón Jiménez.[36]

George Bentham and Joseph Hooker placed it in Cunoniaceae,[37] and this treatment was followed by Adolf Engler and most others.[34] The last comprehensive treatment of Cunoniaceae, however, excludes it from the family.[38] In 2009, Armen Takhtajan placed Gumillea in Simaroubaceae.[39] A 2007 article on Simaroubaceae contains a list of the genera in the family. Gumillea is not on that list, but the authors do not provide a list or section on excluded genera.[40]

Gumillea has also been called a synonym of Picramnia,[14][41] but the ultimate source of this information is obscure and it is not mentioned in either of the recent treatments of Picramnia.[42][43] It is worth noting that on their plate for Gumillea, Ruiz and Pavón showed 11 ovules or immature seeds that had been extracted from a 2-locular ovary. But the ovary in Picramnia has (sometimes 2), usually 3 to 4 locules and there are always two ovules in each locule.[43]

It might be possible to determine the affinities of Gumillea if DNA could be extracted from the existing specimen. DNA has been successfully amplified from specimens of similar age.[44] Any material used in such research, however, will never be replaced.

Apodanthaceae

The family Apodanthaceae comprises 22 to 30 species of endoparasitic herbs. These are distributed into three genera: Pilostyles, Apodanthes, and Berlinianche.[45] Attempts to determine the relationships of Apodanthaceae have produced only uncertain results and they have remained enigmatic,[20][46] until the family was shown to be confidently placed in Cucurbitales[47]

References

  1. 1 2 The Angiosperm Phylogeny Group (2003), "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II", Botanical Journal of the Linnean Society, 141 (4): 399–436, doi:10.1046/j.1095-8339.2003.t01-1-00158.x
  2. ↑ Angiosperm Phylogeny Group (2009). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society 161(2): 105-121.
  3. 1 2 Jesper Kårehed. 2001. "Multiple origin of the tropical forest tree family Icacinaceae". American Journal of Botany 88(12):2259-2274.
  4. ↑ Frederick Lens, Jesper Kårehed, Pieter Baas, Steven Jansen, David Rabaey, Suzy Huysmans, Thomas Hamann, and Eric Smets. 2008. "The wood anatomy of the polyphyletic Icacinaceae s.l. and their relationships within asterids". Taxon 57(2):525-552.
  5. ↑ Timothy M.A. Utteridge and Richard K. Brummitt. 2007. "Leptaulaceae" pages 191-192. In: Vernon H. Heywood, Richard K. Brummitt, Ole Seberg, and Alastair Culham. Flowering Plant Families of the World. Firefly Books: Ontario, Canada. (2007).
  6. ↑ Herbert K. Airy-Shaw, David F. Cutler, and Siwert Nilsson. 1973. "Pottingeria, its taxonomic position, anatomy, and palynology". Kew Bulletin 28(1):97-104.
  7. 1 2 Li-Bing Zhang and Mark P. Simmons (2006), "Phylogeny and delimitation of the Celastrales inferred from nuclear and plastid genes", Systematic Botany, 31 (1): 122–137, doi:10.1600/036364406775971778, S2CID 86095495
  8. ↑ Peter F. Stevens (2001 onwards). Celastrales at Angiosperm Phylogeny
  9. ↑ Jinshuang Ma and Bruce Bartholomew. 2008. "Dipentodontaceae" pages 494-495. In: Zhengyi Wu, Peter H. Raven, and Deyuan Hong (editors). Flora of China volume 11. Science Press: Beijing, China; Missouri Botanical Garden Press: St. Louis, Missouri, USA.
  10. ↑ Vernon H. Heywood, Richard K. Brummitt, Ole Seberg, and Alastair Culham. Flowering Plant Families of the World. Firefly Books: Ontario, Canada. (2007). ISBN 978-1-55407-206-4.
  11. ↑ Andreas Worberg, Mac H. Alford, Dietmar Quandt, and Thomas Borsch. 2009. "Huerteales sister to Brassicales plus Malvales, and newly circumscribed to include Dipentodon, Gerrardina, Huertea, Perrottetia, and Tapiscia". Taxon 58(2):468-478.
  12. 1 2 3 4 5 6 Kenneth J. Wurdack and Charles C. Davis (2009), "Malpighiales phylogenetics: Gaining ground on one of the most recalcitrant clades in the angiosperm tree of life", American Journal of Botany, 96 (8): 1551–1570, doi:10.3732/ajb.0800207, PMID 21628300, S2CID 23284896
  13. ↑ Douglas E. Soltis, Joshua W. Clayton, Charles C. Davis, Matthew A. Gitzendanner, Martin Cheek, Vincent Savolainen, André M. Amorim, and Pamela S. Soltis. 2007. "Monophyly and relationships of the enigmatic family Peridiscaceae". Taxon 56(1):65-73.
  14. 1 2 David J. Mabberley. 2008. Mabberley's Plant-Book third edition (2008). Cambridge University Press: UK. ISBN 978-0-521-82071-4 (see External links below).
  15. ↑ Mac H. Alford. 2007. Samydaceae. Version 6 February 2007. At The Tree of Life Project.
  16. ↑ Charles C. Davis, Maribeth Latvis, Daniel L. Nickrent, Kenneth J. Wurdack, and David A. Baum. 2007. "Floral Gigantism in Rafflesiaceae". Science 315(5820):1812.
  17. ↑ 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.
  18. ↑ Daniel L. Nickrent. 2007. "Cytinaceae are sister to Muntingiaceae (Malvales)". Taxon 56(4):1129-1135.
  19. ↑ Mitrastemonaceae at Parasitic Plant Connection website
  20. 1 2 3 4 Todd J. Barkman, Joel R. McNeal, Seok-Hong Lim, Gwen Coat, Henrietta B. Croom, Nelson D. Young, and Claude W. de Pamphilis. 2007. "Mitochondrial DNA suggests at least 11 origins of parasitism in angiosperms and reveals genomic chimerism in parasitic plants". BMC Evolutionary Biology 7:248.
  21. ↑ Richard K. Brummitt and Martin R. Cheek. 2007. "Hoplestigmataceae" page 167. In: Vernon H. Heywood, Richard K. Brummitt, Ole Seberg, and Alastair Culham. Flowering Plant Families of the World. Firefly Books: Ontario, Canada. (2007).
  22. ↑ Joan W. Nowicke and James S. Miller. 1989. "Pollen morphology and the relationships of Hoplestigmataceae". Taxon 38(1):12-16.
  23. ↑ Maximilian Weigend, Federico Luebert, Marc Gottschling, Thomas L.P. Couvreur, Hartmut H. Hilger and James S. Miller. 2014. "From capsules to nutlets — phylogenetic relationships in the Boraginales". Cladistics 30(5):508-518. doi:10.1111/cla.12061.
  24. 1 2 3 Peter F. Stevens (2001 onwards), Missouri Botanical Garden. Angiosperm Phylogeny.
  25. ↑ Gustavo Lozano-Contreras and Nubia B. de Lozano. 1988. "Metteniusaceae". Monograph 11. In: Polidoro Pinto and Gustavo Lozano-Contreras. (editors). "Flora de Colombia". Instituto de Ciencias Naturales, Universidad Nacional de Colombia: Bogota, Colombia.
  26. ↑ Favio Gonzalez, Julio Betancur, Olivier Maurin, John V. Freudenstein, and Mark W. Chase. 2007. "Metteniusaceae: an early-diverging family in the lamiid clade". Taxon 56(3):795-800.
  27. ↑ Dirk C. Albach, Pamela S. Soltis, Douglas E. Soltis, and Richard G. Olmstead. 2001. "Phylogenetic analysis of Asterids based on sequences of four genes". Annals of the Missouri Botanical Garden 88(2):163-212.
  28. ↑ Birgitta Bremer, Kåre Bremer, Nahid Heidari, Per Erixon, Richard G. Olmstead, Arne A. Anderberg, Mari Kallersjö, and Edit Barkhordarian. 2002. "Phylogenetics of Asterids based on 3 coding and 3 non-coding chloroplast DNA markers and the utility of non-coding DNA at higher taxonomic levels". Molecular Phylogenetics and Evolution 24(2):274-301.
  29. 1 2 Daniel L. Nickrent, Joshua P. Der, and Frank E. Anderson. 2005. "Discovery of the photosynthetic relatives of the "Maltese Mushroom" Cynomorium". BMC Evolutionary Biology 5:38.
  30. ↑ Huei-Jiun Su and Jer-Ming Hu. "The phylogenetic relationships of Balanophoraceae and related Santalales inferred from floral B homeotic genes and nuclear 18S rDNA sequences". no date. no publisher.
  31. ↑ Cynomorium at International Plant Names Index
  32. ↑ Cynomorium At: List of Genera
  33. ↑ Zhi-Hong Zhang, Chun-Qi Li, and Jianhua Li. 2009. "Phylogenetic placement of Cynomorium in Rosales inferred from sequences of the invert repeat region of the chloroplast genome". Journal of Systematics and Evolution 47(4):297-304.
  34. 1 2 H.G. Adolf Engler. 1930. "Cunoniaceae" pages 229-262. In: H.G. Adolf Engler and Karl Prantl (editors). Die Natürlichen Pflanzenfamilien volume 18a. Verlag von Wilhelm Engelmann: Leipzig, Germany.
  35. ↑ Alwyn H. Gentry. 1996. A Field Guide to Woody Plants of Northwest South America. University of Chicago Press Edition (1996). The University of Chicago Press. Chicago, IL, USA.
  36. ↑ Hipólito Ruiz-López and José Antonio Pavón y Jiménez. 1789-1802. Flora Peruviana et Chilensis. Plate CCXLV and volume 3, page 23. Typis Gabrielis de Sancha: Madrid, Spain.
  37. ↑ George Bentham and Joseph Dalton Hooker. 1865. Genera Plantarum volume 1, part 2, page 651. Lovell Reeve & Co.; Williams and Norgate: London, England.
  38. ↑ Jason C. Bradford, Helen C. Fortune-Hopkins, and Richard W. Barnes. 2004. "Cunoniaceae". pages 91-111. In: Klaus Kubitski (editor). The Families and Genera of Vascular Plants volume VI. Springer-Verlag: Berlin;Heidelberg, Germany.
  39. ↑ Armen L. Takhtajan (Takhtadzhian). Flowering Plants second edition (2009). Springer Science+Business Media. ISBN 978-1-4020-9608-2. ISBN 978-1-4020-9609-9 doi:10.1007/978-1-4020-9609-9 (See External links below).
  40. ↑ Joshua W. Clayton, Edwino S. Fernando, Pamela S. Soltis, and Douglas E. Soltis. 2007. "Molecular phylogeny of the tree-of-heaven family (Simaroubaceae) based on chloroplast and nuclear markers". International Journal of Plant Sciences 168(9):1325-1339.
  41. ↑ Lois Brako and James L. Zarucchi. 1993. "Catalogue of the flowering plants and gymnosperms of Peru". Monographs in Systematic Botany from the Missouri Botanical Garden. monograph number 45.
  42. ↑ Edwino S. Fernando and Christopher J. Quinn. 1995. "Picramniaceae, a new family, and a recircumscription of Simaroubaceae". Taxon 44(2):177-181.
  43. 1 2 Klaus Kubitzki. 2007. "Picramniaceae" pages 301-303. In: Klaus Kubitski (editor). The Families and Genera of Vascular Plants volume IX. Springer-Verlag: Berlin;Heidelberg, Germany.
  44. ↑ Katarina Andreasen, Mariette Manktelow, and Sylvain G. Razafimandimbison. 2009. "Successful DNA amplification of a more than 200-year-old herbarium specimen: recovering genetic material from the Linnean era". Taxon 58(3):959-962.
  45. ↑ Albert Blarer, Daniel L. Nickrent, and Peter K. Endress. 2004. "Comparative floral structure and systematics in Apodanthaceae (Rafflesiales)". Plant Systematics and Evolution 245(1-2):119-142.
  46. ↑ Apodanthaceae at Parasitic Plant Connection website
  47. ↑ Filipowicz, N. & Renner, S.S. 2010. The worldwide holoparasitic Apodanthaceae confidently placed in the Cucurbitales by nuclear and mitochondrial gene trees. BMC Evolutionary Biology 10: 219
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