Mesangiospermae

Mesangiospermae
Flower of Liriodendron tulipifera, a Mesangiosperm
Scientific classification
Kingdom:	Plantae
(unranked):	Angiospermae
(unranked):	Mesangiospermae
Clades
Ceratophyllales Chloranthales Eudicots Magnoliidae Monocots

Mesangiospermae is a group of flowering plants, informally called "mesangiosperms". They are one of four clades of angiosperms.^[1] There are about 350,000 species of mesangiosperms.^[2] The mesangiosperms contain about 99.95% of the flowering plants, assuming that there are about 175 species not in this group^[3] and about 350,000 that are.^[2]

Besides the mesangiosperms, the other groups of flowering plants are Amborellales, Nymphaeales, and Austrobaileyales. These constitute a paraphyletic grade called basal angiosperms. The order names, ending in -ales are used here without reference to taxonomic rank because these groups contain only one order. These are the names used in the APG III system of classification for flowering plants.

Name

The mesangiosperms are usually recognized in classification systems that do not assign groups to taxonomic rank. The name Mesangiospermae is a branch-modified node-based name in phylogenetic nomenclature. It is defined as the most inclusive crown clade containing Platanus occidentalis, but not Amborella trichopoda, Nymphaea odorata, or Austrobaileya scandens. ^[4] It is sometimes written as /Mesangiospermae even though this is not required by the PhyloCode. The "clademark" slash indicates that the term is intended as phylogenetically defined.^[1]

Description

In molecular phylogenetic studies, the mesangiosperms are always strongly supported as a monophyletic group.^[5] There is no distinguishing characteristic which is found in all mature mesangiosperms but which is not found in any of the basal angiosperms. Nevertheless, the mesangiosperms are recognizable in the earliest stage of embryonic development.^{[3] [6]} The ovule contains a megagametophyte, also known as an embryo sac, that is bipolar in structure and contains 8 cell nuclei. The antipodal cells are persistent, and the endosperm is triploid.

History

The oldest known fossils of flowering plants are fossil mesangiosperms from the Hauterivian stage of the Cretaceous period.^[7]

Molecular clock comparisons of DNA sequences indicate that the mesangiosperms originated between 140 and 150 Mya (million years ago) near the beginning of the Cretaceous period.^[8] This was about 25 Ma (million years) after the origin of the angiosperms in the mid-Jurassic.^[9]

By 135Mya, the mesangioisperms had radiated into 5 groups: Chloranthales, Magnoliids, Monocots, Ceratophyllales, and Eudicots.^[9] The radiation into 5 groups probably occurred in about 4 million years.

Because the interval of this radiation (about 4 million years) is short in proportion to its age (about 145 million years), it had long appeared that the 5 groups of mesangiosperms had arisen simultaneously. The mesangiosperms were shown as an unresolved pentatomy in phylogenetic trees. In 2007, two studies attempted to resolve the phylogenetic relationships among these 5 groups by comparing large portions of their chloroplast genomes.^[9][10] These studies agreed on the most likely phylogeny for the mesangiosperms. In this phylogeny, the monocots are sister to the clade [Ceratophyllales + eudicots]. However, this result is not strongly supported. The approximately unbiased topology test showed that some of the other possible positions of the monocots had more than 5% probability of being correct. The major weakness of these 2 studies was the small number of species whose DNA was being used in the phylogenetic analysis, 45 in one study and 64 in the other.^[9] This was unavoidable, because complete chloroplast genome sequences are known for only a few plants.

References

1. ^ ^{a b} Philip D. Cantino, James A. Doyle, Sean W. Graham, Walter S. Judd, Richard G. Olmstead, Douglas E. Soltis, Pamela S. Soltis, and Michael J. Donoghue (2007). "Towards a phylogenetic nomenclature of Tracheophyta". Taxon 56(3):822-846.
2. ^ ^{a b} Alan J. Paton, Neil Brummitt, Rafaël Govaerts, Kehan Harman, Sally Hinchcliffe, Bob Allkin, & Eimear Nic Lughadha (2008). "Towards Target 1 of the Global Strategy for Plant Conservation: a working list of all known plant species - progress and prospects". Taxon 57(2):602-611.
3. ^ ^{a b} Peter F. Stevens (2001 onwards). Angiosperm Phylogeny Website In: Missouri Botanical Garden Website. (see External links below).
4. ^ Philip D. Cantino, James A. Doyle, Sean W. Graham, Walter S. Judd, Richard G. Olmstead, Douglas E. Soltis, Pamela S. Soltis, and Michael J. Donoghue. 2007. Electronic Supplement: pages E1-E44. To: Cantino et alii. 2007. "Towards a phylogenetic nomenclature of Tracheophyta". Taxon 56(3):822-846. (see External links below).
5. ^ Douglas E. Soltis, Pamela S. Soltis, Peter K. Endress, and Mark W. Chase (2005). Phylogeny and Evolution of the Angiosperms. Sinauer: Sunderland, MA
6. ^ William E. Friedman and Kirsten C. Ryerson (2009). "Reconstructing the ancestral female gametophyte of angiosperms: Insights from Amborella and other ancient lineages of flowering plants". American Journal of Botany 96(1):129-143. doi:10.3732/ajb.0800311
7. ^ Else Marie Friis, K. Raunsgaard Pedersen, and Peter R. Crane (2006). "Cretaceous angiosperm flowers: Innovation and evolution in plant reproduction". Palaeogeography, Palaeoclimatology, Palaeoecology232(2-4):251-293. doi:10.1016/j.palaeo.2005.07.006
8. ^ T. Jonathan Davies, Timothy G. Barraclough, Mark W. Chase, Pamela S. Soltis, Douglas E. Soltis, and Vincent Savolainen (2004). "Darwin's abominable mystery: Insights from a supertree of the angiosperms". Proceedings of the National Academy of Sciences 101(7):1904-1909.
9. ^ ^{a b c d} Michael J. Moore, Charles D. Bell, Pamela S. Soltis, and Douglas E. Soltis (2007). "Using plastid genome-scale data to resolve enigmatic relationships among basal angiosperms". Proceedings of the National Academy of Sciences 104(49):19363-19368. doi:10.1073/pnas.0708072104
10. ^ Robert K. Jansen, Zhengqiu Cai, Linda A. Raubeson, Henry Daniell, Claude W. dePamphilis, James Leebens-Mack, Kai F. Müller, Mary Guisinger-Bellian, Rosemarie C. Haberle, Anne K. Hansen, Timothy W. Chumley, Seung-Bum Lee, Rhiannon Peery, Joel R. McNeal, Jennifer V. Kuehl, and Jeffrey L. Boore (2007). "Analysis of 81 genes from 64 plastid genomes resolves relationships in angiosperms and identifies genome-scale evolutionary patterns" Proceedings of the National Academy of Sciences 104(49):19369-19374 doi:10.1073/pnas.0709121104

External links

• Angiosperm Phylogeny at Missouri Botanical Garden
• Electronic Supplement to Cantino et alii
• Divergence Times T. Jonathan Davies et alii. (2004). PNAS 101(7):1904-1909.
• chloroplast genome list In: Montreal genomics