| Mesangiospermae | |
|---|---|
 | |
| Diversity of Mesangiosperms | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Mesangiosperms |
| Groups | |
| Synonyms | |
| |
Mesangiospermae is aclade that contains the majority offlowering plants (angiosperms). Mesangiosperms are therefore known as thecore angiosperms, in contrast to the three orders of earlier-diverging species known as thebasal angiosperms:Nymphaeales (includingwater lilies),Austrobaileyales (includingstar anise), andAmborellales. Mesangiospermae includes about 350,000 species,[1] while there are about 175 extant species of basal angiosperms.[2]
It is a name created under the rules of thePhyloCode system ofphylogenetic nomenclature.[3] While such a clade with a similar circumscription exists in theAPG III system, it was not given a name.[4]
Besides the mesangiosperms, the other groups of flowering plants areAmborellales,Nymphaeales, andAustrobaileyales. These constitute aparaphyleticgrade calledbasal angiosperms. Theordernames, ending in -ales are used here without reference to taxonomic rank because these groups contain only one order.[clarification needed]
Mesangiospermae includes the following clades:
Cladogram: Thephylogenetic position of the Mesangiospermae within the angiosperms, as of APG IV (2016)[5]
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The mesangiosperms are usually recognized inclassificationsystems that do not assign groups totaxonomic rank. The name Mesangiospermae is abranch-modifiednode-based name inphylogenetic nomenclature. It isdefined as the most inclusivecrown clade containingPlatanus occidentalis, but notAmborella trichopoda,Nymphaea odorata, orAustrobaileya scandens.[6] It is sometimes written as /Mesangiospermae even though this is not required by thePhyloCode. The "clademark" slash indicates that the term is intended as phylogenetically defined.[3]
Inmolecular phylogeneticstudies, the mesangiosperms are always stronglysupported as amonophyletic group.[7] There is nodistinguishing 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.[2][8] Theovule contains amegagametophyte, also known as anembryo sac, that isbipolar in structure and contains 8cell nuclei. Theantipodal cells arepersistent, and theendosperm istriploid.
The oldest knownfossils of flowering plants are fossil mesangiosperms from theHauterivianstage of theCretaceousperiod.[9]
Molecular clock comparisons of DNA sequences indicate that the mesangiosperms originated between 140 and 150Mya (million years ago) near the beginning of the Cretaceous period.[10] This was about 25Ma (million years) after the origin of the angiosperms in the mid-Jurassic.[11]
By 135Mya, the mesangiosperms hadradiated into 5 groups:Chloranthales,Magnoliids,Monocots,Ceratophyllales, andEudicots.[11] 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 unresolvedpentatomy inphylogenetic trees. In 2007, two studies attempted toresolve thephylogeneticrelationships among these 5 groups by comparing large portions of theirchloroplastgenomes.[11][12] These studies agreed on the most likely phylogeny for the mesangiosperms. In this phylogeny, the monocots aresister to the clade [Ceratophyllales + eudicots]. However, this result is not strongly supported. The approximately unbiased topologytest 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.[11] This was unavoidable, becausecomplete chloroplast genomesequences are known for only a few plants.
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