Amborellaceae

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Amborellaceae, or the Amborella Family, is a family of flowering plants endemic to New Caledonia. The family consists of only a single species, Amborella trichopoda. The Angiosperm Phylogeny Group plant classification system (APG II) recognizes this family, but does not assign it to an order due to uncertainty about its relationship to the family Nymphaeaceae. Some cientists working on the family today assign it to its own order, the Amborellales. It is currently accepted by plant systematists working with APG II as the most basal lineage in the clade angiosperms.

By 'most basal' scientists mean that the Amborellaceae diverged the earliest from all other lineages of flowering plants. Comparing the derived characteristics that all other angiosperms share with each other, but not with the Amborella Family, may give scientists clues to what features early flowering plants had and how these characteristics have evolved through time. One early twentieth century idea of "primitive", or less derived, angiosperms that was accepted until relatively recently was modeled on the Magnolia blossom with numerous parts arranged in spirals on an elongated receptacle rather than the small numbers of parts in distinct whorls of more derived flowers. However, studies of a well-preserved fossil putative aquatic angiosperm, Archaefructus, have raised questions about what characteristics are more ancestral.

Description

The Amborellaceae are sprawling shrubs or small trees with two-ranked leaves without stipules. The leaves have distinctly rippled or wavy margins. The plants are dioecious, and the flowers are small, in terminal cymose inflorescences, with a perianth of undifferentiated sepals and petals arranged in a spiral, rather than in the whorls of more derived angiosperms.

The Amborella Family has parts arranged in a spiral, of an indeterminate number (5-8 perianth parts), numerous stamens without a well-defined stalk or filament, an indeterminate number of free carpels (apocarpous). The more derived families of flowering plants (the eudicots), often have two distinguishable perianth whorls, the calyx and the corolla, each with a well-defined number of parts (4 or 5 is common), stamens on filaments, and compound ovaries with united carpels (syncarpous).

Most basal angiosperm

In a study designed to clarify relationships between the well-sequenced and well-studied model plants such as Arabidopsis thaliana, and the basal angiosperms such as Amborella, Nuphar of the Nymphaeaceae, Illicium, the monocots, and more derived angiosperms, the eudicots, scientists examined the chloroplast genomes and expressed sequence tags of these organisms, and other seed plants to create this cladogram. Note that in this image, the angiosperms are all of the plants not labeled "gymnosperms." This hypothesized relationship of the extant seed plants places Amborella as the sister taxon to all other angiosperms, and shows the gymnosperms as a monophyletic group sister to the angiosperms, supporting the theory that Amborella branched off earliest from all other living angiosperms. The dashed line between Amborella and Nuphar is meant to indicate some uncertainty about the relationship between the Amborellaceae and the Nymphaeaceae, and whether or not they form a clade that is sister to the angiosperms, rather than Amborella alone being a monophyletic group sister to the angiosperms.

Other plant classification systems

The Cronquist system, of 1981, assigned the family

to the order Laurales

in subclass Magnoliidae,

in class Magnoliopsida [=dicotyledons]

of division Magnoliophyta [=angiosperms].

The Thorne system (1992) placed it

in the order Magnoliales, which was assigned

to superorder Magnolianae,

in subclass Magnoliideae [=dicotyledons],

in class Magnoliopsida [=angiosperms].

The Dahlgren system placed it

in the order Laurales, which was assigned

to superorder Magnolianae

in subclass Magnoliideae [=dicotyledons],

in class Magnoliopsida [=angiosperms].

External links

* Amborellaceae in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. Version: 3rd May 2006. http://delta-intkey.com

* NCBI Taxonomy Browser

References

Simpson, M.G. Plant Systematics. Elsevier Academic Press. 2006.
Albert, V.A., et al., Floral gene resources from basal angiosperms for comparative genomics research, 2005. BMC Plant Biology, http://bmc.ub.uni-potsdam.de/1471-2229-5-5/1471-2229-5-5.pdf.

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