Amborella Trichopoda

Buds and staminate ("male") flowers of Amborella
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
Order: Amborellales
Melikyan, A.V.Bobrov, & Zaytzeva[1]
Family: Amborellaceae
Pichon[1]
Genus: Amborella
Baill.[2]
Species: A. trichopoda
Binomial name
Amborella trichopoda
Baill.[3]

Amborella is a monotypic genus of rare understory shrubs or small trees endemic to the main island, Grande Terre, of New Caledonia.[4] The genus is the only member of the family Amborellaceae and contains a single species, Amborella trichopoda Baill.[4] Wood of Amborella lacks the vessels characteristic of most flowering plants.[4] Amborella is of great interest to plant systematists because molecular phylogenetic analyses consistently place it at or near the base of the flowering plant lineage.[5][6] That is, the Amborellaceae represent a line of flowering plants that diverged very early on (about 130 million years ago) from all the other extant species of flowering plants, and, among extant flowering plants, is the sister group to the other flowering plants.[5] Comparing characteristics of this basal angiosperm, other flowering plants and fossils may provide clues about how flowers first appeared—what Darwin called the "abominable mystery".[7]

Description

Amborella is a sprawling shrub or small tree up to 8 m high. It bears alternate or decussate, simple evergreen leaves without stipules.[4][8] The leaves are two ranked, with distinctly serrated or rippled margins, and about 8 to 10 cm long.[8]

Xylem is the tissue that conducts water upwards in a vascular plant. Two of the main components of xylem in flowering plants are tracheids and vessel elements. Tracheid cells are typically pointed at each end, and have "pits" – regions often surrounded by thickenings adding mechanical strength – through which cells make contact with one another and can exchange fluids. The cells of vessel elements have actual perforations, usually on the flattened ends of the cells, through which fluids can be exchanged directly. Vessel elements form well-defined series of cells along the length of the stem; tracheids are arranged less regularly.[9] The xylem of Amborella contains only tracheids; vessel elements are absent.[10] Xylem of this form has long been regarded as a "primitive" feature of flowering plants.[11]


The species is dioecious, that is each flower produces either functional stamens or functional carpels, but not both.[12] At any one time, a plant produces only functionally staminate or carpellate flowers. Staminate ("male") flowers do not have carpels, whereas carpellate ("female") flowers have non-functional "staminodes", structures resembling stamens in which pollen does not develop. Plants may change from one kind to the other. In one study, seven cuttings from a staminate plant produced, as expected, staminate flowers at their first flowering, but three produced carpellate flowers at their second flowering.[13]

The small, creamy white, inconspicuous flowers are arranged in terminal inflorescences of 2 to 30 flowers, borne in the axils of foliage leaves.[14] The inflorescences have been described as "determinate thyrses", with up to three orders of branching, each branch being terminated by a flower. Each flower is subtended by bracts. The bracts gradually transition into a perianth of undifferentiated tepals, making it difficult to determine where a flower actually begins. The tepals are arranged in a spiral or possibly whorled at the periphery.

Carpellate flowers are approximately 3 to 4 mm in diameter, with 7 or 8 tepals. There are 1 to 3 (or rarely 0) well differentiated staminodes and a spiral of 4 to 8 free carpels (apocarpous). Carpels have green ovaries and lack a style. They contain a single ovule with the micropyle directed downwards. Staminate flowers are approximately 4 to 5 mm in diameter, with 6 to 15 tepals. These flowers bear 10 to 21 spirally arranged stamens, which become progressively smaller toward the center. The innermost may be sterile. Stamens have triangular anthers on short broad filaments, and consist of four pollen sacs, two on each side, with a small sterile central connective. The anthers have connectives tips with small bumps and may be covered with secretions.[15] These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity.[13]

Typically, 1 to 3 carpels develop into fruit per flower. The fruit is an ovoid red drupe (approximately 5 to 7 mm long and 5 mm wide) borne on a short (1 to 2 mm) stalk. The remains of the stigma can be seen at the tip of the fruit. The skin is papery, surrounding a thin fleshy layer containing a red juice. The inner pericarp is lignified and surrounds the single seed. The embryo is small and surrounded by copious endosperm.[16]

Phylogeny

This plant is currently accepted by plant systematists as the most basal lineage in the angiosperms clade.[5] By "most basal", botanists 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 Amborella, may suggest 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.

In a study designed to clarify relationships between the well-sequenced and well-studied model plants such as Arabidopsis thaliana, and the basal angiosperms Amborella, Nuphar of the Nymphaeaceae, Illicium, the monocots, and more derived angiosperms, the eudicots, chloroplast genomes and their expressed sequence tags of these organisms and other seed plants were studied to create the cladogram shown below.[17]

extant seed plants

gymnosperms


angiosperms

Amborella




Nuphar




Illicium




monocots



magnoliids



eudicots







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. (There is some uncertainty about the relationship between the Amborellaceae and the Nymphaeales: whether or not the Amborellaceae alone are the monophyletic sister to the extant angiosperms or the Amborellaceae and Nymphaeales form a clade that is the sister group to all other extant angiosperms.)[17]

Because of its evolutionary position at the base of the flowering plant clade, there was support for sequencing the complete genome of Amborella trichopoda to serve as a reference for evolutionary studies. In 2010, the US National Science Foundation began a genome sequencing effort in Amborella, and the draft genome sequence was posted on the

Classification

Amborella is placed alone in the family Amborellaceae. The APG II system recognized this family, but left it unplaced at order rank due to uncertainty about its relationship to the family Nymphaeaceae. In the most recent APG system, APG III, the Amborellaceae are placed in the monotypic order Amborellales at the base of the angiosperm phylogeny.[1]

Older systems

The Cronquist system, of 1981, classified the family:

Order Laurales
Subclass Magnoliidae
Class Magnoliopsida [=dicotyledons]
Division Magnoliophyta [=angiosperms]

The Thorne system (1992) classified it:

Order Magnoliales
Superorder Magnolianae
Subclass Magnoliideae [=dicotyledons]
Class Magnoliopsida [=angiosperms]

The Dahlgren system classified it:

Order Laurales
Superorder Magnolianae
Subclass Magnoliideae [=dicotyledons],
Class Magnoliopsida [=angiosperms].

Ecology

Amborella is typically dioecious, but has been known to change sex in cultivation. Amborella has a mixed pollination system, relying both on insect pollinators and wind.

Conservation

The islands of New Caldedonia are a biodiversity hot-spot, preserving many early diverging lineages of plants, of which Amborella is but one. This preservation has been ascribed to climate stability during and since the Tertiary (), enabling the continued survival of tropical forests, in contrast to the drought conditions which occurred in Australia at the end of the Tertiary. Current threats to biodiversity in New Caledonia include fires, mining, agriculture, invasion by introduced species, urbanization and global warming.[6] The importance of conserving Amborella has been dramatically stated by Pillon: "The disappearance of Amborella trichopoda would imply the disappearance of a genus, a family and an entire order, as well as the only witness to at least 140 million years of evolutionary history."[19] Conservation strategies targeted on relic species are recommended, both preserving a diversity of habitats in New Caledonia and ex-situ conservation in cultivation.[6]

Gallery

References

Further reading

External links

  • The Amborella Genome Sequencing Project
  • Description language for taxonomy
  • Ancient plant provides clues to evolutionary mystery (National Science Foundation)
  • Description language for taxonomy
  • ), article with detailed photos of plants in cultivation
  • Nova "First Flower" (transcript)
  • NCBI Taxonomy Browser
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