| Amborella | |
|---|---|
 | |
| Male specimen | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Order: | Amborellales Melikyan,A.V.Bobrov, &Zaytzeva[4] |
| Family: | Amborellaceae Pichon[4] |
| Genus: | Amborella Baill.[3] |
| Species: | A. trichopoda |
| Binomial name | |
| Amborella trichopoda | |
.svg) | |
| It is endemic to New Caledonia[1] | |
Amborella is amonotypicgenus ofunderstoryshrubs or smalltreesendemic to the main island,Grande Terre, ofNew Caledonia in the southwest Pacific Ocean.[5] The genus is the only member of the family Amborellaceae and the order Amborellales and contains a singlespecies,Amborella trichopoda.[6]Amborella is of great interest to plantsystematists becausemolecular phylogenetic analyses consistently place it as thesister group to all otherflowering plants, meaning it was the earliest group to evolve separately from all other flowering plants.
Amborella is a sprawlingshrub or smalltree up to 8 metres (26 feet) high. It bearsalternate,simpleevergreen leaves withoutstipules.[6][7] The leaves are two-ranked, with distinctly serrated or rippled margins, and about 8 to 10 centimetres (3 to 4 inches) long.[7]
Amborella hasxylem tissue that differs from that of most otherflowering plants. The xylem ofAmborella contains onlytracheids;vessel elements are absent.[8] Xylem of this form has long been regarded as aprimitive feature of flowering plants.[9]
The species isdioecious. This means that each plant produces either male flowers (meaning that they have functionalstamens) or female flowers (flowers with functionalcarpels), but not both.[10] At any one time, a dioecious plant produces only functionally staminate or functionally carpellate flowers. Staminate ("male")Amborella flowers do not have carpels, whereas the carpellate ("female") flowers have non-functional "staminodes", structures resembling stamens in which nopollen develops. Plants may change from one reproductive morphology to the other. In one study, seven cuttings from a staminate plant produced, as expected, staminate flowers at their first flowering, but three of the seven produced carpellate flowers at their second flowering.[11]
The small, creamy white flowers are arranged ininflorescences borne in theaxils of foliage leaves.[12] The inflorescences have been described ascymes, with up to three orders of branching, each branch being terminated by a flower.[12] Each flower is subtended bybracts.[12] The bracts transition into aperianth of undifferentiatedtepals.[12] The tepals typically are arranged in a spiral, but sometimes arewhorled at the periphery.
Carpellate flowers are roughly 3 to 4 millimetres (1⁄8 to3⁄16 in) 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 (apocarpous) carpels. Carpels bear green ovaries; they lack astyle. They contain a single ovule with themicropyle 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, amounting to staminodes. The stamens bear triangular anthers on short broad filaments. An anther consists of four pollen sacs, two on each side, with a small sterile central connective. The anthers have connective tips with small bumps and may be covered with secretions.[13] These features suggest that, as with otherbasal angiosperms, there is a high degree of developmental plasticity.[11]
Typically, 1 to 3 carpels per flower develop into fruit. Thefruit is an ovoid reddrupe (approximately 5 to 7 mm long and 5 mm wide) borne on a short (1 to 2 mm) stalk. The remains of thestigma can be seen at the tip of the fruit. The skin is papery, surrounding a thin fleshy layer containing a red juice. The innerpericarp islignified and surrounds the singleseed. The embryo is small and surrounded by copious endosperm.[14]
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TheCronquist system, of 1981, classified the family:[15][16]
TheThorne system (1992) classified it:[17][18]
TheDahlgren system classified it:[19]
Amborella is the only genus in the family Amborellaceae. TheAPG II system recognized this family, but left it unplaced at order rank due to uncertainty about its relationship to the familyNymphaeaceae. In the more recentAPG systems,APG III andAPG IV, the Amborellaceae comprise the monotypic order Amborellales at the base of the angiospermphylogeny.[4][20]
Currently plantsystematists acceptAmborella trichopoda as the mostbasal lineage in theclade of angiosperms.[20] In systematics the term "basal" describes a lineage that diverges near the base of a phylogeny, and thus earlier than other lineages. SinceAmborella is apparently basal among the flowering plants, the features of early flowering plants can be inferred by comparing derived traits shared by the main angiosperm lineage but not present inAmborella. These traits are presumed to have evolved after the divergence of theAmborella lineage.
One early 20th century idea of "primitive" (i.e. ancestral) floral traits in angiosperms, accepted until relatively recently, is theMagnolia blossom model. This envisions flowers with numerousparts arranged in spirals on an elongated, cone-like receptacle rather than the small numbers of parts in distinct whorls of more derived flowers.
In a study designed to clarify relationships between well-studied model plants such asArabidopsis thaliana, and the basal angiospermsAmborella,Nuphar (Nymphaeaceae),Illicium, themonocots, and more derived angiosperms (eudicots),chloroplast genomes usingcDNA andexpressed sequence tags for floral genes, thecladogram shown below was generated.[21]
This hypothesized relationship of the extant seed plants placesAmborella as thesister taxon to all other angiosperms, and shows the gymnosperms as a monophyletic group sister to the angiosperms. It supports the theory thatAmborella branched off from the main lineage of angiosperms before the ancestors of any other living angiosperms. There is however some uncertainty about the relationship between the Amborellaceae and theNymphaeales: one theory is that the Amborellaceae alone are the monophyletic sister to the extant angiosperms; another proposes that the Amborellaceae and Nymphaeales form a clade that is the sister group to all other extant angiosperms.[21]
Because of its evolutionary position at the base of the flowering plant clade, there was support for sequencing the complete genome ofAmborella trichopoda to serve as a reference for evolutionary studies. In 2010, the US National Science Foundation began a genome sequencing effort inAmborella, and the draft genome sequence was posted on the project website in December 2013.[22]
Amborella is of great interest to plant systematists becausemolecular phylogenetic analyses consistently place it at or near thebase of the flowering plant lineage.[23][24][25] That is, the Amborellaceae represent a line of flowering plants that diverged very early on (more than 130 million years ago) from all the other extant species of flowering plants, and, among extant flowering plants, is thesister group to the other flowering plants.[23] 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".[26] This position is consistent with a number of conservative characteristics of its physiology and morphology; for example, the wood ofAmborella lacks thevessels characteristic of most flowering plants.[6] The genes responsible for floral traits like scent and colors in other angiosperms, have yet to be found.[27] Further, the femalegametophyte ofAmborella is even more reduced than normal femaleangiosperm gametophyte.[28]
Amborella, being anunderstory plant in the wild, is commonly in intimate contact with shade- and moisture-dependent organisms such as algae, lichens and mosses. In those circumstances, somehorizontal gene transfer betweenAmborella and such associated species is not surprising in principle, but the scale of such transfer has caused considerable surprise. Sequencing theAmborella mitochondrial genome revealed that for every gene of its own origin, it contains about six versions from the genomes of an assortment of the plants and algae growing with or upon it. The evolutionary and physiological significance of this is not as yet clear, nor in particular is it clear whether the horizontal gene transfer has anything to do with the apparent stability and conservatism of the species.[29][30]
Amborella is typicallydioecious, but has been known tochange sex in cultivation.[6]Amborella has a mixed pollination system, relying on both insect pollinators and wind.[10]
The islands of New Caledonia are a biodiversity hot-spot, preserving many early diverging lineages of plants, of whichAmborella is but one. This preservation has been ascribed to climate stability during and since theTertiary (66 to 3 million years ago), stability that has permitted the continued survival of tropical forests on New Caledonia. In contrast, drought conditions dominated the Australian climate towards the end of the Tertiary. Current threats to biodiversity in New Caledonia include fires, mining, agriculture, invasion by introduced species, urbanization and global warming.[24] The importance of conservingAmborella has been dramatically stated by Pillon: "The disappearance ofAmborella 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."[31] Conservation strategies targeted onrelict species are recommended, both preserving a diversity of habitats in New Caledonia andex situ conservation in cultivation.[24] TheIUCN conservation status is Least Concern (LC).[1]
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