Amborella
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''Amborella'' is a
monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of understory shrubs or small
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...
s
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
to the main island, Grande Terre, of
New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
in the southwest Pacific Ocean. The genus is the only member of the family Amborellaceae and the order Amborellales and contains a single
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
, ''Amborella trichopoda''. ''Amborella'' is of great interest to plant systematists because molecular phylogenetic analyses consistently place it as the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to all other
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...
s, meaning it was the earliest group to evolve separately from all other flowering plants.


Description

''Amborella'' is a sprawling shrub or small
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...
up to high. It bears alternate,
simple Simple or SIMPLE may refer to: *Simplicity, the state or quality of being simple Arts and entertainment * ''Simple'' (album), by Andy Yorke, 2008, and its title track * "Simple" (Florida Georgia Line song), 2018 * "Simple", a song by John ...
evergreen leaves without stipules. The leaves are two-ranked, with distinctly serrated or rippled margins, and about long. ''Amborella'' has xylem tissue that differs from that of most other
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...
s. The xylem of ''Amborella'' contains only tracheids; vessel elements are absent. Xylem of this form has long been regarded as a primitive feature of flowering plants. The species is dioecious. This means that each plant produces either male flowers (meaning that they have functional
stamen The stamen (: stamina or stamens) is a part consisting of the male reproductive organs of a flower. Collectively, the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filament ...
s) or female flowers (flowers with functional carpels), but not both. 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 no
pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
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. The small, creamy white flowers are arranged in
inflorescence In botany, an inflorescence is a group or cluster of flowers arranged on a plant's Plant stem, stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a mai ...
s borne in the axils of foliage leaves. The inflorescences have been described as cymes, with up to three orders of branching, each branch being terminated by a flower. Each flower is subtended by bracts. The bracts transition into a perianth of undifferentiated tepals. The tepals typically are arranged in a spiral, but sometimes are whorled at the periphery. Carpellate flowers are roughly 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 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, 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. These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity. Typically, 1 to 3 carpels per flower develop into fruit. The
fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...
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 In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...
. The embryo is small and surrounded by copious endosperm.


Taxonomy


History

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


Modern classification

''Amborella'' is the only genus 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 Nymphaeaceae () is a family of flowering plants, commonly called water lilies. They live as rhizomatous aquatic herbs in temperate climate, temperate and tropical climates around the world. The family contains five genera with about 70 know ...
. In the more recent APG systems, APG III and APG IV, the Amborellaceae comprise the monotypic order Amborellales at the base of the angiosperm
phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...
.


Phylogeny

Currently plant systematists accept ''Amborella trichopoda'' as the most basal lineage in the
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of angiosperms. In systematics the term "basal" describes a lineage that diverges near the base of a phylogeny, and thus earlier than other lineages. Since ''Amborella'' 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 in ''Amborella''. These traits are presumed to have evolved after the divergence of the ''Amborella'' lineage. One early 20th century idea of " primitive" (i.e. ancestral) floral traits in angiosperms, accepted until relatively recently, is the '' Magnolia'' blossom model. This envisions flowers with numerous parts 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 as '' Arabidopsis thaliana'', and the basal angiosperms ''Amborella'', '' Nuphar'' (Nymphaeaceae), '' Illicium'', the monocots, and more derived angiosperms (eudicots), chloroplast genomes using cDNA and expressed sequence tags for floral genes, the
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
shown below was generated. 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. It supports the theory that ''Amborella'' 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 the Nymphaeales: 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. 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 project website in December 2013.


Genomic and evolutionary considerations

''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. 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 the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to the other flowering plants. 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". This position is consistent with a number of conservative characteristics of its physiology and morphology; for example, the wood of ''Amborella'' lacks the vessels characteristic of most flowering plants. The genes responsible for floral traits like scent and colors in other angiosperms, have yet to be found. Further, the female gametophyte of ''Amborella'' is even more reduced than normal female angiosperm gametophyte. ''Amborella'', being an understory plant in the wild, is commonly in intimate contact with shade- and moisture-dependent organisms such as algae, lichens and mosses. In those circumstances, some
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
between ''Amborella'' and such associated species is not surprising in principle, but the scale of such transfer has caused considerable surprise. Sequencing the ''Amborella'' 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.


Ecology

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


Conservation

The islands of New Caledonia 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 (), 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. 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." Conservation strategies targeted on relict species are recommended, both preserving a diversity of habitats in New Caledonia and ''ex situ'' conservation in cultivation. The
IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...
conservation status is Least Concern (LC).


References


Further reading

* * * *


External links


The Amborella Genome Sequencing Project


i
L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval.
vi
Description language for taxonomy

Ancient plant provides clues to evolutionary mystery (National Science Foundation)
* National Tropical Botanical Garden (
Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
, United States)
article with detailed photos of plants in cultivation



NCBI Taxonomy Browser
{{Taxonbar, from2=Q310470, from1=Q13418082, from3=Q1142499, from4=Q689001 Monotypic angiosperm genera Angiosperms Endemic flora of New Caledonia Taxa described in 1873 Taxa named by Henri Ernest Baillon Dioecious plants