Amborella

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Amborella
Amborella trichopoda.jpg
Male specimen
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Order: Amborellales
Melikyan, A.V.Bobrov, & Zaytzeva [2]
Family: Amborellaceae
Pichon [2]
Genus: Amborella
Baill. [3]
Species:
A. trichopoda
Binomial name
Amborella trichopoda
New Caledonia in the world (W3).svg
It is endemic to New Caledonia [1]

Amborella is a monotypic genus of understory shrubs or small trees endemic to the main island, Grande Terre, of New Caledonia in the southwest Pacific Ocean. [5] The genus is the only member of the family Amborellaceae and the order Amborellales and contains a single species, Amborella trichopoda. [6] Amborella is of great interest to plant systematists because molecular phylogenetic analyses consistently place it as the sister group to all other flowering plants, meaning it was the earliest group to evolve separately from all other flowering plants.

Contents

Description

Amborella is a sprawling shrub or small tree up to 8 metres (26 feet) high. It bears alternate, simple evergreen leaves without stipules. [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 has xylem tissue that differs from that of most other flowering plants. The xylem of Amborella contains only tracheids; vessel elements are absent. [8] Xylem of this form has long been regarded as a primitive feature of flowering plants. [9]

The species is dioecious. This means that each plant produces either male flowers (meaning that they have functional stamens) or female flowers (flowers with functional carpels), 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 no pollen 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 in inflorescences borne in the axils of foliage leaves. [12] The inflorescences have been described as cymes, with up to three orders of branching, each branch being terminated by a flower. [12] Each flower is subtended by bracts. [12] The bracts transition into a perianth of undifferentiated tepals. [12] The tepals typically are arranged in a spiral, but sometimes are whorled at the periphery.

Carpellate flowers are roughly 3 to 4 millimetres (1⁄8 to 3⁄16 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 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. [13] These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity. [11]

Typically, 1 to 3 carpels per flower develop into fruit. 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. [14]

Taxonomy

History

The Cronquist system, of 1981, classified the family: [15] [16]

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

The Thorne system (1992) classified it: [17] [18]

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

The Dahlgren system classified it: [19]

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. 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. [2] [20]

Phylogeny

Currently plant systematists accept Amborella trichopoda as the most basal lineage in the clade 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. 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 shown below was generated. [21]

extant  seed plants

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. [21]

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. [22]

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. [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 the sister 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 of Amborella lacks the vessels 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 female gametophyte of Amborella is even more reduced than normal female angiosperm gametophyte. [28]

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 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. [29] [30]

Ecology

Amborella is typically dioecious, but has been known to change sex in cultivation. [6] Amborella has a mixed pollination system, relying on both insect pollinators and wind. [10]

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 ( 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 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." [31] Conservation strategies targeted on relict species are recommended, both preserving a diversity of habitats in New Caledonia and ex situ conservation in cultivation. [24] The IUCN conservation status is Least Concern (LC). [1]

Related Research Articles

<span class="mw-page-title-main">Laurales</span> Order of flowering plants

The Laurales are an order of flowering plants. They are magnoliids, related to the Magnoliales.

<span class="mw-page-title-main">Flowering plant</span> Clade of seed plants that produce flowers

Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae. The term 'angiosperm' is derived from the Greek words ἀγγεῖον / angeion and σπέρμα / sperma ('seed'), meaning that the seeds are enclosed within a fruit. The group was formerly called Magnoliophyta.

<span class="mw-page-title-main">Magnoliales</span> Basal order of flowering plants

The Magnoliales are an order of flowering plants. Well-known members of Magnoliales include: magnolias, tulip trees, custard apples, American pawpaw, cherimoyas, ylang-ylang, soursop fruit, and nutmeg.

<span class="mw-page-title-main">Saxifragales</span> Order of flowering plants

Saxifragales is an order of angiosperms, or flowering plants, containing 15 botanical families and around 100 genera, with nearly 2,500 species. Of the 15 families, many are small, with eight of them being monotypic. The largest family is the Crassulaceae (stonecrops), a diverse group of mostly succulent plants, with about 35 genera. Saxifragales are found worldwide, primarily in temperate to subtropical zones, rarely being encountered growing wild in the tropics; however, many species are now cultivated throughout the world as knowledge of plant husbandry has improved. They can be found in a wide variety of environments, from deserts to fully aquatic habitats, with species adapted to alpine, forested or fully-aquatic habitats. Many are epiphytic or lithophytic, growing on exposed cliff faces, on trees or on rocks, and not requiring a highly organic or nutrient-dense substrate to thrive.

<span class="mw-page-title-main">Canellales</span> Order of flowering plants

Canellales is the botanical name for an order of flowering plants, one of the four orders of the magnoliids. It is recognized by the most recent classification of flowering plants, the APG IV system. It is defined to contain two families: Canellaceae and Winteraceae, which comprise 136 species of fragrant trees and shrubs. The Canellaceae are found in tropical America and Africa, and the Winteraceae are part of the Antarctic flora. Although the order was defined based on phylogenetic studies, a number of possible synapomorphies have been suggested, relating to the pollen tube, the seeds, the thickness of the integument, and other aspects of the morphology.

<span class="mw-page-title-main">Nymphaeaceae</span> Family of plants

Nymphaeaceae is a family of flowering plants, commonly called water lilies. They live as rhizomatous aquatic herbs in temperate and tropical climates around the world. The family contains five genera with about 70 known species. Water lilies are rooted in soil in bodies of water, with leaves and flowers floating on or rising from the surface. Leaves are oval and heart-shaped in Barclaya. Leaves are round, with a radial notch in Nymphaea and Nuphar, but fully circular in Victoria and Euryale.

<span class="mw-page-title-main">Austrobaileyales</span> Order of flowering plants

Austrobaileyales is an order of flowering plants consisting of about 100 species of woody plants growing as trees, shrubs and lianas. The best-known species is Illicium verum, commonly known as star anise. The order belongs to the group of basal angiosperms, the ANA grade, which diverged earlier from the remaining flowering plants. Austrobaileyales is sister to all remaining extant angiosperms outside the ANA grade.

<span class="mw-page-title-main">Geraniales</span> Order of flowering plants in the rosid subclade of eudicots

Geraniales is a small order of flowering plants, included within the rosid subclade of eudicots. The largest family in the order is Geraniaceae with over 800 species. In addition, the order includes the smaller Francoaceae with about 40 species. Most Geraniales are herbaceous, but there are also shrubs and small trees.

<span class="mw-page-title-main">Geraniaceae</span> Family of plants

Geraniaceae is a family of flowering plants placed in the order Geraniales. The family name is derived from the genus Geranium. The family includes both the genus Geranium and the garden plants called geraniums, which modern botany classifies as genus Pelargonium, along with other related genera.

<span class="mw-page-title-main">Buxales</span> Order of eudicot flowering plants

The Buxales are a small order of eudicot flowering plants, recognized by the APG IV system of 2016. The order includes the family Buxaceae; the families Didymelaceae and Haptanthaceae may also be recognized or may be included in the Buxaceae. Many members of the order are evergreen shrubs or trees, although some are herbaceous perennials. They have separate "male" (staminate) and "female" (carpellate) flowers, mostly on the same plant. Some species are of economic importance either for the wood they produce or as ornamental plants.

<span class="mw-page-title-main">Plant reproductive morphology</span> Parts of plant enabling sexual reproduction

Plant reproductive morphology is the study of the physical form and structure of those parts of plants directly or indirectly concerned with sexual reproduction.

<span class="mw-page-title-main">Gynoecium</span> Female organs of a flower

Gynoecium is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of pistils and is typically surrounded by the pollen-producing reproductive organs, the stamens, collectively called the androecium. The gynoecium is often referred to as the "female" portion of the flower, although rather than directly producing female gametes, the gynoecium produces megaspores, each of which develops into a female gametophyte which then produces egg cells.

<span class="mw-page-title-main">Trochodendraceae</span> Family of flowering plants

Trochodendraceae is the only family of flowering plants in the order Trochodendrales. It comprises two extant genera, each with a single species along with up to five additional extinct genera and a number of extinct species. The living species are native to south east Asia. The two living species both have secondary xylem without vessel elements, which is quite rare in angiosperms. As the vessel-free wood suggests primitiveness, these two species have attracted much taxonomic attention.

<span class="mw-page-title-main">Hydnoroideae</span> A subfamily of flowering plants comprising parasitic taxa

Hydnoroideae is a subfamily of parasitic flowering plants in the order Piperales. Traditionally, and as recently as the APG III system it given family rank under the name Hydnoraceae. It is now submerged in the Aristolochiaceae. It contains two genera, Hydnora and Prosopanche:

<i>Didymeles</i> Genus of trees

Didymeles is a genus of flowering plants. It is variously treated as the only genus of the family Didymelaceae — or in the family Buxaceae, as in the APG IV system.

<span class="mw-page-title-main">Theophrastoideae</span> Subfamily of flowering plant family Primulaceae

Theophrastoideae is a small subfamily of flowering plants in the family Primulaceae. It was formerly recognized as a separate family Theophrastaceae. As previously circumscribed, the family consisted of eight genera and 95 species of trees or shrubs, native to tropical regions of the Americas.

<i>Haptanthus</i> Genus of shrub

Haptanthus is a monotypic genus containing the sole species Haptanthus hazlettii, a shrub or small tree known only from the locality of Matarras in the Arizona Municipality in Honduras. Its flowers are unique among the flowering plants. A single "female" (carpellate) flower has two branches on either side which carry "male" (staminate) flowers. The flowers are very simple, lacking obvious sepals or petals. The family placement of the genus has been uncertain but based on molecular phylogenetic research, it is included in the family Buxaceae as of September 2014. Very few individuals have ever been found, and its habitat is threatened by logging.

<span class="mw-page-title-main">Basal angiosperms</span> Descendants of most extant flowering plants

The basal angiosperms are the flowering plants which diverged from the lineage leading to most flowering plants. In particular, the most basal angiosperms were called the ANITA grade, which is made up of Amborella, Nymphaeales and Austrobaileyales.

<span class="mw-page-title-main">Mesangiospermae</span> One of two clades of flowering plants

Mesangiospermae is a clade that contains the majority of flowering plants (angiosperms). Mesangiosperms are therefore known as the core angiosperms, in contrast to the three orders of earlier-diverging species known as the basal angiosperms: Nymphaeales, Austrobaileyales, and Amborellales. Mesangiospermae includes about 350,000 species, while there are about 175 extant species of basal angiosperms.

<span class="mw-page-title-main">Pentapetalae</span> Group of eudicots known as core eudicots

In phylogenetic nomenclature, the Pentapetalae are a large group of eudicots that were informally referred to as the "core eudicots" in some papers on angiosperm phylogenetics. They comprise an extremely large and diverse group accounting for about 65% of the species richness of the angiosperms, with wide variability in habit, morphology, chemistry, geographic distribution, and other attributes. Classical systematics, based solely on morphological information, was not able to recognize this group. In fact, the circumscription of the Pentapetalae as a clade is based on strong evidence obtained from DNA molecular analysis data.

References

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  27. ↑ Water lily genome expands picture of the early evolution of flowering plants
  28. ↑ Rudall, Paula (2006). "How many nuclei make an embryo sac in flowering plants?". BioEssays. 28 (11): 1067–1071. doi:10.1002/bies.20488. PMID   17041880.
  29. ↑ Megan Scudellari. Genomes Gone Wild, January 1, 2014 |
  30. ↑ Rice, D. W.; Alverson, A. J.; Richardson, A. O.; Young, G. J.; Sanchez-Puerta, M. V.; Munzinger, J.; Barry, K.; Boore, J. L.; Zhang, Y.; dePamphilis, C. W.; Knox, E. B.; Palmer, J. D. (19 December 2013). "Horizontal Transfer of Entire Genomes via Mitochondrial Fusion in the Angiosperm Amborella". Science. 342 (6165): 1468–1473. Bibcode:2013Sci...342.1468R. doi:10.1126/science.1246275. hdl: 11336/2616 . PMID   24357311. S2CID   2499045.
  31. ↑ Pillon 2008, p. 55. "La disparition d'Amborella trichopoda impliquerait donc la disparition d'un genre, d'une famille et d'un ordre entier, ainsi que le seul témoin d'au moins 140 millions d'années d'histoire évolutive."

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