Amborella

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Amborella
Amborella trichopoda (3065968016) fragment.jpg
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.[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]

Description[edit]

Amborella is a sprawling shrub or small tree up to 8 m high. It bears alternate or decussate, simple evergreen leaves without stipules.[4][7] The leaves are two-ranked, with distinctly serrated or rippled margins, and about 8 to 10 cm long.[7]

Amborella has xylem tissue that differs from that of other flowering plants. 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.[8] The xylem of Amborella contains only tracheids; vessel elements are absent.[9] Xylem of this form has long been regarded as a "primitive" feature of flowering plants.[10]

Amborella trichopoda.jpg

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.[11] 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.[12]

The small, creamy white, inconspicuous flowers are arranged in terminal inflorescences of 2 to 30 flowers,[citation needed] borne in the axils of foliage leaves.[13] 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 typically are arranged in a spiral, but sometimes are whorled at the periphery.

Carpellate flowers are roughly 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 (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. 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 connectives tips with small bumps and may be covered with secretions.[14] These features suggest that, as with other basal angiosperms, there is a high degree of developmental plasticity.[12]

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.[15]

Phylogeny[edit]

Currently plant systematists accept Amborella trichopoda as the most basal lineage in the clade of angiosperms.[5] 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 twentieth 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. However, studies of a well-preserved fossil of a putatively very early aquatic angiosperm, Archaefructus, have raised doubts about which characteristics are more ancestral.[citation needed]

In a study designed to clarify relationships between well-sequenced and 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.[16]

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. 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.[16]

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 January 2012. The species was featured in the second episode of the BBC documentary series How to Grow a Planet, where Professor Iain Stewart described it as the "closest living relative of the first flower to evolve."[17]

Classification[edit]

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 most recent APG system, APG III, the Amborellaceae comprise the monotypic order Amborellales at the base of the angiosperm phylogeny.[1]

Older systems[edit]

The Cronquist system, of 1981, classified the family:[citation needed]

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

The Thorne system (1992) classified it:[citation needed]

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

The Dahlgren system classified it:[citation needed]

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

Genomic and evolutionary considerations[edit]

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".[18] 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.[4]

Amborella in the wild, being an understory plant, commonly is in intimate contact with shade- and moisture-dependent organisms such as algae, lichens and mosses. In the circumstances, it is not surprising in principle to find that there should have been horizontal gene transfer between Amborella and such associated species, but the scale of such transfer has in fact 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.[19]

Ecology[edit]

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[edit]

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.[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."[20] 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[edit]

References[edit]

  1. ^ a b c Angiosperm Phylogeny Group III (2009). "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III". Botanical Journal of the Linnean Society 161 (2): 105–121. doi:10.1111/j.1095-8339.2009.00996.x. 
  2. ^ "IPNI Plant Name Query Results for Amborella". The International Plant Names Index. Retrieved 2013-09-03. 
  3. ^ "IPNI Plant Name Query Results for Amborella trichopoda". The International Plant Names Index. Retrieved 2013-09-03. 
  4. ^ a b c d Große-Veldmann, Bernadette; Korotkova, Nadja; Reinken, Bernhard; Lobin, Wolfram & Barthlott, Wilhelm (2011). "Amborella trichopoda — Cultivation of the most ancestral angiosperm in botanic gardens". The Journal of Botanic Garden Horticulture 9: 143–155. Retrieved 2013-06-22. 
  5. ^ a b c d Soltis, Pamela S. & Soltis, Douglas E. (2013). "Angiosperm Phylogeny: A Framework for Studies of Genome Evolution". In Leitch, Ilia J.; Greilhuber, Johann; Doležel, Jaroslav & Wendel, Jonathan F. Plant Genome Diversity. Volume 2. Springer. pp. 1–11. ISBN 978-3-7091-1160-4. 
  6. ^ a b c d Pillon, Yohan (2008). Biodiversité, origine et évolution des Cunoniaceae : implications pour la conservation de la flore de Nouvelle-Calédonie (PhD) (in French & English). University of New Caledonia. Retrieved 2013-06-22. 
  7. ^ a b Simpson, M.G. (2010). Plant Systematics (2nd ed.). Elsevier.  p. 186
  8. ^ Foster, A.S. & Gifford, E.M. (1974). Comparative Morphology of Vascular Plants (2nd ed.). San Francisco: W.H. Freeman. ISBN 978-0-7167-0712-7.  p. 577ff.
  9. ^ Carlquist, Sherwin John & Schneider, Edward Lee (2001). "Vegetative anatomy of the New Caledonian endemic Amborella trichopoda: relationships with the Illiciales and implications for vessel origin". Pacific Science 55 (3): 305–312. doi:10.1353/psc.2001.0020. Retrieved 2013-06-23. 
  10. ^ Sporne, K.R. (1974). The Morphology of Angiosperms. London: Hutchinson. ISBN 978-0-09-120611-6.  p. 98.
  11. ^ Thien, Leonard B.; Sage, Tammy L.; Jaffré, Tanguy; Bernhardt, Peter; Pontieri, Vincenza; Weston, Peter H.; Malloch, Dave; Azuma, Hiroshi; Graham, Sean W.; McPherson, Marc A.; Rai, Hardeep S.; Sage, Rowan F. & Dupre, Jean-Louis (2003). "The Population Structure and Floral Biology of Amborella trichopoda (Amborellaceae)". Annals of the Missouri Botanical Garden (Missouri Botanical Garden Press) 90 (3): 466–490. doi:10.2307/3298537. 
  12. ^ a b Buzgo, Matyas; Soltis, Pamela S. & Soltis, Douglas E. (2004). "Floral Developmental Morphology of Amborella trichopoda (Amborellaceae)". International Journal of Plant Sciences 165 (6): 925–947. doi:10.1086/424024. 
  13. ^ Rudall, Paula J; Sokoloff, Dmitry D.; Remizowa, Margarita V.; Conran, John G.; Davis, Jerrold I.; Macfarlane, Terry D. & Stevenson, Dennis W (2007). "Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent angiosperm lineage". American Journal of Botany 94 (7): 1073–1092. doi:10.3732/ajb.94.7.1073. 
  14. ^ Endress, Peter K. & Igersheim, Anton (2000). "The Reproductive Structures of the Basal Angiosperm Amborella trichopoda (Amborellaceae)". International Journal of Plant Sciences. Current Perspectives on Basal Angiosperms (The University of Chicago Press) 161 (S6): S237–S248. doi:10.1086/317571. 
  15. ^ Floyd, S.K. & Friedman, W.E. (2001). "Developmental evolution of endosperm in basal angiosperms: evidence from Amborella (Amborellaceae), Nuphar (Nymphaceae), and Illicium (Illiciaceae)". Plant Systematics and Evolution 228: 153–169. doi:10.1007/s006060170026. 
  16. ^ a b Albert, Victor A.; Soltis, Douglas E.; Carlson, John E.; Farmerie, William G.; Wall, P. Kerr; Ilut, Daniel C.; Solow, Teri M.; Mueller, Lukas A.; Landherr, Lena L.; Hu, Yi; Buzgo, Matyas; Kim, Sangtae; Yoo, Mi-Jeong; Frohlich, Michael W.; Perl-Treves, Rafael; Schlarbaum, Scott E.; Zhang, Xiaohong; Tanksley, Steven D; Oppenheimer, David G; Soltis, Pamela S; Ma, Hong; dePamphilis, Claude W. & Leebens-Mack, H. (2005). "Floral gene resources from basal angiosperms for comparative genomics research". BMC Plant Biology 5. 
  17. ^ BBC Two, 14 Feb 2012, How to Grow a Planet - Episode 2. The Power of Flowers by Professor Iain Stewart
  18. ^ Friedman, W. E. (2008-12-11). "The meaning of Darwin's 'abominable mystery'". American Journal of Botany 96 (1): 5–21. doi:10.3732/ajb.0800150. PMID 21628174.  edit
  19. ^ Megan Scudellari. Genomes Gone Wild, January 1, 2014 |[1]
  20. ^ 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."

Further reading[edit]

  • Bailey, I.W. & Swamy, B.G.L. (1948). "Amborella trichopoda Baill., a new morphological type of vesselless dicotyledon". Journal of the Arnold Arboretum 29: 245–254. 
  • Endress, P.K.; Igersheim, A. (2000). "The reproductive structures of the basal angiosperm Amborella trichopoda (Amborellaceae).". International Journal of Plant Science. 161(supplement): S237–S248. 
  • Soltis, Douglas E.; Albert, Victor A.; Leebens-Mack, Jim; Palmer, Jeffrey D.; Wing, Rod A.; dePamphilis, Claude W.; Ma, Hong; Carlson, John E.; Altman, Naomi; Kim, Sangtae; Wall, P. Kerr; Zuccolo, Andrea & Soltis, Pamela S. (2008). "The Amborella genome: an evolutionary reference for plant biology". Genome Biology 9 (3): 402.1–402.6. doi:10.1186/gb-2008-9-3-402. 
  • Zuccolo, Andrea; Bowers, John E.; Estill, James C.; Xiong, Zhiyong; Luo, Meizhong; Sebastian, Aswathy; Goicoechea, José L.; Collura, Kristi; Yu, Yeisoo; Jiao, Yuannian; Duarte, Jill; Tang, Haibao; Ayyampalayam, Saravanaraj; Rounsley, Steve; Kudrna, Dave; Paterson, Andrew H.; Pires, J. C.; Chanderbali, Andre; Soltis, Douglas E.; Chamala, Srikar; Barbazuk, Brad; Soltis, Pamela S.; Albert, Victor A.; Ma, Hong; Mandoli, Dina; Banks, Jody; Carlson, John E.; Tomkins, Jeffrey; dePamphilis, Claude W.; Wing, Rod A. & Leebens-Mack, Jim (2011). "A physical map for the Amborella trichopoda genome sheds light on the evolution of angiosperm genome structure". Genome Biology 12 (5): R48. doi:10.1186/gb-2011-12-5-r48. 

External links[edit]