Pan-Angiospermae P. D. Cantino et M. J. Donoghue in Taxon 56, E23. 2007
Amborellales Melikyan, A. V. Bobrov et Zaytzeva in Manitz et Hellwig (eds), 14th Symp. Biodiv. Evolutionsbiol.: 122. 1 Sep 1999; Amborellanae M. W. Chase et Reveal in Bot. J. Linn. Soc. 161: 123. 2009
Genera/species 1/1
Distribution New Caledonia.
Fossils Unknown.
Habit Dioecious, evergreen shrubs or small trees (rarely somewhat climbing).
Vegetative anatomy Mycorrhiza? Root morphology? Roots diarch. Phellogen? Medulla non-septated. Secondary lateral growth present. Vessels absent. Imperforate tracheary xylem elements tracheids with scalariform to circular bordered pits, non-septate? Wood rays uniseriate to multiseriate (usually one or two, sometimes three to five cell layers thick, with upright cells), heterocellular. Axial parenchyma apotracheal diffuse or paratracheal scanty. Pericycle with hippocrepomorphic sclereids. Sieve tube plastids Ss type, with approx. ten starch grains. Nodes 1:2 (unilacunar with two leaf traces). Wood rays and axial parenchyma cells with dark-staining substances. Mucilage cells and cells with ethereal oils absent. Crystals?
Trichomes Hairs unicellular or multicellular, uniseriate, eglandular, or absent.
Leaves Alternate (distichous, also on young branches), simple, entire, with conduplicate? ptyxis. Stipules and leaf sheath absent. Petiole vascular bundle transection arcuate; bundle branched near apex. Venation pinnate; freely terminating vein endings common at margins, with no marginal vein. Stomata (brachy)paracytic or anomocytic (also other types). Cuticular wax crystalloids as irregular platelets (chemically dominated by nonacosan-10-ol?). Mucilaginous idioblasts absent. Idioblasts with ethereal oils absent. Sclereids abundant. Leaf margin serrate, more or less undulating, with chloranthoid teeth.
Inflorescence Axillary, double botryoid. Accessory flowers sometimes present.
Flowers Actinomorphic, fairly small. Floral bracts spiral, successively grading into tepals. Hypanthium present. Epigyny. Tepals five to eight, spiral, free, undifferentiated. Nectaries absent? Disc absent.
Androecium Stamens twelve to 21, spiral, tepaloid. Filaments free; outer filaments adnate to tepal bases. Staminal vascular bundle branched towards apex. Microsporangia adaxial, four, introrse, longicidal (dehiscing by longitudinal slits); connective somewhat prolonged, with secretory apex. Tapetum secretory? Female flowers usually with one to three staminodia (rarely absent).
Pollen grains Microsporogenesis successive. Pollen grains inaperturate to ulcerate (anaulcerate with endexinous operculum and slightly distinct margin), shed as monads, bicellular? at dispersal. Exine probably tectate, without infratectal elements, cupulate, gemmate, undulating, with or without verrucae; ectexine granulate.
Gynoecium Carpels four to six (to eight), somewhat stipitate, whorled, apocarpous; carpel ascidiate, occluded by secretion (carpellary margins not connate), at apex not completely fused at anthesis; extragynoecial mucilaginous compitum present (at anthesis covered by mucilage and facilitating pollination in multiple carpels from pollen grains present on one stigmatic zone). Closure by transverse slit occurring together with longitudinal slit. Ovaries superior, unilocular. Style absent. Stigmatic area somewhat decurrent, with two distinct flanges, with multicellular papillae, Wet type. Pistillodium possibly present.
Ovules Placentation median (marginal, on lower ventral part of carpellary wall). Ovule one per carpel, hemianatropous (almost orthotropous), pendulous, bitegmic, crassinucellar. Micropyle endostomal. Outer integument four or five cell layers thick, annular. Inner integument three? cell layers thick. Megagametophyte monosporic with chalazal megaspore, 9-nucleate and eight-celled (Amborella type), bipolar, large as mature, with three persistent synergids; one of three cells at micropylar end dividing into two cells: one cell becoming third synergid and one cell becoming egg cell, hence four cells constituting egg apparatus; one of these two daughter cells positioned chalazal to remaining three cells and differentiating into thin-walled egg cell. Filiform apparatus and rough endoplasmic reticulum present in each synergid. Two synergids each containing a single large nucleus; third synergid containing two smaller nuclei: pollen tube nucleus plus degenerated synergid nucleus. Polar nuclei situated at chalazal end and fusing into secondary nucleus of central cell. Antipodal cells three, persistent during fertilization, often ephemeral (rarely proliferating). Polar nuclei alternatively secondary nucleus of central cell situated adjacent to egg cell. Polar nuclei fusing prior to fertilization. Double fertilization taking place. Endosperm development cellular. Primary endosperm nucleus present in chalazal region. First division producing pseudotransverse cell wall near chalazal pole, producing two unequal endosperm regions. Subsequent divisions uniseriate in micropylar region and in chalazal region divisions in several planes. Endosperm cells triploid. Endosperm haustoria? Embryogenesis?
Fruit An assemblage of small stalked drupelets. Pyrene surface pitted; pits with resinous substances. Main part of ligneous layer mesocarpic in origin (in contrast to typical drupes).
Seed Aril absent. Testa membranous, not distinctly defined. Exotesta? of polygonal thin-walled cells; cell walls not lignified. Mesotesta and endotesta unspecialized. Tegmen unspecialized. Perisperm absent. Endosperm copious, proteinaceous and oily. Embryo small, basal, well differentiated, chlorophyll? Cotyledons two. Germination phanerocotylar.
Cytology n = 13
DNA “DEAER” motif present in AP3- and P1-homologues; these motifs may have occurred prior to the duplication yielding the AP3- and PI-lineages (subsequently modified after the divergence of Amborella) and thus may be ancestral among angiosperms. Exon 5’ in PI-homologues 42 bp (ancestral character state among angiosperms).
According to Bergthorsson & al. (2004), the mitochondrial genome of the Amborella lineage has received copies of 20 of its 31 mitochondrial protein genes (e.g. atp1) from different embryophytes (land plants), in particular angiosperms but also six gene transfers from at least three bryophyte donors. However, these results have been criticized by, e.g., Goremykin & al. (2009).
Phytochemistry Very insufficiently known. Flavonol glycosides (kaempferol glycosides) and proanthocyanidin present. Ethereal oils not found. Aluminium accumulated.
Use Unknown.
Systematics Amborella (1; A. trichopoda New Caledonia: central montane parts of Grand Terre).
Amborella trichopoda on the island of New Caledonia is sister to all other extant angiosperms, as repeatedly shown by DNA analyses (see, e.g., Lockhart & Penny 2005, Soltis & al. 2011, Drew & al. 2014). However, Nymphaeales have often been recovered as sister-group to Amborella (e.g. Kim & al. 2004; Goremykin & al. 2013; Xi & al. 2014). Thus, using 310 more slowly evolving nuclear genes, Xi & al. (2014) found a sister-group relationship between Amborella trichopoda and Nymphaeales. Recently, Simmons (2017) confirmed the hypothesis that Amborella is sister to the remaining extant angiosperms.
Amborella lacks vessels (Bailey & Swamy 1948). The tracheid end walls have pit membranes with circular pores which are larger than plasmodesmata (Carlquist & Schneider 2001).
The 9-nucleate megagametophyte, which is unique in Amborella, may be the most archaic embryo-sac among extant angiosperms. A third synergid is produced together with the egg cell following the final division during the maturation of the megagametophyte (Friedman & Ryerson 2008). This is the main difference between the Amborella type and the Polygonum type of megagametogenesis. The two haploid polar nuclei or the diploid result of their fusion centre in the close vicinity of the egg cell. The fertilization seems to proceed according to the most common type among angiosperms. The pollen tube enters the ovule through the micropyle and subsequently penetrates the egg apparatus through a degenerate synergid. An extragynoecial mucilaginous compitum is present and the pollen tube, growing through the stigmatic mucilage and down a stylar canal formed from the inner margins of free ascidiate carpels, is provided with wall and plug of callose. Fertilization takes place within 24 hours after germination of the pollen grain. Thus, the progamic phase is comparatively short in the basalmost angiosperm Amborella. Double fertilization most probably takes place, since two sperms are released and a free sperm nucleus has been observed inside the central cell (Williams 2009). The endosperm development is initiated by an oblique cell division, resulting in a large micropylar cell and a smaller chalazal cell.
Literature
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