50839214 origin of angiosperms
TRANSCRIPT
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Origin of Angiosperms
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Geologic time scale Pollen evidence: 126 mya (Lower
Cretaceous Period) Macrofossil evidence
(“Archaeofructus”): 130 mya (Lower Cretaceous)
WHERE: West Gondwana (South America and Africa)
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Archaefructus liaoningensis
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Pioneer plants in the Cretaceous: figs, planes (Platanus), and magnolias
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Possible reasons for rapid radiation of angiosperms:
Co-evolution with many insect groups
Shorter life cycleDiverse habitat types and
edaphic factors
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Angiosperm diversity• at least 260,000 living species classified in 453 families (Judd et al., 2002; APG II, 2003; Soltis et al., 2005)
• epiphytes, floating and rooted aquatics, terrestrial (herbs, shrubs, vines, lianas, giant trees), parasitic forms
•Variation in size, longevity & overall form
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Angiosperm synapomorphies (derived, shared characters):
1) Ovules enclosed within carpels
2) Double fertilization
3) Stamens with two pairs of pollen sacs
4) Features of gametophyte structure and development
5) Phloem tissue (sieve tubes and companion cells)
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Isoetes-Monocotyledon theory (Campbell, 1928)
Similarities in: habitat (humid tropics); habit; embryo & anatomy of older sporophyte
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Gnetalean ancestry (Wettstein, 1901)
Similarities: two cotyledons, vessels in 2ndary wood, two integuments, net-veined leaves
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Coniferalian ancestry of Amentiferae (Eichler, 1875;
Engler & Prantl, 1924) Similarity in: the inflorescence (simple & naked flowers) of amentifers (Casuarinaceae, Salicaceae, Fagaceae) and the conifers; seeds of Juniperus resembles that of Amentifers.
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Pteridospermean ancestry (Long, 1966; Cronquist)
Reticulate venation; monopodial branching; presence of cambium; microsporophylls & megasporophylls on the same plant; stelar structure; absence of vessels in certain angiosperms, like Winteraceae & Nymphaeaceae.
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Caytonialean ancestry
(Thomas, 1925; Stebbins, 1974)
Caytoniales- fossil plants in the Middle Jurassic; had angiosperm-like anthers similar to branched stamens in Ricinus & Hypericum; ovules enclosed by cupule; netlike venation
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Caytonia. (A) Sagenopteris foliage; (B) Caytonanthus microsporophyll with synangia; (C) Caytonia megasporophyll and longitudinal section of cupule showing ovule position and orientation.
Hypericum stamen
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The Woody Magnoliid Hypothesis -- (Doyle and Donoghue)
That the earliest angiosperms had similar morphology to living members of Laurales & Magnoliales; LONG-HELD PARADIGM; supported by Takhtajan, Hutchinson, Bessey, and Stebbins; also known as “Euanthial theory”
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The strobilus-like flower and primitive carpels of Magnolia.
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The Paleoherb Hypothesis (Taylor and Hickey, paleobotanists)
That the basal angiosperms are tropical paleoherbs, a group of flowering plants with uncomplicated flowers and a mix of monocot and dicot features, & rapid life cycle.
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PALEOHERB HYPOTHESIS
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Aristolochiales
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Piperales
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Two competing hypotheses
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Amborella hypothesis Endemic to cloud forest of New Caledonia
described in the mid-19th century by Baillon (1869); classified as one of the basal angiosperms, together w/ Laurales (Cronquist, 1981)
Has spirally-arranged floral organs
Carpels closed only by secretion rather than by fused tissue, an ancestral feature (i.e., plesiomorphy)
Without vessels
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Amborella trichopoda
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Amborella
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Sarcandra chloranthoides
Austrobaileya
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Austrobaileya
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“Leaf-fish” stamen
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Ceratophyllum hypothesis (Chase et al.,1993
Ceratophyllum as sister to all other angiosperms based on molecular data (rbcL gene)
Submerged plants, flowers small and dioecious
Pistillate flower
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Tricolpate pollen
Monosulcate pollen
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Examples of primitive angiospermsEnglerian school: Simple plants are
primitive (Amentiferae) & complex ones are advanced
Based on progressive evolution
Ranalian school: Ranales group is
primitive Evolution
proceeded in progressive & retrogressive manner
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Winteraceae(Cronquist, 1988; Thorne, 1983): Drimys, Bubbia, Exospermum- vesselless, conduplicate carpel
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Magnoliaceae (Hutchinson, 1959)- elongated floral axis, spirally-arranged stamens & carpels; laminar stamen; very small embryo & abundant endosperm
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Magnolia purpurea
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Ranales: Ranunculus
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Ranales: Berberidaceae