50839214 origin of angiosperms
TRANSCRIPT
Origin of Angiosperms
Geologic time scale Pollen evidence: 126 mya (Lower
Cretaceous Period) Macrofossil evidence
(“Archaeofructus”): 130 mya (Lower Cretaceous)
WHERE: West Gondwana (South America and Africa)
Archaefructus liaoningensis
Pioneer plants in the Cretaceous: figs, planes (Platanus), and magnolias
Possible reasons for rapid radiation of angiosperms:
Co-evolution with many insect groups
Shorter life cycleDiverse habitat types and
edaphic factors
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
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)
Isoetes-Monocotyledon theory (Campbell, 1928)
Similarities in: habitat (humid tropics); habit; embryo & anatomy of older sporophyte
Gnetalean ancestry (Wettstein, 1901)
Similarities: two cotyledons, vessels in 2ndary wood, two integuments, net-veined leaves
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.
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.
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
Caytonia. (A) Sagenopteris foliage; (B) Caytonanthus microsporophyll with synangia; (C) Caytonia megasporophyll and longitudinal section of cupule showing ovule position and orientation.
Hypericum stamen
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”
The strobilus-like flower and primitive carpels of Magnolia.
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.
PALEOHERB HYPOTHESIS
Aristolochiales
Piperales
Two competing hypotheses
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
Amborella trichopoda
Amborella
Sarcandra chloranthoides
Austrobaileya
Austrobaileya
“Leaf-fish” stamen
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
Tricolpate pollen
Monosulcate pollen
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
Winteraceae(Cronquist, 1988; Thorne, 1983): Drimys, Bubbia, Exospermum- vesselless, conduplicate carpel
Magnoliaceae (Hutchinson, 1959)- elongated floral axis, spirally-arranged stamens & carpels; laminar stamen; very small embryo & abundant endosperm
Magnolia purpurea
Ranales: Ranunculus
Ranales: Berberidaceae