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