BIOLOGY 3404F EVOLUTION OF PLANTS Fall 2008 Lecture 16 Tuesday November 18, 2004 Chapter 20 Evolution of the Angiosperms, part

A word about fossil dates: The age of fossils is frequently indicated simply by the layer (stratum) of rocks in which they occur (e.g., Upper Devonian, or Late Devonian). Calendar-year age estimates of these strata often vary, or are corrected over time with additional evidence. As an example, Archaefructus is a recent find from China from the Cretaceous. When it was first published in Science as the earliest Angiosperm, it was dated at 142 MYA, but this was later revised to MYA

The first Angiosperms The earliest definite angiosperm fossils are from the Cretaceous, approximately 130 MYA. Angiosperms dominated the flora by 90 MYA, and most existing families were present by 75 MYA. There are scientists who believe that angiosperms originated much earlier, in the late Permian ( MYA). Fossils of Gigantopterids from this time have not conclusively been linked to reproductive parts. Many scientists consider them to be seed-ferns, but others point out that they had primitive vessels, a eustele, and produced the compound oleanane, now known only from Angiosperms and Gnetales.

Gigantopterids, II It is possible that the Gigantopterids are the common ancestor of both Angiosperms and Gnetales, explaining some of the other similarities in the two groups, including double fertilization, lack of archegonia in Gnetum and Welwitschia, presence of vessels, and similarity of gnetid strobili to some angiosperm inflorescences. This would also help solve Darwin’s “abominable mystery” of the sudden appearance of angiosperms in the fossil record.

Carpels and flowers The earliest angiosperms, including Archaefructus, did not have what we would recognize as flowers. First, they invented the carpel. This is the fundamental female reproductive unit of angiosperms, and was derived from a leaf (perhaps similar to the cupule of some early gymnosperms, but fully enclosing the ovule or ovules). Carpels in primitive angiosperms were imperfectly fused, and make a physical intermediate between a folded leaf and fused pistil.

Bevhalstia pebja, ~ the world’s oldest flower (130 MYA)

Early pollen Early Angiosperms had pollen grains with one aperture (slit or pore, termed monocolpate), as do cycads and Ginkgo. Among extant angiosperms, monocolpate pollen is found in water lilies (Nymphales), “woody magnoliids” (Magnoliales and Laurales, e.g., Magnolia and the tulip tree Liriodendron), Canellales (e.g., Drimys, or Winter’s bark, which lack vessels), Piperales (including Piper – B&W pepper – and Peperomia of the Piperaceae; and Aristolochia – Dutchman’s Pipe – and Asarum – wild ginger – of the Aristolochiaceae), which are now regarded as the basal angiosperms, and also in the monocots.

Monocolpate pollen of maize, a monocot

Tricolpate (modern) pollen The “eudicots” are characterized by pollen with three apertures (tricolpate; however, not all eudicots have this). Tricolpate pollen goes back approximately 127 MY. Flattened, trilobal, tricolpate grains with three furrows, identified as Quercus nigra From

Origins of Angiosperms There are (at least) two competing hypotheses for the origins of angiosperms: the “paleoherb hypothesis” and the “wood magnoliid hypothesis”. [see paper by Stuessy 2004, linked on web site] The “paleoherb hypothesis” suggests that the earliest angiosperms were a group of tropical flowering plants with uncomplicated flowers and a mix of monocot and dicot features (see Ceratophyllum essay, p. 526). “Paleoherbs” are a polyphyletic group of non-woody basal angiosperms, and include the water lilies, Ceratophyllum, and Aristolochia.

The “woody magnoliid hypothesis” suggests that early angiosperms had a morphology similar to living members of the Magnoliales and Laurales: plants that are small to medium-sized trees with long broad leaves and large flowers with indeterminate numbers perianth parts; carpels are imperfectly fused, and make a physical intermediate between a folded leaf and fused pistil. Recent molecular studies support this latter hypothesis (which was conceived long before molecular phylogenetics) and point, in particular to the monotypic order Amborellales (see tree handouts from AmJBot) See Liriodendron, Nymphaea and Primitive Carpels and Stamens demonstration sheets in Lab 11.

Amborella trichopoda, a New Caledonian shrub and basal Angiosperm

Flowers of Amborella - carpels incompletely fused, no vessels

Ceratophyllum, with many “primitive” characters because it is aquatic

The “Ceratophyllum hypothesis” for the origin of Angiosperms

What Made Angiosperms so Successful? Many early angiosperms had traits adaptive to resisting drought and cold: small, tough leaves; vessels; tough seed coat. Being deciduous evolved early, and is also adaptive for harsh conditions. Other factors include evolution of sieve tube elements (efficient carbohydrate transport), specialized pollination and seed dispersal mechanisms (often animal-dependent), enormous chemical diversity (defenses against disease organisms and herbivores), and mycorrhizal relationships (both ecto- and endomycorrhizae, the latter much more widespread).

Angiosperm success II Importantly, insects diversified in the Cretaceous too, and although many ate plants (driving evolution of protective chemicals), many others came to act in pollination or seed dispersal. There is an excellent discussion of flower- insect coevolution in text.

Fruits Fruits: This year, we will skip over fruit terminology but please read about fruit dispersal mechanisms and biochemical coevolution.

Additional resources Read about the earliest angiosperm, Archaefructus: This plant had carpels, but no true flowers (still no insect pollination) Read about Gigantopterids, oleanane, and the possible origin of angiosperms in the Permian (250 MYA): html and Look at Pollen grains and terminology: y/byType.html

How many species? Approximately 250,000 species of flowering plants (versus gymnosperms) 175,000 eudicots and 70,000 monocots Five largest plant families are: 1.Asteraceae (composites, eudicots, 25k) 2.Orchidaceae (orchids, monocots, 20k) 3.Fabaceae (legumes, eudicots, 18k) 4.Rubiaceae (coffee/madder, eudicots, 10k) 5.Poaceae (grasses, monocots, 9k) [Note: The exact numbers and order are not important, or even agreed upon]