Transcript Lecture 1 Thursday Jan. 4, 2001

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 120-125 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 (245-250 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 http://www.crimescene.com/joan/evidence.test.results.html
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 flowerinsect 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:
• http://www.sciencemag.org/cgi/content/full/296/5569/899 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):
http://www.stanford.edu/dept/news/report/news/april4/acsflowers
-44.html and
http://www.amjbot.org/cgi/content/abstract/86/11/1563
Look at Pollen grains and terminology:
• http://www.geo.arizona.edu/palynology/polkey.html
• http://www.biologie.uni-hamburg.de/b-online/e27/6.htm
• http://geography.berkeley.edu/ProjectsResources/PollenKe
y/byType.html
How many species?
•
•
•
Approximately 250,000 species of flowering
plants (versus 700-800 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]