Evolution of Plants
David Baum
Game plan
• What are “plants” and how did they evolve?
• Differences between plant and animal evolution
• Some stories of plant evolution
What are the three most
important* events in the
evolution of life on earth?
1. Oxygenic photosynthesis (cyanobacteria)
2. Invasion of land (plants)
3. Human agriculture and technology
*Profoundly
affecting the globe’s chemistry and ecology
Early land plants were low to
the ground
They became larger, more complex, and
acquired a vascular system
Time
Multiple origins of “trees”
Crane and Leslie (2013)
Why?
http://www.earthhistory.org.uk/recolonisation/vegetation-in-devonian
An evolutionary arm’s race
• The Red Queen principle
Now, here, I see it takes all
the running you can do, to
keep in the same place. If
you want to get somewhere
else, you must run at least
twice as fast as that!
(Through the Looking Glass,
Lewis Carroll)
Competition for light is a very
important driver of plant evolution
Problems that plants faced
• Gain light, water, nutrients
• Escaping predators (once animals invaded land)
• Sex!
If you want to know more:
Botany 130, 300, 305, 401, 500
Fern sperm cell
Are there differences between
plant and animal evolution?
• Very few – plants are excellent “model
systems”
• But..
– Greater diversity in sexual systems
• Abundant asexuality
– More chemistry less behavior
– Maybe more evolution by “hopeful monsters”
Examples of “hopeful
monsters?”
Rudall PJ, Bateman RM. 2003.
Trends Plant Sci. 8(2):76-82.
Rudall PJ, Bateman RM. 2002.
Biol Rev Camb Philos Soc.77(3):403-441
Are flowers monsters?
Pollen cone
Living gymnosperms
have unisexual cones
Seed cone
Are flowers monsters?
Megasporophyll
Microsporophyll
(stamen)
A flower is a bisexual “cone”
(although unisexual flowers have
evolved in many groups)
If so: quite successful!
• ~300,000 species of flowering plants
• Dominate all land ecosystems (and several
aquatic ones)
• Provide all food resources for humans
Are there differences between
plant and animal evolution?
• Very few – plants are excellent “model
systems”
• But..
– Greater diversity in sexual systems
• Abundant asexuality
– More chemistry less behavior
– Maybe more evolution by “hopeful monsters”
– Polyploidy
Why is polyploidy common in
plants?
• Tolerance of different ploidy levels during
development (dosage compensation?)
• Often make unreduced gametes (“rescues”
meiotic problems)
Chester et al. 2010. Genes 1(2), 166-192.
Why is polyploidy common in
plants?
• Tolerance of different ploidy
levels during development
(dosage compensation?)
• Often make unreduced
gametes (“rescues” meiotic
problems)
• Allows for “instant speciation”
http://www.amjbot.org/content/95/6/713/F1.large.jpg
Example of “instant
speciation”
• Spartina maritima grew in British salt marshes
• A different species, S. alterniflora arrived ~1816,
probably from boats arriving from America
– This was figured out partly by talking to thatchers in the
1830’s who noted that the new form of cordgrass had
appeared some 20 years earlier
Example of “instant
speciation”
• In 1879 two botanist brothers, H. & J.
Groves, found a plant that confused
them:
– “Spartina which . . .would be rather
more S. alterniflora… yet which we now
consider to be [S. maritima]”
• In 1880 they concluded they had
found a new species, which they
called S. townsendii
• It was not fertile
• Examination of the herbarium records
showed that it had appeared by 1870
Example of “instant
speciation”
• Fertile “S. townsendii” appeared and was
eventually named as a new species S. anglica.
• Stapf (1910) noted: ‘towards the end of the eighties
(c. 1890) something occurred that favoured the
spreading of the grass’ – this is now presumed to
represent the origin of S. anglica.
• Stapf (1926) hypothesized that S. anglica and S.
townsendii are hybrids of S. maritima X S.
alterniflora
Example of “instant
speciation”
• S. anglica spread quickly around Southampton
harbor
• It gradually reached other estuaries – especially
during World War II – perhaps because of training
with landing craft
http://www.thinkdefence.co.u
k/2011/11/uk-militarybridging-world-war-ii-africaand-northwest-europe/
Example of “instant
speciation”
• But if this is true: how come “…it is fertile and
breeds true to type” (Hutchins, 1930)?
[S. anglica]
Hypothesis
S. maritima
2a chromosomes
S. townsendii
a+ b
chromosomes
(sterile)
S. alterniflora
2b chromosomes
Hypothesis
S. townsendii
a+ b
chromosomes
(sterile)
Chromosome
doubling
S. anglica
2a + 2b
This hypothesis explains a few things
• Why the S. townsendii arose suddenly
• Why it was successful (“hybrid vigor”)
• Why it was sterile
• Why S. anglica arose suddenly
• Why it was successful
• Why it is fertile
But is it true?
C. Leonard Huskins (1930)
Professor of Genetics at McGill
S. maritima, 2n = 56 (“2a”)
S. alterniflora, 2n = 70
(“2b”)
126
S. anglica, 2n =
(Became Professor of Botany at UWMadison in 1945)
Huskins (1930) Genetica
There are many other examples
of polyploid species in plants
• Often generate competitively superior
species
• Many crops have arisen by hybrid
polyploidy (including wheat and potato)
• Means that plants have lots of “extra” genes
that can acquire new function
• All vascular plants have undergone multiple
rounds of polyploidy in their ancestry
Pollination Stories
Pollination (only occurs in seed
plants) avoids the need for
motile sperm
• Pollen is a minute
male plant
• Can be carried by
wind (rarely water)
• More commonly
animals do it
– Insects
– Birds
– Mammals
Pollen needs to deliver the
gametes to the egg cells
Stigma
Pollen tubes grow through
plant tissues – navigated
chemically
Pollen tubes grow through
plant tissues – navigated
chemically
Plants have evolved diverse
ways to get pollen from one
flower to another
• Wind
• Water (rare)
• Animals
– Mutualistic (give a reward)
– Parasitic (trick the animal)
How do you think this evolved?
• What else would you like to know?
At the other extreme: Figs and
fig wasps
figs are “tomb blossoms”
Implications
• There is a one-to-one relationship between a
fig species and its wasp pollinator species
• Predicts cospeciation: that the figs and wasp
Prediction:
• One-to-one species association
• Cospeciation
A
a
Fig phylogeny
C
c
B
b
D
d
E
e
F
f
Wasp phylogeny
Actual result
Host
switching
Weiblen and Bush (2002)
• Plant evolution is similar to other
multicellular eukaryotes
• Arm’s race for light
• Polyploidy is especially important
• Coevolution with animals for pollination
(and dispersal) is important
• (Also, abundant coevolution with fungi and
bacteria)
Feel free to contact me:
dbaum@wisc.edu