Download Plant evolution and speciation

4/10/12
Plant evolution and speciation
BY2204 EVOLUTION
Trevor Hodkinson
Plant Sciences Moderatorship
Some evol. processes shared with other
organisms
(natural selection; allopatric speciation).
Some more common in plants than
animals
(sympatric speciation, hybridisation &
polyploidy).
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Natural selection - explains
close fit of organisms to
environment (adaptation)
nothing else can explain such a
match
Cacti are succulent
E.g. 1
Trifolium repens
(white clover)
Some individuals
produce cyanide for
grazing protection
however this makes them
less frost tolerant
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Release hydrogen cyanide (HCN) when tissue
is broken (HCN not present in intact tissue -it is
created upon breaking)
Two components in a cell
1)Cyanogenic glucoside (one compartment)
2)Cyanogenic β-glucosidase (hydrolytic
enzyme) in another compartment
When the enzyme meets glucoside -HCN is produced
Cyanide
producing
frost sensitive
white clover
Variation within a
species maintained
by natural selection
(grazing and temperature;
cyanide makes them less
frost tolerant)
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E.g. 2 Evolution of metal
tolerance
Strong selection pressure due to high
toxicity of soils
Metal tolerant populations (within a
species) have evolved -ecotypes
-speciation has been documented
•  Copper-cobalt outcrops- High Cu, Co
(Zambia & Congo)
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European metal mines eg. Glendalough, Co. Wicklow
Ecotypes of
grasses
Festuca,
Agrostis,
Deschampsia
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Normal populations
Poor growth
Population from copper mine
Good growth
Pratt 1934. Transplanted populations of
Silene vulgaris (campion) onto toxic copper
soil. Showed evolution of tolerance.
tolerant population on
the mine and nontolerant off the mine
Metal tolerant populations of sweet
vernal grass (Anthoxanthum). Sharp
boundary in tolerance caused by strong
selection against non-tolerant individuals
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Allopatric speciation in plants
Allopatric speciation with
geographic separation of
populations
Allopatric speciation
-At time A populations can interbreed
-At time C pre- and/or post-zygotic
reproductive barriers have evolved
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Allopatric
speciaton
Allopatric speciation facilitated
by the founder effect
A small
subset
founds a
new colony
the new
gene pool is
a different
composition
Single locus -two alleles A, a
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Allopatric
speciation
on islands
New gene pools
(genetic drift/
founder effect)
and new selection
pressures
combine to cause
speciation
PACIFIC ISLAND SPECIATION &
DISPERSAL
zoochory
SPECIES RICH
Hawaii
(animals)
Galapagos
anemochory
(wind)
Pitcairns
hydrochory
(water)
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Adaptive radiation
‘numerous diversely adapted
species evolving from a
common ancestor’
Hawaii
Volcanic islands of recent origin
(<100mya)
90% of all plant species are endemic
and c.4000km from the mainland
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Adaptive radiation
in Hawaii
28 species in three genera of silverswords
have evolved from one colonisation event
by Madia sativa from the mainland USA
Argyoxiphium
Dubautia
Wilkesia
Radiation of leaf form
Hawaiian silverswords
(tarweeds)
Madia sativa
Argyoxiphium
Dubautia
(ancestral
silversword)
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Sympatric speciation
Speciation without geographic
separation of populations
Sympatric speciation is
controversial in animals
Eg, Cichlid fish (Cichlidae)
(1,800 species).
Text book example of
adaptive radiation. 850
species in lake Malawi
alone
Lobochilotes labiatus at Nkondwe Island, Lake Tanganyika, Tanzania.
Have they evolved by
sympatric speciation? No
geographic barriers. Sexual
selection, niche
specialisation, feeding
adaptations?
(Staufer et al. 2007, in Hodkinson &
Parnell, Reconstructing The Tree of Life,
Chapter 14.).
Placidochromis milomo at Nkhomo Reef, Lake Malawi, Malawi.
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Sympatric speciation not
controversial in plants
e.g. occurs via hybridisation
Helianthus
annuus
Hybrid
H. annomalus
sunflower
H. petiolatus
Reproductively
isolated from its
two parental
species
Platanus allopatric and
sympatric speciation
P. occidentalis
(north American)
P. orientalis
(European)
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Platanus
orientalis
New square
TCD
Recent hybrid of P. occidentalis
and P. orientalis
Common ancestor c. 30mya
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Formation of Platanus orientalis and P.
occidentalis by allopatric speciation
Formation of P. xhybrida (London
plane) by hybridisation/sympatric
speciation
Polyploidy & polyploid speciation
(a type of sympatric speciation)
Polyploids contain more than two genomes
(a diploid has two genomes
eg. humans -one genome from each parent)
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Happlopappus
Ophioglossum
Diploid 4 chromosomes
Extreme polyploid
(2 sets of 2)
1260 chromomsomes
Polyploids
Rare in animals
Common in plants
95% of ferns are polyploid
80% of flowering plants (angiosperms) are
polyploid
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Polyploids
Autopolyploids –have more
than 2 genetically ‘identical’
genomes
Allopolyploids –combine
genomes from more than one
ancestral species
Same species
Different species
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S. maritima
S. alterniflora
S. anglica
Spartina anglica (saltmarsh grass)
A vigorous allopolyploid hybrid of native S. maritima
(Europe) and introduced S. alterniflora (USA)
S. maritima 60
x
S. alterniflora 62
Hybridisation
S. townsendii 62 (Sterile hybrid; no seed)
Doubling of
chromosome number
S. anglica 122 (Fertile polyploid)
S. anglica is an allotetraploid
Chromosome number provided –one pair lost in S. anglica
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Types of polyploid (allo -or autopolyploid) , x=haploid
chromosome set (ie one set).
diploid 2x
triploid 3x
tetraploid 4x
pentaploid 5x
hexaploid 6x
Sometimes these can interbreed and we get a
‘polyploid complex’
Miscanthus –biomass crop
TCD botanists
•  Miscanthus x
giganteus
•  Example of hybrid
speciation in a
polyploid complex
•  Sterile but maintained
by vegetative
reproduction (clones)
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M. ×giganteus metaphase chromosomes
2n=3x=57
x=19 (haploid
number; one set)
Triploid
Allopolyploid or
autopolyploid?
Hodkinson et al. (2002) American Journal of Botany 89: 279-86
M. sacchariflorus (76)
x
M.sinensis (38)
(tetraploid)
(diploid)
M. x giganteus (38+19=57)
(allotriploid, sterile)
Chromosome doubling would restore fertility
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Conclusions
Evolutionary processes in plants are
dominated by:
•  Natural selection
• Genetic drift and the founder effect
• Allopatric and sympatric speciation
• Hybridisation and polyploidy (which can
also cause sympatric speciation).
Walley et al. 1974. Sowed 1000 seeds of
bent grass (Agrostis) from normal bent
grass on a copper contaminated soil
4 individuals were tolerant
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Genetic drift and bottlenecks are
powerful forces in evolution
a bottleneck
can cause a
change in
allele
frequency
by random
processes
Random
genetic drift
loss of white and
increase in black
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