do plants evolve differently?
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Do plants evolve differently?. Thorsten Reusch IFM-GEOMAR Kiel. Eelgrass megaclone, Aland Islands. Fotograph: Chris Boström,. Genetic variation as fuel for evolution. selection recombination drift ultimate input: mutation. usual estimates of mutation rates - PowerPoint PPT PresentationTRANSCRIPT
Do plants evolve differently?
Eelgrass megaclone, Aland Islands. Fotograph: Chris Boström,
Thorsten ReuschIFM-GEOMAR Kiel
Genetic variation as fuel for evolution
•selection•recombination•drift
•ultimate input: mutation
•usual estimates of mutation rates10-7 -10-9 per nucelotideper generation
Common views about mutation
•somatic mutation-evolutionary dead end-often deleterious, e.g. may cause cancer-will not be passed into next generation-no change of allele frequencies•meiotic (~germline) mutations-evolutionary change
animal centered view originating from
overinterpretation of Weismann’s germline
concept
Weismann’s germline concept 1892
Das Keimplasma: eine Theorie der Vererbung. The germ plasm: a theory of heredity 1892
August Weismann1834-1914
•soma cell lines are separate from germline from the onset of ontogeny•only germline cells keep totipotency
zygote embyo
The Weismann ‘barrier’
•during modern synthesis provided ‘final blow’ against Lamarckism and pangenesis ideas of Darwin
John Maynard Smith: The theory of evolution 1993
soma
germline
protein
genetic information
Darwin’s pangenesis theory
...was actually pretty close to Lamarckian ideas
•Pangenesis view introduces gemmules that are derived from all body cells and are the carrier of hereditary changes•gemmules collectively migrate to reproductive organs•information from gemmules is somehow tranferred to germ cells
the Weismann barrier
•central to Modern Evolutionary Synthesis!
“Weismann is the second most notable evolutionary theorist of the nineteenth century, after Charles
Darwin”.
Ernst Mayr
Weismann was instrumentalized to save Darwin from Lamarckism
But: the Weismann barrier is not general
... and this is why botanists never really liked it
plant meristematic tissue can re-differentiate into reproductive organs (flowers)
Botany online Univ Hamburg
flower bud
apical meristem producing leaves
Genetic individual or genet (=clone)
Module or ramet
moreover plants are modular
Many plants (+ animals) are facultative clonal asexuals to a degree
mutation
somatic mutation may affect one module
clone tree
...and thus defective mutants (e.g. chlorophyll-)may be nourished by neighboring twigs/modules
Consequences of plant modular growth
chlorophyll-defective somatic mutants in mangrove
Klekowski & Godfrey Nature 1989
=> plants may carry a ‘somatic’ genetic load
Evidence for other somatically derived variation?
Agricultural and horticultural varieties
normal
pink Fosters grapefruit
Genetic mosaics also possible
Rio red ‘chimera’ Texas, 1990
Meristem composed of (two) different genotypes
somatic mutation leads to genetic mosaic
meristemtic tissueof vascular plants is stratified,i.e. consisting of independently dividing sublineages
spatial arrangement of meristematic tissues...
Variegated Pelargonium
...determines type of mosaic
Genetic detection of mosaics with microsatellitesD
NA
-Seq
uen
ce
allele (TC)11
allele (TC)10
...because mutation rates much higher (10-3 - 10-4)
Genetic mosaics in grapevine ....
Moncada et al Genome 2006
...allow for the separation of different Cabernet Sauvignon cultivars
Mosaic genotypes as mixtures of bi-allelic genotypes
step-wiseMutation
125:131:133
125:133
125:131
Are there any genetic mosaics in the wild?
hypotheses
•the larger + older the plant, the more important become somatic mutations and genetic mosaics(and cultivars propgated by grafting are very old clones)
•because sexual reproduction restores genetic uniformity by introducing a single-cell phase (zygote), sexual reproduction should be negatively correlated with mosaics
vegetative propoagation
sexual reproduction
marine angiosperm Zostera marina
seeds ininflorescens
pollenfemale flowers
Z. marina may from very large + old clones
in Archipelago and Aland Sea (Baltic) clones are 160 m + in extension and possibly >1000 yr old
Reusch et al 1999
Using clonal richness as a surrogate for sexual /vegetative reproduction
Low clonal richness
High clonal richness
Detection of mosaics using 2 hypervariable microsats
site Kolaviken, Archipelago Sea, Finnland
mosaic genotype
normal bi-allelic genotype
normal bi-allelic genotype
mosaic genotype
normal bi-allelic genotype
normal bi-allelic genotype
locu
s G
A17H
locu
s 35
Detection of clones using 7 less polymorphic loci
genotypeA
B
C
D
C
C
‘normal’ bi-allelic genotypes can be recovered
step-wiseMutation
sea
gra
ss le
af
basal meristematic tissue
The data set: 36 European populations
ArchipelagoSea
12
Aland-Islands
5
South Portugal
4
NorthSea 8
SW Baltic 5
Nramets = 20-60
Sampling together with Chris Boström, Jeanine Olsen, Ester Serrao
The less sex, the more mutations
Sex No sex
Clonal Richness
Min
imal
Mut
atio
ns p
er c
lone
1 .8 .6 .4 .2 0
0
1
2
3
4 Exponential function, R2 = 0.64; P < 0.001
Reusch & Boström, Evol Ecol in press
Mean Clonal Richness
Min
imal
Mut
atio
ns p
er c
lone
1 .8 .6 .4 .2 0
0
0.5
1
1.5
Exponential function,R2 = 0.97; P = 0.007
SW Baltic5
Archipleago, 12
Aland5
North Sea8 Portu
4
Correlation also present on the basis of subregions
Sex No sex
Reusch & Boström, Evol Ecol in press
The less sex the fewer mutation-free genotypes
Sex No sex
Reusch & Boström, Evol Ecol in press
So what..
microsatellite loci are selectively neutral(tested for Zos mar Oetjen & Reusch Mol Ecol 2007)
nevertheless, microsatellite polymorphism may indicate potential of processes at level of selected genes
depending on the age of plants or plant clones, input by somatic mutations may be much larger than input by meiotic mutations (e.g. modelling by Orive J theor Biol 2001)
Mutation without meiosis and dispersal stages
...is expected to lead to high between-population differentiation
complete population separation
random walk allele length
0
0.1
0.2
0.3
0.4
0.1 1 10 100
0
0.1
0.2
0.3
0.4
1 10 100
0
0.1
0.2
0.3
0.4
1 10 100 1000
0
0.1
0.2
0.3
0.4
1 10 100
gene
tic
dist
ance
(th
eta)
geographic distance between pop pairs (log-scale)
SW Baltic
FinlandS Portugal
Wadden Sea
0
1
0
1
clon
al r
ichn
ess
Isolation-by-distance in 4 European coastal regions
ns
p<0.01p<0.01
ns
Adaptive implications
adaptive genetic variation may also arise by somatic mutations, and reside as mosaics within genets or plant modules (cf cultivars)
how come that oldest clones of Zostera marina can survive under drastical environmental change in past 4000 yrs in northern Baltic?
testable hypothesis: mutation + selection may have happened within clones
Trees are just three-dimensional clones
www.inkart.con
Genetic mosaic hypothesis
Why are large, long-lived trees not overrun by pests and pathogens?
hypothesis: somatic mutation create mosaics at the level of branches
Witham & Slobotkin 1981; Gill 1988
Do plants evolve differently?- Yes
....but many clonal animals (19 phyla) do so as well
Do plants evolve differently?- Yes
1 because they do not posses a germline
2 because many of them grow to very large size (i.e. have time to accumulate somatic mutations)
3 because plants grow in modular fashion, opening the possibility for within ‘individual’ selection + variation
4 hierarchical levels of selection needs more attention in population genetics
Kollinpää
Lökholm
Sandö
VänöRyssholm
Holma
Högsåra
Husskär, Långören, Sandskär
Kakskerta, Eerikinvallanniemi
Äppelö Rankoskär
Hinderbengtsviken
Sandö
Prästö
HenriksbergPlagen
Turku/Åbo
Fårö
Ängsö
Utö
Empty red circles indicate mutation sites, filled red circles indicate all(?) other sites that have been genotyped through the years...
Åland
FINLAND
The old Danish itinerary in the1250s
Other medieval sailing routes, just marked one of very many