Download Gene flow, hybridization, and evolution in in situ

The population genetics of gene
flow and hybridization
Implications for in situ managed
populations of crop wild relatives
and landraces
Norman Ellstrand
Professor of Genetics
University of California - Riverside
CWRs and landraces
are evolutionarily
l i
il dynamic
d
i
Gene flow’s role?
The
Th population
l ti genetics
ti off gene flflow**
Population genetics primer
– Relationship of gene flow to the other evolutionary
forces
When do immigrant
alleles p
persist and spread?
g
p
What does it all mean for in situ conservation of
plant genetic resources?
Hybridization = Intertaxon gene flow
*Hybridization
Population genetics (microevolution)
Focuses on 4 evolutionary “forces” and their
interactions
1. Mutation: spontaneous allelic or cytogenetic change
source of all genetic variation
rate (μ): generally, 10-4-10-6 / generation, but varies with
organism, gene, etc.
2 Selection:
2.
S l ti
genetically--based differences in reproductive success
genetically
many kinds, directional (Darwinian), disruptive, stabilizing,
balancing, etc.
1-5% selective disadvantage (s) of immigrant allele not
unusual
3. Drift: chance events that alter allele frequencies
generally, drift becomes more important as population size (N)
decreases
founder effect, skewed sex ratio, bottleneck, chronically small
populations, etc.
4 Gene Flow (migration)
4.
allele frequency changes due to immigration of
individuals or gametes
– e.g., spores, sperm, seeds, pollen, eggs, etc.
generally, tends to homogenize populations
gene flow rate (m) 11-5% / generation
not unusual for outcrossing plant
populations separated by >100 m
Relationship of gene flow to the
other evolutionary forces
Gene flow vs. opposing mutation:
– Gene flow is more important
p
than mutation
when m > μ
Gene flow
mutation
Relationship of gene flow to the
other evolutionary forces
Gene flow vs. opposing selection:
– Gene flow is more important
p
than selection
when m > s
Gene flow
selection
Relationship of gene flow to the
other evolutionary forces
Gene flow and selection in concert:
– augment
g
each other,, speeding
p
g adaptive
p
evolution
Selection
Selection & Gene Flow
Gene flow
Relationship of gene flow to the
other evolutionary forces
Gene flow vs. drift:
– One or more successful immigrants
g
p
per everyy
other generation (Nm > 0.5) are sufficient to
counteract drift
Gene flow
drift
Relationship of gene flow to the
other evolutionary forces
Summary
Summary
Gene flow vs.
Mutation: Gene flow prevails when m > μ
Mutation:
- Selection
Selection:: Gene flow prevails when m > s
+ Selection
Selection:: Gene flow augments selection
Drift:: Gene flow prevails when Nm > 0.5
Drift
Gene flow prevails frequently
For detailed discussion of assumptions,
see C. 4, Ellstrand. 2003. Dangerous Liaisons? Johns Hopkins University press
Implications for in situ
management
Generally, plant conservation managers
will like to see some immigrant alleles
persistence and spread
p
p
– and others g
go
extinct
When do immigrant alleles persistence and
spread – and others go extinct?
Persistence and spread of immigrant allele
depends on …
Whether gene flow is oneone-time or recurrent
How that allele affects fitness
Whether that allele is tightly linked to other
alleles with strong fitness effects
One locus expectations:
p
Immigrant allele fate X generations after a
single unliateral gene flow episode
single,
‰ Neutral allele – persists, more or less, in its
original frequency as immigrant.
‰ E.g., if the initial frequency of an immigrant
allele is 5%, it should persist at ca. 5%.
‰ Detrimental allele – decreases to extinction
‰ Beneficial allele – increases to fixation
IImportant
t t assumption:
ti
The sink population is large (N
N >> 100);
that is, drift is negligible.
One locus expectations:
p
Immigrant allele fate after X generations of
recurrent unilateral gene flow
recurrent,
‰ Neutral allele – persists and increases to match its
frequency in the source population.
‰ e.g., if an immigrant allele has a 5% frequency in its
source population, will evolve to 5% in the sink population.
‰ Detrimental allele – persists in “migration-selection”
“migration selection”
equilibrium
‰ Beneficial allele – increases to fixation
IImportant
t t assumption:
ti
The sink population is large (N
N >> 100);
that is, drift is negligible.
Multi-locus expectations:
Multip
Immigrant allele fate after X generations
following unilateral gene flow
‰ If an immigrant allele is tightly linked to an allele at another
locus with a stronger fitness effect, its fate will be largely
determined by “hitch-hiking” with that allele.
¾ e.g., selective sweep
‰ Genomic
G
i linkage
li k
varies
i with
ith ““recombination
bi ti system”
t ”
Ö Apomixis – linkage more or less absolute
Ö Selfing – linkage decays relatively slowly
Ö Outcrossing – linkage decays relatively quickly
Some possible consequences of
unintended gene flow for the recipient
population. #1
Some interesting documented consequences
Changes
g in diversity
y ((increase OR decrease))
Evolution of new species
Evolution of increased weediness/invasiveness
Extirpation/Extinction via …
– genetic swamping
– outbreeding depression
Some possible consequences of
unintended gene flow for the recipient
population. #2
One hypothetical???
yp
consequence
q
CWRs as a repository
p
y for “heirloom” crop
p alleles?
Gene flow as a potential tool:
Some examples
“Genetic rescue” (inc. “assisted migration”?)
– Adding diversity to a genetically depauperate
population
– Should population genetics
Use scalpel,
p , not a chainsaw
Genetic pest management
(hypothetical, at the moment)
– Gould,
G ld G
Gressel,
l and
d many more
When is unintended gene flow
important in plant conservation?
Straightforward rule of thumb:
Gene flow becomes an important
p
concern in
plant conservation ONLY when it changes
SUBSTANTIALLY compared
to recent g
gene
p
flow levels
(Ellstrand & Elam. 1993. Annual Review of Ecology & Systematics)
Gene flow becomes an important concern in
plant conservation ONLY when it changes
SUBSTANTIALLY compared to recent gene
fl
flow
llevels
l
What is a SUBSTANTIAL
S S
change?
?
“No” gene flow to “some” gene flow (or vice versa)
“Some” gene flow to “lots” of gene flow (or vice versa)
Et cetera
How can you estimate “recent gene flow levels”?
Population
p
g
genetic structure statistics ((e.g.,
g , FST) reflect
historic, not current, gene flow levels
Alternatively, information from ethnobotany, history,
breeding system, dispersal ecology, etc.
Qualitative “Levels”
Levels of Gene Flow
No gene flow or essentially “no” gene flow
–m<μ
“Some” gene flow (stochastically
significant)
– ca. 5% > m > μ
“Lots of” gene flow (adaptively significant)
– m > ca
ca. 5%
Conclusions
Gene flow
G
fl
is
i a common and
d potentially
t ti ll
important evolutionary force
Generally, an unmanaged immigrant allele
persist in a p
population
p
will p
Plant conservation managers should be
mindful of gene flow’s
flow s potential as a
problem, as a tool, or something to be left
alone
In conservation, only SUBSTANTIAL gene
fl
flow
changes
h
are worthy
th off concern
Thank you!
Funding from
John Simon Guggenheim Memorial Foundation Fellowship
and prior support from
USDA NSF
USDA,
NSF, EPA
EPA, SJFR,
SJFR Fulbright Fellowship
Fellowship, etc
etc.
“Team Ellstrand” Crop Gene Flow Scientists
Janet Clegg
Terrie Klinger
Lesley
y Blancas
Sylvia Heredia
Caroline Ridley
S b
Subray
H
Hegde
d
Roberto Guadagnuolo
Melinda Zaragoza
Detlef Bartsch
Diane Marshall
Janet Leak
Leak--Garcia
P
Pesach
hL
Lubinsky
bi k
Diane Elam
Paul Arriola
Jutta Burger
g
Bernie Devlin
Karen Goodell
J
Joanne
H
Heraty
t