Download Hybridization and Conservation

Hybridization and Conservation
Allendorf, Chapter 17
1. Case Study: The New Zealand
Black Stilt
2. What is hybridization, and what
causes it?
3. What are the potential
consequences of hybridization?
4. How do we recognize hybrids
and introgressed individuals?
1. Case Study: The New Zealand Black Stilt
The New Zealand black stilt
- formerly bred throughout New Zealand
- now occurs in only one river basin due to
predation and loss of breeding habitat.
The pied stilt
- self-introduced from Australia to the
South Island in the early 1800s
- spread to the North Island in the 1900s
Hybrids
- were first reported in the late 1800s
- are now widespread.
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Will hybridization lead to genetic
extinction of the black stilt?
2. What is hybridization, and what causes it?
It may occur naturally, e.g.:
Hybridization may be defined generally as “mating between
individuals from two populations, or groups of populations, which
are distinguishable on the basis of one or more heritable
characters” (Harrison 1990).
- many (10%+) species hybridize occasionally
(e.g Darwin's finches)
- many sister-taxa form hybrid zones,
where hybrids are frequent (e.g. firebellied toads)
- many species appear to have arisen from
polyploidization following hybridization
(e.g. sunflowers)
It can also result from anthropogenic changes, especially
3. What are the potential consequences of hybridization?
- species introductions (e.g., bull vs. rainbow trout)
Hybridization from anthropogenic causes has contributed
directly or indirectly to many extinctions....
- habitat fragmentation (e.g., red vs. white mulberry)
- e.g., cutthroat trout vs. rainbow trout
- habitat degradation (e.g., cichlids in Lake Victoria)
- population decline leading to "desperation" (e.g. speckled teal)
- New Zealand grey duck vs. mallard duck
....and appears to be increasing
However, hybridization can either increase or decrease fitness:
Individuals meet
Do not breed
Fertilization occurs
No offspring produced
F1 progeny produced
F1 have low survival
F1 have high survival
F1 have low fertility
F1 have high fertility
F2 or BC have low survival
F2 or BC have high survival
F2 or BC have low fertility
F2 or BC have high fertility
hybrid swarm
with genetic
swamping
Fitness effects are largely unpredictable, and depend on
- intrinsic and extrinsic factors
Intrinsic Causes
Demographic
swamping or
extinction
vortex
genomic
extinction
If BCs are fertile, hybridization can lead to a "genomic
ratchet", because the introgressed DNA cannot be removed.
Extrinsic Causes
Fitness
- masking of deleterious
Increase
recessive alleles
(hybrid vigor - increase in heterozygote
or heterosis) advantage
- introduction of
new adaptations
Fitness
- chromosomal
Decrease
incompatabilities
(outbreeding - loss of coadapted
depression)
gene complexes
- loss of local
adaptations
- time (generations)
Generally, the more karyotypically, genetically or ecologically
different the organisms are, the more negative the outcome
(Fig. 17.3):
Potentially, hybridization could lead a species into an extinction
vortex.
Hybrid Vigor
- The shape of the
fitness relationship
varies both among
and within species.
outbreeding
depression
population
decline
outbreeding
limited
mate
choice
4. How do we recognize hybrids and
introgressed individuals?
Molecular methods now enable identification of hybrids and BCs:
allozymes
¸ PCR ¸ NGS
Theoretically,
- computer programs (e.g. NewHybrids) enable assignments of
individuals to different hybrid classes,
Generation [P1]
F1
50
BC1
~25
BC2
~12.5
BC3
~ 6.2
...
[P2]
50
~75
~87.5
~93.8
- e.g. Peruvian and blue-footed boobies (Taylor et al. 2012)
Until the 1960s, hybrids were
identified from morphology, but
- not all variation is additive
Hybridization is much more extensive that previously recognized!
1. The Black Stilt, Revisited
Molecular analyses revealed limited genetic introgression
et al. 2010, Mol. Ecol.19: 5090) .
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Morphological hybrids are now culled.
(Steeves