PLANT OF THE DAY! Tamarix (salt cedar) 50-60 species Family Tamaricaceae
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PLANT OF THE DAY!
•Tamarix (salt cedar)• 50-60 species •Family Tamaricaceae•native to dry areas of Eurasia and Africa.
•Introduced to North America as ornamental shrub in 19th century•Planted extensively during great depression to prevent soil erosion
•Second worst invasive species in USA•Colonizes riparian habitats, displacing native vegetation and consume precious water resources
•Most common invasive in USA is a hybrid of two species that do not grow in the same areas of Asia
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Hybridization and Speciation
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Big Questions
How frequent is hybridization in plants?
What is the role of hybridization in plant evolution?
“The major point to be stressed with respect to the role of hybrids in diversity is that they are intermediate. . . For this reason, the ultimate contributions to overall diversity made by hybrids must be small or negligible.”
Wagner (1969, p. 785)
“. . . in such [hybridizing] populations, the raw material for evolution brought in by introgression must greatly exceed the new genes produced directly by mutation.”
Anderson (1949, p. 102)
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Terms
What is hybridization?
mating between different varieties or species
What is Introgression?
the movement of genes flow from one variety or species to another by repeated backcrossing
Hybrid Oaks
Introgression
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How frequent is hybridization?
Whitney et al. 2010
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What are the evolutionary consequences of hybridization?
• Merger of species (destructive role)
• Stable hybrid zones, balanced by selection against hybrid and gene flow into the zone (neutral role)
• The origin and transfer of adaptations (creative role)
• The reinforcement of reproductive barriers (creative role)
• The birth of new hybrid lineages (creative role)
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Merger of species (destructive role)
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Stable hybrid zones, balanced by selection against hybrid and gene flow into the zone (neutral role)
Steep clines indicate strong selection against hybrids (Whibley et al. 2006)
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The origin and transfer of adaptations (creative role)
Ray flowers and outcrossing regained through introgression in Senecio (Kim et al. 2008)
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The reinforcement of reproductive barriers (creative role)
Darwin - reproductive barriers byproduct of adaptation
Wallace - selection against hybrids favored development of reproductive barriers
New PhytologistVolume 197, Issue 4, pages 1095-1103, 16 JAN 2013 DOI: 10.1111/nph.12119http://onlinelibrary.wiley.com/doi/10.1111/nph.12119/full#nph12119-fig-0001
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The reinforcement of reproductive barriers (creative role)
Darwin - reproductive barriers byproduct of adaptation
Wallace - selection against hybrids favored development of reproductive barriers
New PhytologistVolume 197, Issue 4, pages 1095-1103, 16 JAN 2013 DOI: 10.1111/nph.12119http://onlinelibrary.wiley.com/doi/10.1111/nph.12119/full#nph12119-fig-0001
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Reinforcement in Phlox
Phlox drummondii and Phlox cuspidata are annual herbs native to east and central Texas.
Throughout most of their ranges, they have light-blue flowers characteristic of the Phlox clade.
Where these two species co-occur P. drummondii has dark-red flowers
Robin Hopkins
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Reinforcement in Sunflowers
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Dune Sunflowers from Great Sand Dunes NP, CO
200-230 m mega-dunes
Sand sheet: typical H. petiolaris5 km
Non-dune
Dune
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Reinforcement: Estimating reproductive barrier strength
Kate OstevikKate Ostevik
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Pollen Competition
Dune mothers are choosier than non-dune mothers
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Kinds of Hybrid Speciation
Homoploid Hybrid Speciation•Rare•Reproductive isolation difficult to achieve
2x 2x
2x
X
Polyploid Hybrid Speciation(Allopolyploidy)•Common•Reproductive isolation byproduct of genome doubling
2x 2x
4x
X
reproductive isolation
reproductive isolation
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Model for Homoploid Hybrid Speciation
Interspecific hybridization
Fertility / viability selection
Stabilization of fertile & viable hybrid segregates
Reproductive isolation facilitated by karyotypic divergence (recombinational model) hybrid trait causes ecological divergence hybrid trait causes assortative mating spatial isolation
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Recombinational Model
New homokaryotype confers partial isolation with parentals
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Frequency of Hybrid Speciation (open habitat available for hybrids)
•Fertility controlled by two underdominant loci
•Ecological performance controlled by two loci with additive effects
•Three different habitats and ecological selection occurred at seedling stage
Heredity (2000) 84, 441–4510.1 0.3 0.5 0.7 0.9
0.1
0.3
0.5
0.7
0.9
0
0.1
0.2
0.3
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0.5
prop
ortio
n of
rep
licat
es
ecological selection
fertilityof F1 hybrid
Hybrid speciation
Alex Buerkle
Hybrid Speciation Model (open habitat available for hybrids)
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Hybrid speciationFrequency of Hybrid Speciation (no open habitat available for hybrids)
0.9
0.7
0.5
0.3
0.1
0.1 0.3 0.5 0.7 0.9
F1 Hybrid Fertility
Ec
olo
gic
al S
ele
cti
on
Stable Hybrid Zone
Adaptive Introgression
Hybrid SpeciationBuerkle et al. (2003)
Alex Buerkle
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CONDITIONS FAVORING HOMOPLOID HYBRID SPECIATION
•Little spatial isolation between parental species, but substantial isolation of hybrid species.
•Open habitat for hybrid species.
•Strong ecological selection favoring hybrid lineage in new habitat.
•Weak postzygotic isolation between parental species, but strong isolation of hybrid species.
•Hybrid trait causes assortative mating (not modeled)
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EMPIRICAL EVIDENCE: SPATIAL ISOLATION(allopatric origin of oxford ragwort, Senecio squalidus (Abbott, 2000, 2002)
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H. deserticoladesert floor
H
x
Helianthus annuusmesic soils
H. petiolaris sandy soils
H. anomalussand dune
H. paradoxus salt marsh
P
P
H
H
EMPIRICAL EVIDENCE: ECOLOGICAL ISOLATION
Reciprocal transplant experiments indicate that synthetic and natural hybrids favored in hybrid habitats.
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TESTING THE IMPORTANCE OF ECOLOGY IN HYBRID SPECIATION
•Are the stabilized hybrid species ecologically divergent from their parents?
•Are the hybrid species favored in the hybrid habitats?
•Is there evidence of parallel hybrid speciation?
YES - for all but one species tested
YES - for Helianthus
YES - for Argyranthemum, Helianthus, Pinus
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HYBRID TRAITS CAUSE ASSORTATIVE MATING
Helianthus deserticola (flowering time)Rieseberg 1991
Heliconius heurippa (wing pattern)Maverez et al. 2006
Iris nelsonii (flower color)Arnold 1993
Penstemon clevelandii (flower color)Wolf et al. 1998
Xiphorus clemenciae (swordtail)Meyer et al. 2006
H. deserticoladesert floor
H
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EMPIRICAL EVIDENCE: KARYOTYPIC EVOLUTIONDistribution of hybrid and parental Helianthus species
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Parents
H. annuus x H. petiolaris: 8 translocations / 3 inversions
collinear with collinear with novel gene both parents one parent order
H. anomalus: 6 linkages 3 linkages 8 linkagesH. deserticola: 6 5 6H. paradoxus: 6 4 7
24 of 29 new chromosomal changes in hybrid species associated with linkage groups already rearranged in parents (P = 0.009)
Hybrids
Origins of rearrangements
1/3 sorting of pre-existing rearrangements2/3 novel rearrangements
Comparative Linkage Mapping - summary
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Conclusions - Homoploid Hybrid Speciation
•Generally good match between theory & empirical data
•spatial isolation of hybrid species predicted & 12/16 hybrid species parapatric or allopatric with parents
•Ecological divergence of hybrid species predicted & all but one hybrid species exhibit some degree of ecological divergence
•Karyotypic evolution predicted to contribute to reproductive independence of hybrid lineage & 6/14 hybrid species exhibit karyotypic divergence
•Hybrid traits frequently cause assortative mating
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How does hybridization create novel or extreme phenotypes?
•Natural populations of organisms often contain cryptic variation that cannot be predicted from the phenotype of the population.•Cryptic variation is released in crosses through the expression of extreme or “transgressive” phenotypes
Mechanism = complementary gene action
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Unanswered Questions
• Is hybridization an important extinction threat?
• Does introgression often contribute to the evolution of invasive species?
• Is reinforcement frequent? Have we been looking at the wrong traits?
• Have most species experienced one or more episodes of hybridization in their evolutionary history (i.e., is strict allopatric speciation rare)?