plant of the day! tamarix (salt cedar) 50-60 species family tamaricaceae native to dry areas of...
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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
Hybrid Speciation
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
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
Recombinational Model
New homokaryotype confers partial isolation with parentals
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
0.4
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)
Decay of genetic distance (Gst) between hybrid species and one of its parental species
Eco
logi
cal S
elec
tion
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
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)
EMPIRICAL EVIDENCE: SPATIAL ISOLATION(allopatric origin of oxford ragwort, Senecio squalidus (Abbott, 2000, 2002)
EMPIRICAL EVIDENCE: SPATIAL ISOLATION
Allopatric:Gila seminude (DeMarais et al., 1992)Paeonia sinjiangensis (Sang and Zhang, 1999)Senecio squalidus (Abbott et al., 2000)
Parapatric:Argyranthemum sundingii (Brochmann, 1987) Helianthus anomalus (Rieseberg, 1991)Helianthus deserticola (Rieseberg, 1991)Helianthus paradoxus (Rieseberg et al., 1990)Invasive sculpin (Nolte et al. 2005)Lycaeides butterflies (Gompert et al. 2006)Iris nelsonii (Arnold, 1993)Pinus densata (Song et al., 2003)Penstemon clevelandii (Wolfe et al., 1998)
Sympatric:Daphnia mendotae (Taylor et al., 1996)Lonicera fly (Schwartz et al., 2005)Pungu maclareni (Schliewen & Klee, 2004)Heliconius heurippa (Mavarez et al. 2006)
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.
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
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
EMPIRICAL EVIDENCE: KARYOTYPIC EVOLUTIONDistribution of hybrid and parental Helianthus species
H. deserticola
H. paradoxus
H. annuus
H. anomalus
H. petiolaris
1.6%
18.5%
2.2%
1.6%
1.5%
2.1%
0.18%
0.78%
0.96%
13.8%
Line thickness proportional to % seed set
Crossing relationships among hybrid and parental Helianthus species
annuus
deserticola
paradoxus
petiolaris
anomalus
LG2
LG2
LG2A
LG2B
LG2-8 LG2-8
Comparative mappingof linkage group 2In Helianthus
1152229
1282
925279
10657081011
423
1035
333
1152
333
332
1065249
1035
120
332229279
1011
984
1065
249
1028
377120
996
996
229
1147984
7083771035423E-12
103
16
328
1065
229
11471028925
671671423E-12103
16
328
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
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
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
H. deserticoladesert floor
H
x
Helianthus annuusmesic soils
H. petiolarissandy soils
H. anomalussand dune
H. paradoxus salt marsh
P
P
H
H
Field Experiments:
Hybrid species favored in hybrid habitats
Empirical Evidence: Ecological Divergence
Most extreme traits could be re-created in experimental hybrids
1% of experimental hybrids classified as H. anomalus
6.5% of experimental hybrids classified as H. deserticola
0.2% of experimental hybrids classified as H. paradoxus
Re-creating the birth of a new species in the greenhouse
Conclusions - Transgressive Segregation
• Crosses between genetically divergent populations frequently release cryptic phenotypic variation, referred to as transgressive variation
• Transgressive segregation may provide a means for large or difficult evolutionary transitions requiring simultaneous changes at multiple genes or traits.
• This possibility is illustrated by the ecological divergence of several sunflower species.
• Hybridization’s role in adaptation is probably under-estimated.