plant introductions & evolution: hybrid speciation and gene transfer
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The Environment InstituteWhere ideas grow
Richard Abbott
Plant Introductions & Evolution: Hybrid Speciation and Gene Transfer
Plant Introductions & Evolution:Hybrid Speciation and
Gene Transfer
Richard Abbott - St Andrews University, UK
“…invasive species cost the British economyapproximately £2bn a year…”
“…approximately 15% of the aliens within Europeare known to have some impact on the environment or economy - and this problem goes across all taxonomic groups."
“There are almost 11,000 alien species inEurope and the trend of new arrivals isshowing no signs of levelling out.”
“Invasive species are one of the greatest threats facing biodiversity today.”
BBC News 13 October 2008
Plant Introductions- points of entry
Level of Plant Invasion in Europe
(% aliens)
Chytry et al. (2009) Diversity & Distributions.15: 98-107
Plant Species in Britain & Ireland(after Preston et al. 2002)
Total Number of Species 2711
Native 1363 (50.28%)
Native/Alien* 44 (1.62%)
Naturalised Aliens* 1304 (48.10%)
* Species introduced after AD 1500
Invasives are models for studying
(i) Evolution in response to environmental change
(ii) Speciation and Gene transfer following hybridization with other species
EVOLUTIONARY CONSEQUENCESOF INVASIONS
Hybrid speciationSpecies BSpecies A
F1 hybrid
X
2n=10 2n=10
2n=10
2n=10
Homoploid hybrid species
2n=20
Chromosomedoubling
Allopolyploid species
Origin of a new homoploid hybridspecies - Senecio squalidus
Oxford Ragwort (Senecio squalidus)
Oxford Ragwort (Senecio squalidus) in the UK
Brought to Oxford Botanic Gardens from Mount Etna, Sicily, 1700
Escaped and spread around UK via railway network
The Senecio hybrid zone on Mount Etna, Sicily.
Senecio aethnensis
Senecio chrysanthemifolius
Hybrid zone
3000m
1000m
2000m
0m
Morphological differences between S. chrysanthemifolius and S. aethnensis
Leaf shape and texture Flower head size
Differences between S. squalidus and its Sicilian relatives
Intermediate morphology - distinct from wild hybrids on Mt. Etna
Urban habitats: railways, walls, motorways
Resolved:
2600 m
13 markers diagnostic of S. aethnensis
150 m
13 markers diagnostic of S. chrysanthemifolius
Surveyed RAPD variation for species diagnostic markers
Ancestry of S. squalidusplants in UK
Ancestry of plants alongaltitude gradient, Mt Etna
2600 m
150 m
James JK, Abbott RJ (2005)Evolution 59: 2533-2547
PCo 1 (40.6%)
PCo
2 (1
0.3%
)
S. chrysanthemifolius Hybrids S. aethnensis S. squalidus
Principal Coordinate Plot – RAPD Variation
James JK, Abbott RJ (2005)Evolution 59: 2533-2547
HYBRID ORIGINS OF NEW TAXA IN SENECIO
1792
S. aethnensis x S. chrysanthemifolius(2n=20) (2n=20)
S. squalidus(2n=20)
Species B
F1
Bc1
Bc2
Species A
Bc3
Bc4
25% B
50% B
12.5% B
6.25% B
3.12% B
Introgression - Gene Transfer
Movement of genes fromone species to anotherby recurrent backcrossingof hybrid to a parent
Introgression (Gene transfer):
• Many examples based on analyses of neutral markers
• Very few examples involve genes affecting fitness
• Few examples where hybridizing species differ in ploidy and/or mating system
Hybridizes with native Groundsel (S. vulgaris)
Oxford Ragwort (Senecio squalidus)
2n = 20
2n = 40
X
F1(2n = 30)
sterile
S. vulgaris S. squalidus
Self-compatible Self-incompatible
Effects of interspecific hybridization on gene expression
S. vulgarisS. x baxteriS. squalidus
0.1
1.0
10
Nor
mal
ised
Exp
ress
ion
(Log
Sca
le)
S. squalidus F1 S. vulgaris
‘Transcriptome shock’ in F1 hybrid. Normalized microarray expression data for 475 cDNA clones identified as showing significant differences in expression between F1 and one or both progenitors. Hegarty et al. (2005) Molecular Ecology 14: 2493-2510
(2n=20) (2n=40)
S. squalidusWaste-sites, Roadsides,
Walls
S. vulgarisAgricultural land
Waste-sites,Gardens
X
Hybrid evolution in Senecio
New Products
S. eboracensis (2n=40)Only in York
1979
S. cambrensis (2n=60)N.Wales & Edinburgh
1948
Radiate S. vulgaris (2n=40)Widespread in UK
1832
Radiate Groundsel (S. vulgaris var hibernicus )
Outcrossing rates of Non-Radiate (NN) and Radiate (RR) plants
Outcrossing rates
Non-Radiate Radiate
1 - 15% 6 - 36%
Finding genes that produce ray florets
• QTL analysis
• Microarray analysis
• Candidate gene approach √
Ray floret
Disc floret
CYCLOIDEA AS A CANDIDATE GENE
Snapdragon(Antirrhinum majus)
1 gene is largely responsible for change in flower shape: Cycloidea
Encodes a transcription factor
Luo et al. (1996) Nature 383: 794-9Luo et al. (1999) Cell 99: 367-76
• 6 cycloidea-like genes (RAY1-6) amplified in S. vulgaris
• 2 (RAY1 and RAY2) expressed in outer floret primordia
Semi-quantitative RT-PCR showing RAY1 and RAY2expression in young flower heads of RR and NN S. vulgaris
RAY Cleaved Amplified Polymorphic Sequences (CAPS)
Taq1 digest EcoR1 digest
• Linkage analysis: No recombinants for RAY1 or RAY2found among >700 F2 offspring of R/R x N/N cross
• Linkage confirmed by bulk segregant analysis of R/Rand N/N genotypes: in each case no recombinantsfound among 2,800 chromosomes
• RAY1 and RAY2 are tightly linked and associated with RAY
DNA sequences of RAY1 and RAY2 genes associated with flower head forms
N and N1 - Non-radiate sequencesR and R1 - Radiate sequences
Radiate S. vulgaris contains the R sequence found in S. squalidus
Confirms Radiate S. vulgaris received the R sequence from S. squalidus
Transformation studiesDo the RAY1 and RAY2 genes control development
of ray florets in S. vulgaris flower head?
• Developed transformation system for S. vulgarisusing Agrobacterium tumefaciens strain GV3101 and a Kanamycin resistance screen
• Took sequences of RAY1 and RAY 2 genes fromNon-radiate S. vulgaris (i.e. N alleles) andinserted them with 35S constitutive promoterinto Radiate S. vulgaris
RAY1
RAY2
Transformants
‡ Expression of RAY2 N allele in Radiate S. vulgaris produces tubular ray florets
Both genes, RAY1 and RAY2, affect ray floret development
* Expression of RAY1 N allele in Radiate S. vulgaris inhibits ray floret production
Kim et al. (2008) Science 322: 1116-1119
Control * ** ‡
Conclusions
• We have isolated two genes RAY1 and RAY2 that control the development of ray florets in the flower heads of Senecio vulgaris
• Radiate alleles of RAY1 and RAY2 are tightly linked and were introgressed from the diploid S. squalidus to generate the radiate variant of S. vulgaris
• Radiate S. vulgaris has a greater outcrossing rate than the non-radiate variant
• This difference in outcrossing rate between the two morphs of S. vulgaris will affect their relative fitness in polymorphic populations
0.01
RAY2b-ARAY2b-C
RAY2b-BRAY2a-R
RAY2a-NRAY2a-NaRAY2a-R2
RAY2a-R2aRAY2a-R1RAY2a-R1a
100
100
100
100
68
60
Clade 2(RAY2a)
Clade 1(RAY2b)
Maximum likelihood phylogeny ofRAY2 sequence variation
Chapman & Abbott (2009) New Phytologist
• Clades 1 and 2 represent two copies of RAY2 gene (RAY2a and RAY2b)
• Both copies are found in S. vulgaris (tetraploid). Diploids contain only RAY2a
Distribution of RAY2a-Rand R1 alleles in S. squalidus
Note: Only the ‘R’ allele has been introgressed into radiate S. vulgaris
Chapman & Abbott (2009) New Phytologist
Guildford
SouthamptonExmouth
Oxford
Birmingham
ManchesterLeeds
Edinburgh
Aberdeen
Key:R R1
0%
20%
40%
60%
80%
100%
NIC1(755m)
RAN1(755m)
MON1(1045m)
SAP4(1364m)
SAP2(1613m)
SAP0(1915m)
PRO2(2061m)
ET3(2287m)
R
R2
R1
Relative frequencies of RAY2a alleles in Senecio populations on Mount Etna
HYBRID ORIGINS OF NEW TAXA IN SENECIO
x S. vulgaris(2n=40)
S. baxteri(2n=30)
x S. vulgaris(2n=40)
S. vulgaris (Radiate)(2n=40)
1832
S. eboracensis(2n=40)
19791792
S. aethnensis x S. chrysanthemifolius(2n=20) (2n=20)
S. squalidus(2n=20)
S. cambrensis(2n=60)
1948
Acknowledgements:
St Andrews University: John Innes Centre:
Mark Chapman Rico CoenAmanda Gillies Pilar CubasJuliet James Min-Long Cui
Minsung Kim
Funded by NERC & BBSRC
Andy Lowe
The Environment InstituteWhere ideas grow
www.adelaide.edu.au/environmentPhone: +61 8 8303 5379
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