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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

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