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Plant Introductions & Evolution: Hybrid Speciation and Gene Transfer


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Professor Richard Abbott presents a seminar entitled "Gene transfer and plant evolution: What we have learnt from Senecio." Richard has been at St Andrews University since October 1971 and currently holds a Chair in Plant Evolution. He is also an Editor of New Phytologist, and Associate Editor of Molecular Ecology, and Plant Ecology & Diversity. Richard’s main research focus is on the evolutionary consequences of hybridization in plants using the genus Senecio (Asteraceae) as a system for study.

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Plant Introductions & Evolution: Hybrid Speciation and Gene Transfer

  1. 1. The Environment Institute Where ideas grow Richard Abbott Plant Introductions & Evolution: Hybrid Speciation and Gene Transfer
  2. 2. Plant Introductions & Evolution: Hybrid Speciation and Gene Transfer Richard Abbott - St Andrews University, UK
  3. 3. “There are almost 11,000 alien species in Europe and the trend of new arrivals is showing no signs of levelling out.” “…approximately 15% of the aliens within Europe are known to have some impact on the environment or economy - and this problem goes across all taxonomic groups." “…invasive species cost the British economy approximately £2bn a year…” “Invasive species are one of the greatest threats facing biodiversity today.” BBC News 13 October 2008
  4. 4. Plant Introductions - points of entry
  5. 5. Level of Plant Invasion in Europe (% aliens) Chytry et al. (2009) Diversity & Distributions. 15: 98-107
  6. 6. 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
  7. 7. EVOLUTIONARY CONSEQUENCES OF INVASIONS Invasives are models for studying (i) Evolution in response to environmental change (ii) Speciation and Gene transfer following hybridization with other species
  8. 8. Hybrid speciation Species A Species B X 2n=10 2n=10 F1 hybrid 2n=10 Chromosome doubling 2n=10 2n=20 Homoploid hybrid species Allopolyploid species
  9. 9. Origin of a new homoploid hybrid species - Senecio squalidus Oxford Ragwort (Senecio squalidus)
  10. 10. 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
  11. 11. The Senecio hybrid zone on Mount Etna, Sicily. 3000m 2000m Senecio aethnensis 1000m Hybrid zone 0m Senecio chrysanthemifolius
  12. 12. Morphological differences between S. chrysanthemifolius and S. aethnensis  Leaf shape and texture  Flower head size
  13. 13. Differences between S. squalidus and its Sicilian relatives  Intermediate morphology - distinct from wild hybrids on Mt. Etna  Urban habitats: railways, walls, motorways
  14. 14. Surveyed RAPD variation for species diagnostic markers Resolved: 13 markers diagnostic of S. aethnensis 2600 m 13 markers diagnostic of S. chrysanthemifolius 150 m
  15. 15. Ancestry of plants along Ancestry of S. squalidus altitude gradient, Mt Etna plants in UK 2600 m 150 m James JK, Abbott RJ (2005) Evolution 59: 2533-2547
  16. 16. Principal Coordinate Plot – RAPD Variation PCo 2 (10.3%) PCo 1 (40.6%) S. chrysanthemifolius Hybrids S. aethnensis S. squalidus James JK, Abbott RJ (2005) Evolution 59: 2533-2547
  17. 17. HYBRID ORIGINS OF NEW TAXA IN SENECIO S. aethnensis x S. chrysanthemifolius (2n=20) (2n=20) S. squalidus (2n=20) 1792
  18. 18. Introgression - Gene Transfer Species B Species A 50% B F1 25% B Bc1 Movement of genes from 12.5% B Bc2 one species to another by recurrent backcrossing of hybrid to a parent 6.25% B Bc3 3.12% B Bc4
  19. 19. 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
  20. 20. Hybridizes with native Groundsel (S. vulgaris) Oxford Ragwort (Senecio squalidus)
  21. 21. S. vulgaris S. squalidus Self-compatible Self-incompatible X F1 (2n = 30) sterile 2n = 40 2n = 20
  22. 22. Effects of interspecific hybridization on gene expression 10 Normalised Expression (Log Scale) 1.0 0.1 S. squalidus S. x baxteri S. vulgaris 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
  23. 23. Hybrid evolution in Senecio S. squalidus S. vulgaris Waste-sites, Roadsides, X Agricultural land Waste-sites, Walls Gardens (2n=20) (2n=40) New Products S. cambrensis (2n=60) S. eboracensis (2n=40) Radiate S. vulgaris (2n=40) N.Wales & Edinburgh Only in York Widespread in UK 1948 1979 1832
  24. 24. Radiate Groundsel (S. vulgaris var hibernicus )
  25. 25. Outcrossing rates of Non-Radiate (NN) and Radiate (RR) plants Outcrossing rates Non-Radiate Radiate 1 - 15% 6 - 36%
  26. 26. Finding genes that produce ray florets • QTL analysis • Microarray analysis • Candidate gene approach √
  27. 27. Ray floret Disc floret
  28. 28. 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: 794-9 Luo et al. (1999) Cell 99: 367-76
  29. 29. • 6 cycloidea-like genes (RAY1-6) amplified in S. vulgaris • 2 (RAY1 and RAY2) expressed in outer floret primordia
  30. 30. Semi-quantitative RT-PCR showing RAY1 and RAY2 expression in young flower heads of RR and NN S. vulgaris
  31. 31. RAY Cleaved Amplified Polymorphic Sequences (CAPS) Taq1 digest EcoR1 digest • Linkage analysis: No recombinants for RAY1 or RAY2 found among >700 F2 offspring of R/R x N/N cross • Linkage confirmed by bulk segregant analysis of R/R and N/N genotypes: in each case no recombinants found among 2,800 chromosomes • RAY1 and RAY2 are tightly linked and associated with RAY
  32. 32. DNA sequences of RAY1 and RAY2 genes associated with flower head forms N and N1 - Non-radiate sequences R 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
  33. 33. Transformation studies Do the RAY1 and RAY2 genes control development of ray florets in S. vulgaris flower head? • Developed transformation system for S. vulgaris using Agrobacterium tumefaciens strain GV3101 and a Kanamycin resistance screen • Took sequences of RAY1 and RAY 2 genes from Non-radiate S. vulgaris (i.e. N alleles) and inserted them with 35S constitutive promoter into Radiate S. vulgaris RAY1 RAY2
  34. 34. Transformants Control * * * ‡ * Expression of RAY1 N allele in Radiate S. vulgaris inhibits ray floret production ‡ Expression of RAY2 N allele in Radiate S. vulgaris produces tubular ray florets Both genes, RAY1 and RAY2, affect ray floret development Kim et al. (2008) Science 322: 1116-1119
  35. 35. 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
  36. 36. Maximum likelihood phylogeny of RAY2 sequence variation RAY2b-A Clade 1 RAY2b-C (RAY2b) RAY2b-B RAY2a-R RAY2a-N 100 100 RAY2a-Na Clade 2 68 RAY2a-R2 (RAY2a) 100 RAY2a-R2a 60 RAY2a-R1 100 RAY2a-R1a 0.01 • 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 Chapman & Abbott (2009) New Phytologist
  37. 37. Distribution of RAY2a-R and R1 alleles in S. squalidus Aberdeen Edinburgh 100% 80% R 60% Leeds R2 40% Manchester R1 20% Key: Birmingham 0% R R1 NIC1 RAN1 MON1 SAP4 SAP2 SAP0 PRO2 ET3 Oxford (755m) (755m) (1045m) (1364m) (1613m) (1915m) (2061m) (2287m) Guildford Relative frequencies of RAY2a alleles in Southampton Senecio populations on Mount Etna Exmouth Chapman & Abbott (2009) New Phytologist Note: Only the ‘R’ allele has been introgressed into radiate S. vulgaris
  38. 38. HYBRID ORIGINS OF NEW TAXA IN SENECIO S. aethnensis x S. chrysanthemifolius (2n=20) (2n=20) S. squalidus S. vulgaris x S. eboracensis (2n=20) (2n=40) (2n=40) 1979 1792 S. baxteri x S. vulgaris (2n=30) (2n=40) S. vulgaris (Radiate) S. cambrensis (2n=40) (2n=60) 1832 1948
  39. 39. Acknowledgements: St Andrews University: John Innes Centre: Mark Chapman Rico Coen Amanda Gillies Pilar Cubas Juliet James Min-Long Cui Andy Lowe Minsung Kim Funded by NERC & BBSRC
  40. 40. The Environment Institute Where ideas grow Phone: +61 8 8303 5379