Plant evolution and speciation BY2204 EVOLUTION

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1 Plant evolution and speciation BY2204 EVOLUTION Trevor Hodkinson Plant Sciences Moderatorship Some evol. processes shared with other organisms (natural selection; allopatric speciation). Some more common in plants than animals (sympatric speciation, hybridisation & polyploidy). 1

2 Natural selection - explains close fit of organisms to environment (adaptation) nothing else can explain such a match Cacti are succulent E.g. 1 Trifolium repens (white clover) Some individuals produce cyanide for grazing protection however this makes them less frost tolerant 2

3 Release hydrogen cyanide (HCN) when tissue is broken (HCN not present in intact tissue -it is created upon breaking) Two components in a cell 1)Cyanogenic glucoside (one compartment) 2)Cyanogenic β-glucosidase (hydrolytic enzyme) in another compartment When the enzyme meets glucoside -HCN is produced Cyanide producing frost sensitive white clover Variation within a species maintained by natural selection (grazing and temperature; cyanide makes them less frost tolerant) 3

4 E.g. 2 Evolution of metal tolerance Strong selection pressure due to high toxicity of soils Metal tolerant populations (within a species) have evolved -ecotypes -speciation has been documented Copper-cobalt outcrops- High Cu, Co (Zambia & Congo) 4

5 European metal mines eg. Glendalough, Co. Wicklow Ecotypes of grasses Festuca, Agrostis, Deschampsia 5

6 Normal populations Population from copper mine Poor growth Good growth Pratt Transplanted populations of Silene vulgaris (campion) onto toxic copper soil. Showed evolution of tolerance. tolerant population on the mine and nontolerant off the mine Metal tolerant populations of sweet vernal grass (Anthoxanthum). Sharp boundary in tolerance caused by strong selection against non-tolerant individuals 6

7 Allopatric speciation in plants Allopatric speciation with geographic separation of populations Allopatric speciation -At time A populations can interbreed -At time C pre- and/or post-zygotic reproductive barriers have evolved 7

8 Allopatric speciaton Allopatric speciation facilitated by the founder effect Single locus -two alleles A, a A small subset founds a new colony the new gene pool is a different composition 8

9 Allopatric speciation on islands New gene pools (genetic drift/ founder effect) and new selection pressures combine to cause speciation PACIFIC ISLAND SPECIATION & DISPERSAL SPECIES RICH Hawaii zoochory (animals) Galapagos anemochory (wind) Pitcairns hydrochory (water) 9

10 Adaptive radiation numerous diversely adapted species evolving from a common ancestor Hawaii Volcanic islands of recent origin (<100mya) 90% of all plant species are endemic and c.4000km from the mainland 10

11 Adaptive radiation in Hawaii 28 species in three genera of silverswords have evolved from one colonisation event by Madia sativa from the mainland USA Argyoxiphium Dubautia Wilkesia Radiation of leaf form Hawaiian silverswords (tarweeds) Madia sativa Argyoxiphium Dubautia (ancestral silversword) 11

12 4/10/12 Sympatric speciation Speciation without geographic separation of populations Sympatric speciation is controversial in animals Eg, Cichlid fish (Cichlidae) (1,800 species). Text book example of adaptive radiation. 850 species in lake Malawi alone Lobochilotes labiatus at Nkondwe Island, Lake Tanganyika, Tanzania. Have they evolved by sympatric speciation? No geographic barriers. Sexual selection, niche specialisation, feeding adaptations? (Staufer et al. 2007, in Hodkinson & Parnell, Reconstructing The Tree of Life, Chapter 14.). Placidochromis milomo at Nkhomo Reef, Lake Malawi, Malawi. 12

13 Sympatric speciation not controversial in plants e.g. occurs via hybridisation Helianthus annuus sunflower H. petiolatus Hybrid H. annomalus Reproductively isolated from its two parental species Platanus allopatric and sympatric speciation P. occidentalis (north American) P. orientalis (European) 13

14 Platanus orientalis New square TCD Recent hybrid of P. occidentalis and P. orientalis Common ancestor c. 30mya 14

15 Formation of Platanus orientalis and P. occidentalis by allopatric speciation Formation of P. xhybrida (London plane) by hybridisation/sympatric speciation Polyploidy & polyploid speciation (a type of sympatric speciation) Polyploids contain more than two genomes (a diploid has two genomes eg. humans -one genome from each parent) 15

16 Happlopappus Ophioglossum Diploid 4 chromosomes (2 sets of 2) Extreme polyploid 1260 chromomsomes Polyploids Rare in animals Common in plants 95% of ferns are polyploid 80% of flowering plants (angiosperms) are polyploid 16

17 Polyploids Autopolyploids have more than 2 genetically identical genomes Allopolyploids combine genomes from more than one ancestral species Same species Different species 17

18 S. maritima S. alterniflora S. anglica Spartina anglica (saltmarsh grass) A vigorous allopolyploid hybrid of native S. maritima (Europe) and introduced S. alterniflora (USA) S. maritima 60 x S. alterniflora 62 Hybridisation S. townsendii 62 (Sterile hybrid; no seed) Doubling of chromosome number S. anglica 122 (Fertile polyploid) S. anglica is an allotetraploid Chromosome number provided one pair lost in S. anglica 18

19 Types of polyploid (allo -or autopolyploid), x=haploid chromosome set (ie one set). diploid 2x triploid 3x tetraploid 4x pentaploid 5x hexaploid 6x Sometimes these can interbreed and we get a polyploid complex Miscanthus biomass crop TCD botanists Miscanthus x giganteus Example of hybrid speciation in a polyploid complex Sterile but maintained by vegetative reproduction (clones) 19

20 M. giganteus metaphase chromosomes 2n=3x=57 x=19 (haploid number; one set) Triploid Allopolyploid or autopolyploid? Hodkinson et al. (2002) American Journal of Botany 89: 279 M. sacchariflorus (76) x M.sinensis (38) (tetraploid) (diploid) M. x giganteus (38+19=57) (allotriploid, sterile) Chromosome doubling would restore fertility 20

21 Conclusions Evolutionary processes in plants are dominated by: Natural selection Genetic drift and the founder effect Allopatric and sympatric speciation Hybridisation and polyploidy (which can also cause sympatric speciation). Walley et al Sowed 1000 seeds of bent grass (Agrostis) from normal bent grass on a copper contaminated soil 4 individuals were tolerant 21

22 Genetic drift and bottlenecks are powerful forces in evolution a bottleneck can cause a change in allele frequency by random processes Random genetic drift loss of white and increase in black 22