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Polyploidy & Evolution

Presented by: Anirudh R. Acharya,M.Sc – II semester,

Department of Zoology, University of Mysore.

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Polyploidy Origin of Polyploidy Causes Types Role in plant evolution

◦ Establishment◦ Variation ◦ Mechanism

In animals Conclusion Reference

Contents:

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Mutation.

Ploidy or Ploidy level – number of complete sets of chromosome in an organism.

Polyploidy - having more than two sets of chromosomes.

What is Polyploidy??

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Winkler (1916) introduced the term polyploidy.

Winge (1917): Polyploids developed from gametes that were so unlike that chromosomes could not pair and zygote would perish. He speculated that in such zygotes chromosomes might split longitudinally, each making a pair. This allowed the zygotes to develop the hybrid carrying 2x parental number of chromosomes.

Karpechencko (1927): From the cross between Raphanus and Brassica shown that meiotic failure resulted unreduced gametes.

Buxton and Darlington (1931): From the study on Digitalis concluded that “the doubling of the chromosome number is directly determined by the omission of reduction.”

Most cases the chromosome increase is due to the formation of unreduced gametes. Some cases chromosome doubling in somatic tissues were reported. For example: Primula kewensis and Nicotiana digluta.

Origin of Polyploidy:

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What are the causes of Polyploidy??

Polyploidy

Union of unreduced gametes

Meiosis disturbance Endomitosis

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Diploid spores (2n)

Diploid ova of same

plant (2n)

Tetraploid Zygote (4n)

Tetraploid plant (4n)

Autopolyploid: The chromosomes of one genome within an organism or species are simply duplicated, the resulting polyploid is an autopolyploid.

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Diploid spores (2n)

Diploid ova of another plant species (2n)

Tetraploid Zygote (4n)

Tetraploid plant

species (4n)

Allopolyploidy:

The genome duplication occurs during a cross of

two different species, the resulting organism is

referred to as an allopolyploid.

Allopolyploids may have varying degrees

of heterozygosity depending on the divergence of the parental genomes.

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Arisen from parents with partially divergent chromosome arrangements such that some chromosomal regions are homologous and others are homeologous.

Segmental allopolyploid:

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Modes of Speciation

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For eudicot, which comprises 75% of the angisoperms, the basal ancestral chromosome number (x) is probably = 8.

Most, if not all, angiosperms have gone through one or more cycle of chromosome doubling.

After polyploidization, diversification continued with subsequent cycles of chromosomal doubling.

Therefore, polypolids are a great source of new genetic variation and evolution of novel genotypes/germplasms.

Role in plant evolution:

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PlantProbable ancestral

haploid number

Chromosomenumber Ploidy level

domestic oat 7 42 6n

peanut 10 40 4n

sugar cane 10 80 8n

banana 11 22, 33 2n, 3n

white potato 12 48 4n

tobacco 12 48 4n

cotton 13 52 4n

apple 17 34, 51 2n, 3n

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Evolutionary history of diploid and allotetraploid cotton (Gossypium)

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Environmental factors that affect 2n gamete formation include sudden changes in temperature (heat or cold treatment), dehydration, x-rays, uv light, infections, etc., and can induce chromosome doubling (Sax, 1937)

 Other factors - Superior vegetative (clonal) growth, perennial life history, niche separation, assortative mating, and other fitness differences.

Once 2n gamete production exceeds a certain threshhold, the tetraploids are able to replace diploids.

Will it get affected by external factors??

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Several factors play role in the establishment and survival of polyploids. They are:

Gametic screen: 2n gametes must be produced.

Sterility screen: Seed must be produced, though seed set is often low.

Vigor screen: Plants must be able to compete with neighbors, including ancestral species.

What are the factors for persistence??

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Polyploids can acquire variation both through ◦ Mechanisms of population genetics◦ Chromosomal rearrangements◦ Epigenetic phenomenon

Stebbins (1971) considered that the primary mechanism by which polyploids gain variation was via hybridization with related diploid species.

Allopolyploids were considered to be the major force in evolution, whereas autopolyploids were maladaptive and extremely rare in the nature.

How new variations occur??

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Potential causes of novel variation in polyploids. The merger of chromosomes from two diploid genomes (red and blue) into a tetraploid genome can cause

1. Increased variation of dosage-regulated gene effects and expression (magnitudes of allelic effects and expression shown by size of blocks for three loci)

2. Altered regulatory interactions (trans-acting regulatory factors shown as dimeric proteins, with heterodimers not functioning properly)

3. Genetic changes affecting gene expression (e.g., insertions, deletions, translocations and gene conversions)

4. And epigenetic changes (repression or derepression of gene expression caused by genome interaction of chromatin modeling factors, which could also trigger movement of transposable elements).

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Hybrid vigor - The hybrid offspring of two inbred genetically different varieties produces higher yields than either one of the two parental lines,

Arabidopsis suecica (Comai et al., 2000; Madlung et al., 2002) - seed lethality.

“Genome shock” that may result from the union of different genomes (McClintock, 1985).

Are they all advantageous??

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Arabidopsis (Mittelsten Scheid et al. 1996; Lee and Chen 2001; Madlung et al. 2002; Chen et al. 2003; Mittelsten Scheid et al. 2003)

Brassica (Song et al. 1995; Chen and Pikaard 1997; Pires et al. 2003)

Triticum (Ozkan et al. 2001; Shaked et al. 2001; Kashkush et al. 2003)

Nicotiana (Kenton et al. 1993; Lim et al. 2000; Mette et al. 2002)

Rapid genomic and epigenetic changes:

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Rarer because sex chromosome acts as barrier.

XXY, XYY – Intersex

Causes genetic imbalance. Klinefelter syndrome, 47, XXY, or XXY syndrome Down's syndrome or trisomy 21)

WGD – Whole genome duplication

Polyploidy in Animals:

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Parthenogenesis

Snout beetle – Curculionidae of 18 parthenogenetic species one was diploid, rest were all 3n, 4n, 5n (Suomalainen, 1951)

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Heteronotia binoei – Australian Gecko

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Daphnia – Cyclic parthenogeneis

Allopolyploidy

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50 species – Whiptail lizards 1/3 of total parthenogenetic C. tesselatus Parker and Selander, 1976

Cnemidophorus

C. tesselatus

C. tigris

C. septemvittatus

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Poecilia formosa Cobitis taenia

Octodontidae

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African clawed frog Silurana –

10, tetraploid (4n=36) 5, octaploid (8n=72) 2, dodecaploid (12n=108)

Natural hydridization

Xenopus & Silurana:

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Two whole genome duplication took place, in vertebrate lineage before gnathostome and cyclostome split (Kuraku, 2009)

There was a third round of genome duplication specific to teleost fish.

This gave animals better chances of survival and development.

Conclusion:

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As a consequence, geneticists described evolution simply as a change in gene frequencies in populations, totally ignoring the fact that evolution consists of the two simultaneous but quite separate phenomena of adaptation and diversification.

-Ernst W. Mayr

Questions & Comments

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References: Michael L. Arnold, 2006, Evolution through genetic exchange, 1st edition,

Oxford University press, New York.

F. Harvey Pough, R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, K.D. Wells, 2004, Herpetology, 3rd edition, Pearson Prentice Hall publishers, USA.

Richard Robinson, 2003, Genetics, Voulme III, 1st edition, Macmillan reference USA, Thomson Gale publishers.

Michael Hutchins,2011, Grzimek’s Animal Life Encylopedia – Evolution, Cengage learning, USA.

Nature review, Polyploid evolution in angiosperms.

Thomas G. Ranney, Polyploidy: From evolution to new plant development,

Mountain Horticultural Crop Extension Centre, North Carolina State University, USA.

Polyploid.org

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Thank you..