How Meiosis has driven Genome Variation in Plants is all about.

1. Heartily Welcome
You all

2. Liverwort
HornwortFern
cones
Meiosis-Driven Genome Variation in
Plants

3. Contents
• Introduction
• Mitosis & Meiosis
• How meiosis evolved?
• Insight of Genes for meiotic events in models
• Meiosis-driven genome variation
• Conclusion
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4. • Meiotic cell division is a complex and dynamic process with
molecular and cellular events:
 DNA and chromosome replication,
 Synapsis and recombination,
 Chromosome segregation, and
 Cytokinesis.
• Involved in gametogenesis
Word Meiosis from Greek root meaning “ to diminish” 1st described by
Van Beneden in 1883 in the Ascaris
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Meiosis is a conserved cytological process and serves as a physical foundation for Mendelian genetics

5. • Meiosis includes two successive divisions of
the nucleus with one round of DNA
replication and leads to the formation of
gametes with half of the chromosomes of the
mother cell during sexual reproduction.
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7. Independent assortment
Anaphase I
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9. In humans
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10. • Diploid somatic cells of Fungi, Animals & Plants chromosomal crossing over
known as ‘Mitotic crossing over’
Significant contrasts between meiotic and mitotic recombination:
1. Mitotic recombination takes at very much lower frequency than in meiosis.
2. In mitotic cells Crossing over between sister chromatids is fairly frequent ,
meiosis is structured to promote crossing over between non-sister chromatids.
3. As in yeast cells, mitotic recombination is mediated efficiently by either of two
recA homolog's, rad51 and Dmc1, while meiotic exchange between homolog's
requires Dmc1 specifically
Neale and Keeney , 2006
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11. • Earliest eukaryotic species were single-cell haploid forms, having single set of
chromosomes, and propagated by mitosis.
• Simplest contemporary eukaryotes: Protists and fungi, exhibit the mitotic propagation
of both haploid and diploid states, diploidy is almost certainly a derived state.
• Very first diploid cells could have first arisen either by cell fusion or by
endomitosis.
• Hurst and Nurse (1991): First diploids probably arose via rare endomitotic errors
rather than by cell fusion.
• Either route to early diploid states is possible since non-sexual cell and nuclear
fusions can occur independently of sex (‘‘parasexuality’’).
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12. Liverwort
Hornwort
Club mosses
Cone bearing plants
Green Algae
Ancestor
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13. • All the key molecules employed in eukaryotic mitosis are of
Prokaryotic homolog's
 Actins, required for daughter cell separation in eukaryotes.
 Tubulins, required in eukaryotes for the mitotic spindle and movement
of chromosomes; and
• SMC molecules must for chromosome condensation and
sisterchromatid cohesion, members of the so-called Structural
maintenance of chromosomes (SMC) family.
Hirano 2005 and Erickson 2007
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14. • Tubulin family, FtsZ genes, which were first discovered in
Escherichia coli later found in many prokaryotes.
• Product of Ftsz involved in initiation of septum formation.
• minB locus regulate site of septum formation
• MinC/MinD/MinE determines location of septum formation
• The homologs of the SMC proteins are found throughout the
Eubacterial and Archaebacterial kingdoms.
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15.  For genetic recombination recA family of proteins molecules, in
prokaryotes and eukaryotes.
Aboussekhra et al., 1992;
Shinohara et al., 1992;
 Recombinational capacity is found throughout the prokaryotes
and considerably predate eukaryotes so meiosis arisen from pro..
Levin 1988;
Cavalier-Smith 2002;
Marcon and Moens 2005.
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16. • Suppression of Kinetochore splitting in MI was key innovation
in meiosis.
Cavalier-Smith,2002
• Selection pressure on homologous pairing and high levels of intergenic
recombination are prime force for origin of meiosis.
Fisher1930; Muller 1932; Maynard Smith 1978;Crow 1988
;
• Efficient DNA ds breaks and repair system.
Bernstein 1977,1988
Argueso et al.,2008
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17. • Genetic, cytological, and immunochemical characterization done
in :
 Saccharomyces cerevisiae,
 Arabidopsis thaliana, &
 Drosophila melanogaster etc.
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• SPO11 gene, required for meiotic recombination by catalyzing DNA ds
Breaks

20.  Mutation of Spoll-1
(i) Drastic reduction in meiotic recombination and ascospore viability & no
significant effect on vegetative growth or mitotic exchange.
(ii) The low spore survival results from aneuploidy.
(iii) It is epistatic to other recombination (Rec) mutants, such as rad51, rad52,
and rad57, that do not produce viable spores in the presence of spol3-1
alone but do so when spoll-1 is also present
(iv) It does not block either pre-meiotic DNA synthesis or synaptonemal
complex formation
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21. Cont…
• Unlike the RAD genes, which can affect both meiotic and
mitotic recombination as well as DNA repair, the SPO11
function appears to be meiosis specific and unrelated to repair
• RNA analyses demonstrate that wild-type SPO11 gene acts relatively at
early in the meiotic recombination process or soon after the time of
chromosome pairing.
CATHERINE L. ATCHESON et al.,1987
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22. Arabidopsis thaliana 2n=2x=10
• S.cerviceae that initiates meiotic recombination.
• The Model plant A.thaliana possesses at least 3 Spo11 homologues.
• Meiotic progression altered by EMS by which AtSpo11-1 gene was disrupted.
• Leading to non-synapsed chromosome in Prophase –I and non-functional
gametes due to random chromosome distribution in meiosis –I
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23. 6 weeks Wild type
6 weeks mutant type
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24. Wild Type (97%)
Pollen fertility
Mutant Type spo11-1
(11%) Pollen fertility
Differential Interference Contrast (DIC) Microscopy observation
Mature embryo sacs
were observed:
Only 3% female
gametophyte were
differentiated Intermediate stages
were seen:
 Incomplete
differentiation
lead to 2-4 nuclei
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25. Irregular
Tetrads and
Polyads
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31. • Structure Maintenance of Chromosomes (SMC)
• MRE11
• RAD51 homologs
• BRCA2
• MSH4
• MER3
• ZIP1
• Genes that encode components unique to plants, such as POOR
HOMOLOGOUS SYNAPSIS 1 and AMEIOTIC 1, have been cloned.
Hamant O, Ma H, Cande WZ 2006
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32. • Knowledge of meiosis-driven genome variations
Genome evolution and
Genetic variability in plants and evolution in plants.
 Facilitates plant genome research
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Meiosis-driven genome variation

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35. • Asynapsis: Homologous chromosomes may fail to synapse
• Desynapsis: Homologous chromosomes synapse with each other normally, but
the association cannot be held until Anaphase I and they separate
prematurely.
• Both lead to univalents , they either get lost or are randomly transmitted to
daughter cells, resulting in chromosomally unbalanced gametes and
eventually aneuploids in the offspring
 Misdivision, such as transverse division, lead to Telocentric/ Acrocentric, and
Acentric chromosomes or Isochromosomes in wheat.
Sears, E.R.1952
Asynapsis and desynapsis induced aneuploidy in
plants
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36. • For accurate segregation Proper Pairing, Synapsis, and
Recombination of homologous chromosomes at meiosis I are
prerequisites.
• Synapsed Bivalents held together until Anaphase I by the
chiasmata resulting from crossing-over and cohesin
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37. • Cohesin Rec8 is required for reductional chromosome segregation
at meiosis.
Watanabe, Y., Nurse, P. 1999
The rice Os-Rad21-4, an orthologue of yeast Rec8 protein, is required
for efficient meiosis.
Zhang, L.2006
The Arabidopsis SYN1 cohesion protein is required for sister chromatid arm
cohesion and homologous chromosome pairing.
Cai, X et al.,2003
AtREC8 and AtSCC are essential to the monopolar orientation of the
Kinetochores during meiosis
Chelysheva, L. et al.,2005
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38. Genes: Asynapsis and Desynapsis
CROP ASYNAPSIS DESYNAPSIS ASSOCIATED REFERENCES
Soybean st2, st3, st8 st4 ast6 and
*st7 and st5
Male & female
sterility
Palmer, R.C., Kilen, R.C.,1987
Palmer, R.G., Horner,
H.T.,2000
Kato, K.K.,2003
Maize phs1 dy and dsy1 Telomere
misplacement
Bass, H.W.,2003
Arabidopsis
thaliana
asy1 dsy1 Male fertility Chaudhury, A.M.,et al.,1994
Single recessive genes & Duplicated recessive factors*
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39. • In many plant spp. higher recombination frequency at gene-rich
regions than repetitive DNA sequences.
 At a number of gene loci in plants:
• In Rice (Oryza sativa) wx locus and
• In Maize (Zea mays) A1 locus and bronze locus
 In Arabidopsis at the csr1 locus intragenic recombination rate was
found almost same as the genome average
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40. • Meiotic recombination can be :
 Intergenic recombination
 Intragenic recombination
Generators of genetic diversity : Plays important role
in gene evolution
Fisher 1930; Muller 1932;
Maynard Smith 1978; Crow 1988
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41. Intragenic Recombination
• First described in Drosophila melanogaster in 1940,
• In 1955, fungus Neurospora crassa.
• Results in disruption of a functional gene or generation of new
alleles at the gene locus.
• Reciprocally or via a non-reciprocal process called „gene conversion‟
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42. Intragenic recombination : Evolution of disease
resistance genes
Maize-Rust
Flax-Rust
Arabidopsis-Downy Mildew
Tomato-Leaf Mold
Wheat –Leaf Rust
Citrus- Tristeza
Maize rust resistance gene complex Rp1
 Recombination within Rp1 gene complex, mostly within the coding
regions, led to new rust resistance specificities.
Richter, T.E 1995, Sun, Q.,2001
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43. Flax (Linum usitatissimum)
>30 rust resistance genes, designated K, L, M, N, and P, have
been identified
• Extensive analysis of the L and M loci suggests :
 The L locus contains a single gene with multiple alleles
Anderson, P.A 1997.
Bent, A. F 1996,
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44.  Sequence and genetic analysis of 13 natural and in vitro engineered
recombinant alleles at the L locus by intragenic recombination resulted in
mutant alleles that confer novel resistance specificities.
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Islam, M.R 1991,
Mayo, G.M.E 1980,

45. Arabidopsis downy mildew resistance gene RPP8
McDowell et al., 1998
Tomato (Lycopersicon) leaf mold resistance gene Cf-5.
Dixon, M.S.et al.,1988
Wheat (Triticum) leaf rust resistance gene Lr21
Citrus Citrus Tristeza virus resistance gene Ctv
Yang, Z.et al.,2003
 Revealed occurrence of the recombination within the coding
region of disease resistance genes in plants.
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46. Intragenic recombination: Generates allelic diversity
• Sorghum (Sorghum vulgare) dwarf mutant dw3 mutant allele was
generated by a direct duplication of an 882 bp interval in Exon 5.
• Instability by loss of the duplication due to unequal crossing-over
between the duplicated regions.
• The intragenic unequal crossing-over event reverted the mutant
allele dw3 to the wild type allele Dw3.
Multani, D. S. et al.,2003
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47. Results in
• Deletions,
• Duplications,
• Inversions, and
• Translocations
 Disrupt the function of a gene,
 Generate new alleles at a gene
locus, and
 Cause variations in the genome
structure and size by counter
acting genome expansion
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Impact

48. unequal crossing-over
• Unequal alignment of homologous chromosomal regions
and then unequal crossing-over due to sequence
similarity among members of a gene family or other
tandem repeats.
• Unequal crossing-over was first documented at the
Bar locus in Drosophila melanogaster in the 1920s.
Sturtevant, A.H.,1925
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49. Unequal crossovers : Evolution of disease resistance
gene families
• In Tomato Cf-4 and Cf-9 reside within a large gene family on the short arm
of chromosome 1.
• Arabidopsis downy mildew resistance locus RPP5.
Parker, J.E et al.,1997
• In Soybean (Glycine max)Intragenic unequal crossing over in nucleotide-
binding-site/leucine-rich-repeat-like sequence lead to deletion of the
phytophthora resistance gene Rps4
Sandhu, D.et al.,2004
• Rsv1 conditioning resistance to soybean mosaic virus.
Hayes, A.J. 2004
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50. Unequal recombination: Mechanisms for
genome contraction
• >50% of nuclear DNA is Repetitive DNA sequences, Including
transposable elements and variable number of tandem repeats (VNTRs),
comprise a large portion of the genome in plant species.
Flavell, R.B.,1974
• Unequal crossing-over occurred within LTR (long terminal repeat)-
containing retrotransposons due to the LTRs at both ends of the element.
removes the internal domain of the retroelement and leads to solo LTRs
Bennetzen, J.L.,1997
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51. • In the rice genome ratio of solo LTRs to intact LTR retroelements varies
with the retrotransposon families.
Vitte, C., Panaud, O. 2003
• In Rice Oryza sativa L. Formation of solo-LTRs through unequal
homologous recombination counterbalances amplifications of LTR
retrotransposons.
• Over 190 Mb of LTR retrotransposon sequences have been removed from
the Rice genome in the last eight million years
Ma J., et al.,2004
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52. • In Maize genome retroelements found to be intact, a mechanism
to counterbalance genome expansion
SanMiguel, P., et al.,1996
• Rapid genome divergence at orthologous low molecular weight
Glutenin loci of the A genome of wheat by sequence deletion in
the LTR retrotransposons due to unequal recombination
Wicker, T., et al., 2003
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53. MEIOTIC CHROMOSOME SEGREGATION
• Orientation of sister Kinetochores
• At both meiotic divisions govern chromosome
segregation and transmission throughout meiosis.
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54. • Monopolar attachment of microtubules to the sister
Kinetochores at metaphase I is prerequisite to ensure accurate
segregation of homologous chromosomes at meiosis I
Watanabe, Y.,2004
• Bipolar attachment of the microtubules from opposite poles to
the sister Kinetochores ensures equational division of sister
chromatids at meiosis II.
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55. MEIOTIC RESTITUTION & POLYPLOIDY
• Meiotic restitution significant driving force behind speciation in plants.
• Normal meiosis includes
The first division is reductional and
The second is equational.
• Failure of the first or second division leads to the formation of restitution
nuclei with unreduced chromosomes.
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Ramanna, M.S., Jacobsen, E. 2003

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58. • Wheat msg gene affect megasporogenesis leading to
unreduced egg.
Joppa, L.R., 1987
• Barley triploid inducer (tri) gene.
Finch, R.A. M.D. Bennett.1979
• Maize elongate (el) gene induces chromosome doubling.
Rhoades, M.M., Dempsey, E. 1966
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59. • In Potato
1. parallel spindles (ps),
2. premature cytokinesis-1 (pc-1) and
3. premature cytokinesis-2 (pc-2)
4. desynaptic (ds-1) and
5. synaptic mutants (sy-1,sy-2, sy-3, and sy-4)
Jongedijk, E., Ramanna, M.S.1988
6. omission of the second division (os)
Werner, J E., Peloquin, S.J.1990
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60. • In diploid Alfalfa (Medicago sativa):
 Two single recessive genes,
1. rp (restitution pollen) and
2. jp (jumbo pollen)
 conditioning 2n pollen formation by disorientation of
spindles at metaphase II and failure of cytokinesis at the
second division
McCoy, T.J 1982 & 1983
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61. • In Angiosperms, >30% of species were estimated to be of polyploid
origin.
Stebbins, G.L. 1950 & 1971
• Polyploidization is considered one of the major processes involved in
plant genome expansion
Bennetzen, J.L. 1997& 2002
• Functioning of the unreduced gametes produced through meiotic
restitution lead to polyploidy.
Ramanna, M.S.,2003
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POLYPLOIDY

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68. Lilly (Lillium regale) 2n=2x=24

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