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Population and Chromosome Genetics - Leitch > Reading - Genetic regulation of meiosis in polyploid species > Flashcards

Reading - Genetic regulation of meiosis in polyploid species Flashcards

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Biology 101 flashcards
1
Q

Meiosis is a fundamental process for all sexual species with direct relevance to natural selection

A
  • leads to the formation of gametes
  • contributes to genome stability
  • generates genetic diversity
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2
Q

Meiosis relies on the interrelated events of

A
  • homologous chromosome recognition
  • intimate association
  • synapsis
  • recombination
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3
Q

Correct segregation of chromosomes requires

A

the formation of stable bivalents at metaphase I

  • result from physical connection between homolgoues (chiasmata)
    • results from sister chromatid cohesion and meiotic crossovers
      • between homologous chromosomes,

need for proper disjunction at the first division

NEED AT LEAST ONE CROSSOVER BETWEEN HOMOLOGOUS CHROMOSOMES

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4
Q

Cytological diploidization

A

the process by which meiosis in polyploids leads to

chromosomally and genetically balanced gametes

critical for polyploid speciation

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5
Q

Autopolyploid

A

having more than 2 sets of chromosomese, all derived from the same species

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6
Q

Allopolyploid

A

having 2 or more complete sets of chromosomes derived from different species

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7
Q

Meiosis in autopolypoloids

A
  • more than 2 copies of each chromosome which have the same chance of recombining
  • form multivalents at MI
  • chromosome missegregation at anaphase I
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8
Q

Meiosis in autopolypoloids

Santos

A

established autotetraploids lines of A. thaliana showed fewer multivalents than newly synthesized ones

→ partial cyotolgoical diploidization over 13 generations

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9
Q

Meiosis in allopolypoloids

A
  • requires a nonrandom assortment of chromosomes into pairs
  • crossovers exclusively between homologues despite early promiscuity between homeologuges
  • diploid-like meiotic behavior results from divergence between homeologous chromosomes
    • involves the rearrangement of large chromosome fragments
    • or from the activity of Pairing homeologous (Ph) genes

in allopolyploids the homologous chromosomes within each parental sub-genome should pair faithfully during meiosis = disomic inheritance

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10
Q

Wheat

A
  • T. aestivum
  • allopolyploid (allohexaploid)
  • T. turgidum x Aegilops tauschii
  • behave as diploids at meiosis
  • strictly bivalent forming have disomic inheritance (one set of chromosomes from each parent)
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11
Q

The diploid-like behavior of wheat is ensured by

A

a multigene system whic includes a locus with a major dominant effect

  • Ph1 on the long arm of chromosome 5B
  • contains a cluster of CDK-related genes
  • 7 CDK genes on 5B, 5 on 5A, 2 on 5D
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12
Q

Ph1 may act as a master coordinator

A
  • premeiotic chromosome arrangement
  • chromatin organization
  • chromosome synapsis
  • recombination
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13
Q

Transcription of CDK-like genes

A
  • most from 5A and 5D aren’t transcribed when Ph1 is present
  • expressed when Ph1 is absent

→ Ph1 regulates the overall activity of these CDK-like genes

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14
Q

TaASY1

(T. aestivum asynapsis)

A
  • encodes a protein associated with the synaptonemal complex
  • absence of Ph1 → 20-fold increased transcription of TaASY1
  • Ph1 could also control the transcription of meiotic genes that contribute to the fideilty of synpasis or crossover formation
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15
Q

Ph1 vs. TaASY1

A
  • Ph1 required to suppress interactions between homologous chromosomes
  • TaASY1 for synapsis and crossover
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16
Q

Ph2

A
  • also shown to suprress crossovers between homeologous chromosomes
  • but when mutated to ph2 there were very few homeologous bivalents at MI (but active Ph1 could’ve masked)
  • combined with the ph1b mutation, ph2b mutation was shown to induce a slight increase in crossovers between homeologues
17
Q

Brassica napus

A
  • allotetraploid
  • Brassica rapa x Brassica oleracea
  • complete diploid-like meiotic behavior, only bivalents
  • haploids show different meiotic behavior at MI with 2 phenotypes
    • 8.1 - 13.8 univalents
    • 2.35 - 5.75 univalents
  • these 2 meiotic phenotypes were shown to be inherited in a seemingly Mendelian fashion, compatible with the sesgregation of a major gene called Pairing regulator in B-napus (PrBn)
18
Q

Nicolas et al.

A
  • surveyed chromosomal rearrangements in 2 progenies of B. napus haploids
  • the 2 meiotic behaviors observed at MI reflect differences in recombinaiton between homeologous chromosomes
  • PrBn shown to have an effect on the frequency but not the distribution of crossovers between homeologous chromosomes during meiosis in B. napus haploids
19
Q

What occurs at the onset of polyploid formation?

A
  • meiotic aberrations in most newly formed autopolyploid and allopolyploid plants
    • negative consequences for fertility and early demographic success
  • T. mirus and T. miscellus (allotetraploids)
    • unbalanced genomic composition
    • some infertile

→ newly formed polyploids can persist for c. 40 generations, sometimes despite early bottlenecks of reduced fertility

  • window of opportunity for selection to promote increased cytological diploidization by favoring the most fertile individuals among which those with a most regular meiosis are likely to be overrepresented
  • fertility selection on neopolyploids results in increased bivalent formation
    • selection of Ph-like genes
20
Q

Where do Ph-like genes come from?

A
  • idiosyncratic structure = Ph1 locus of wheat is likely to be the result of a chromosomal rearrangement that occurred at the onset of tetraploid wheat formation
  • molecular and cell biological characterization of Ph1 - suggests involvement in proper chromosome pairing and recombination in vertebrates and to control the expression of Homolog pairing Hop1
  • effect of Ph1 on synapsis is a conserved mechanism that’s been adjusted to polylploidy
21
Q

Recurrent polyploid

A

rule, not exception

22
Q

All polyploid wheat species show

A

Ph1 activity and a conserved structure at the Ph1 locus

23
Q

Martinez

A
  • analyzed meiotic prophase and MI in haploid from different bread wheat cultivars
  • found significant dfiferences in synapsis and recombination between homeologues
24
Q

Genetic diploidization

A
  • duplicated gene copies of some functional categories have been shown to be preferentially retained or lost
  • recombination between nonhomologous chromosomes in meiotic mutantas of maize and A. thaliana - which both display extensive duplicated regions within their genomes
25
Q

Diploid-like meiotic behavior of allopolyploids

A

Ph genes

  • Ph1 in wheat
  • Ph1 with CDK-related genes
    • 7 on 5B
    • 5 on 5A
    • 2 on 5D

Ph1 locus with an effect on

  • premeiotic chromosome arrangement
  • chromatin organization
  • chromosome synapsis
  • recombination

TaASY1 - controls the transscription of meiotic genes for fidelity of synapsis or crossover formation

26
Q

TaASY1

A

synaptonemal complex

  • absence of Ph1 → increase in TaASY1

Ph1 suppresses interactions between homeologous chromosomes

TaASY1 for synapsis and crossover → bivalents

27
Q

Ph2

A

also suppresses homeologous crossovers BUT few homeologous crossovers in ph2 mutant

  • ph1b mutation and ph2b mutation induce slight increase in crossovers between homeologues
  • Ph2 on 3Ds
  • Ph2 different effect from Ph1, involved in progression of synapsis
  • Ph1 has suppressors acting on it
28
Q

B. napus haploids show 2 different phenotypes of numbers of univalents at MI

A

inherited in a Mendelian fashion with the PrBn gene

  • Nicolas - 2 meiotic behaviors at MI = difference in recombination between homeologous chromosomes
    • PrBn affects frequency (not distribution) of crossovers between homeologous chromosomes
      • may be dosage sensitive
29
Q

Newly formed polyploids can persist despite early bottlenecks of reduced fertility

A
  • selects for most fertile
  • usually results in increaed bivalent formation
    • Ph-like genes

Ph-like genes

  • chromosomal rearrangements
  • conserved across kingdoms
    • control Hop1 (similar to Asy1)
30
Q

Accessory B chromsomes

A
  • reduce crossover formation between nonhomologous chromsomes
  • compensate for absence of Ph1

Population and Chromosome Genetics - Leitch (6 decks)

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