Reading - Genetic regulation of meiosis in polyploid species Flashcards
Meiosis is a fundamental process for all sexual species with direct relevance to natural selection
- leads to the formation of gametes
- contributes to genome stability
- generates genetic diversity
Meiosis relies on the interrelated events of
- homologous chromosome recognition
- intimate association
- synapsis
- recombination
Correct segregation of chromosomes requires
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,
- results from sister chromatid cohesion and meiotic crossovers
need for proper disjunction at the first division
NEED AT LEAST ONE CROSSOVER BETWEEN HOMOLOGOUS CHROMOSOMES
Cytological diploidization
the process by which meiosis in polyploids leads to
chromosomally and genetically balanced gametes
critical for polyploid speciation
Autopolyploid
having more than 2 sets of chromosomese, all derived from the same species
Allopolyploid
having 2 or more complete sets of chromosomes derived from different species
Meiosis in autopolypoloids
- more than 2 copies of each chromosome which have the same chance of recombining
- form multivalents at MI
- chromosome missegregation at anaphase I
Meiosis in autopolypoloids
Santos
established autotetraploids lines of A. thaliana showed fewer multivalents than newly synthesized ones
→ partial cyotolgoical diploidization over 13 generations
Meiosis in allopolypoloids
- 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
Wheat
- 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)
The diploid-like behavior of wheat is ensured by
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
Ph1 may act as a master coordinator
- premeiotic chromosome arrangement
- chromatin organization
- chromosome synapsis
- recombination
Transcription of CDK-like genes
- 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
TaASY1
(T. aestivum asynapsis)
- 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
Ph1 vs. TaASY1
- Ph1 required to suppress interactions between homologous chromosomes
- TaASY1 for synapsis and crossover
Ph2
- 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
Brassica napus
- 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)
Nicolas et al.
- 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
What occurs at the onset of polyploid formation?
- 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
Where do Ph-like genes come from?
- 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
Recurrent polyploid
rule, not exception
All polyploid wheat species show
Ph1 activity and a conserved structure at the Ph1 locus
Martinez
- analyzed meiotic prophase and MI in haploid from different bread wheat cultivars
- found significant dfiferences in synapsis and recombination between homeologues
Genetic diploidization
- 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
Diploid-like meiotic behavior of allopolyploids
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
TaASY1
synaptonemal complex
- absence of Ph1 → increase in TaASY1
Ph1 suppresses interactions between homeologous chromosomes
TaASY1 for synapsis and crossover → bivalents
Ph2
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
B. napus haploids show 2 different phenotypes of numbers of univalents at MI
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
- PrBn affects frequency (not distribution) of crossovers between homeologous chromosomes
Newly formed polyploids can persist despite early bottlenecks of reduced fertility
- 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)
Accessory B chromsomes
- reduce crossover formation between nonhomologous chromsomes
- compensate for absence of Ph1
