Transcript ppt - Barley World

Polyploidy – so many options
Impacts of Ploidy Changes
• Changes in chromosome number and structure
can have major health impacts e.g. trisomy 21
• Polyploidy in cultivated and domesticated plants
is widespread and of evolutionary and economic
importance
Polyploidy – Pros and Cons
• Advantages
 Vigor effects – heterotic boost from divergent parental
genomes
 Redundancy – masking of recessive alleles
 Buffering capacity
• Disadvantages
 Changes in cell structure & shape – doubling genome
content increases cell volume
 Problems in cell division – mitosis and meiosis
 Changes in gene expression
 Epigenetic instability
Alternation of Generations
The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x)
1 pair homologous chromosomes
0 sets of homoeologous chromosomes
AA
A A
VAVA
2n = 2x = 14
30,000 genes
2 pairs of homologous chromosomes
2 sets of homoeologous chromosomes
AABB
A ABB
VAVAVBVB
2n = 4x = 28
60,000 genes
3 pairs of homologous chromosomes
3 sets of homoeologous chromosomes
AABBDD
A ABBDD
VAVAVBVBVDVD
2n = 6x = 42
90,000 genes
Euploidy
• An organism with an exact multiple of a basic
chromosome number (x)
 Can be diploid (2x), triploid (3x), tetraploid (4x) …..
• Barley in the sporophytic generation is 2n = 14
 n = 7 in the gametophtyic generation
 The base number (x) = 7 = n for a diploid
• Potato in the sporophytic generation is 2n = 48
 n = 24 in the gametophytic generation but x = 12
2n=2x=14
2n=4x=48
Aneuploidy
• Not euploid – more or less
chromosomes than a multiple of the
basic number
• Monosomic – loss of a chromosome,
(2n-1)
• Trisomic – addition of a chromosome,
(2n+1)
• Of value in genetic studies
Polyploidy
• More than two basic sets of chromosomes
 Autopolyploidy – 3 or more copies of each
chromosome in the basic number
 Allopolyploidy – 2 or more copies of ancestral
genomes giving 4 or more copies of the basic number
Polyploid Formation
 Genome duplication
 Failure of spindle fibers in
meiosis or mitosis
 Must lead to balance in order
to achieve euploidy and longterm viability
 Imbalanced gametes =
aneuploidy
Polyploids - Bivalent Pairing for viability
• Trivalent + of homologous chromosome pairing
in autopolyploids = sterility
• Homoeologous pairing in allopolyploids = sterility
• If non-bivalent pairing, gametes will not all get
the same number of chromosomes
• Must have bivalent pairing for fertility: polyploids
behaving as diploids
Autopolyploids
• Three or more homologues for each chromosome
in the basic number
 Even numbers (4x, 6x etc.) can be fertile
• Potato 2n = 4x = 48; Alfalfa 2n = 4x = 32
• Pairs of homologs = bivalents and normal meiosis
 Odd numbers (3x, 5x, …) = sterile or abnormal
• Banana 2n = 3x = 33
• The complete chromosome complement cannot form into pairs
and normal meiosis is disrupted
New Autopolyploids
• Can be synthesized by the use of colchicine to
double the chromosome complement
• Colchicine interferes with spindle formation in cell
division
• A 2n homozygous cell undergoes replication of each
chromosome during S phase of mitosis giving 2
copies of each
• No spindle at Anaphase and all can migrate to the
same cell to give a homozygous tetraploid
New Autopolyploids
• Can also create triploids by crossing related
tetraploid with a diploid
• Newly synthesized autopolyploids generally sterile
 Formation of multivalents disrupts meiosis
 Advantage in breeding some crops
• Seedless watermelon 2n = 3x =33
Genetics & Breeding of Autopolyploids
• Potentially very complex as up to 4 copies of an
allele at each gene can be present
 Nulliplex, simplex, duplex, triplex, quadriplex …..
Cross
Nulliplex
(N)
Simplex (S)
Duplex (D)
Triplex (T)
Nulliplex
(aaaa)
All N
Simplex
(Aaaa)
1S : 1N
1D : 2S : 1N
Duplex
(AAaa)
1D : 4S : 1N
1T:5D:5S:1N
1Q:8T:18D:
8S:1N
Triplex
(AAAa)
1D : 1S
1T : 2D: 1S
1Q:5T:5D:1S
1Q : 2T : 1D
All D
1T : 1S
1Q : 4T : 1D
1Q : 1T
Quadriplex
(AAAA)
Quadriplex
(Q)
All Q
It’s all Bananas
• Cultivated bananas derived from diploid species
Musa acuminata (A) and Musa balbisiana (B)
• Most edibles are triploids with genomes of AAA
(desert), AAB (plantains), and ABB (Cooking)
• Irregular pairing means bananas are seedless
 Good for the consumer but problematic for the
breeder and maintainer
• Evidence of pairing between homoeologous
chromosomes from A and B genomes
• 90% desert bananas are cv. Cavendish
Sequencing to the rescue?
• Previous breeding efforts have looked at mutation
• Now major effort resulted in sequencing a wild
Musa acuminata genome (AA)
http://banana-genome.cirad.fr/
Seedless Watermelons
• An infertile triploid created from 4x and 2x parents
Tetraploid Inbred
AAAA
x
Diploid Inbred
AA
Triploid F1
AAA
Grow with Fertile Diploid
to stimulate seedless fruit production
Seedless Watermelons
• The consumer benefits but breeding is more
difficult and hence expensive
 Development of suitable tetraploids
 Selection against sterility and fruit abnormalities
 Reduced seed yield for seed company
 Grower devotes up to 33% field to 2x pollinator
Allopolyploids
• An individual with chromosome sets from two or
more different but related species
• Interspecific hybridization followed by
chromosome doubling
 Spontaneous (natural forms)
 Colchicine (synthesized forms)
• Behave like diploids due to bivalent pairing
 Homologs within each ancestral species pair even
though homoeologous genomes may be collinear
Incipient allopolyploids – sterile unless
there is doubling
• Interspecific hybrids have just one copy of each
genome
 AA x BB → AB
• Haploid number of chromosomes from each
species
• Gametes get the wrong number of
chromosomes and hence infertility
• Use colchicine to double the chromosome
complement
Polyploidy in the Triticeae
Sporophytic
generation
Gametophytic
generation
Ploidy Level
Genome
Formula
2n = 14
n=7
2x (diploid) e.g.
Emmer wheat
2n = 2x = 14
2n = 28
n = 14
4x (tetraploid) e.g.
Durum wheat
2n = 4x = 28
2n = 42
n = 21
6x (hexaploid)
e.g. Bread wheat
2n =6x = 42
The Evolution of Bread Wheat
(and barley)
Hordeum spontaneum
Wild barley 2n = 2x =14
AA=BB=DD 2n=2x=14
Hordeum vulgare
Cultivated barley 2n = 2x =14
The Bread Wheat Genome
Wheat Pairing
• Ph1 locus on 5B affects
pairing in wheat
• Promotes homologous
pairing
• Blocks homoeologous
pairing
• Gene has been cloned
For more information see
https://www.jic.ac.uk/staff/graham-moore/Wheat_meiosis.htm
Triticale an allo-hexa/octaploid
• Wheat (durum or bread) Rye hybrid
Bread Wheat
AABBDD
x
Infertile F1
ABDR
Fertile F1
AABBDDRR
Rye
RR
Brassicas – The Triangle of U
(Woo Jang-choon = Nagahara U)
Haploids
• Single basic set of chromosomes
 Maize – n=10; bread wheat – n=21; barley – n=7
• Haploid plants can be nurtured to grow
 Only have the basic chromosome content (n) so are
infertile – meiotic irregularities
Doubled Haploids
• Doubling the haploid chromosome content gives
two exact copies
 No heterozygotes – “instant inbred lines”
 Sample pollen or egg cells from F1 plants
• A random sample of all the possible products of the first
round of segregation from meiosis
• Shorten the breeding cycle
• Immortal genetic populations for research
 Can make doubled haploids at any stage in the
selfing process (e.g. F1, F2, F3)
Doubled Haploids
•
•
•
•
•
Production of ‘Instant’ Inbreds
Shortens Breeding Cycle
Makes Selection More Effective
Can make Pure Seed Production Easier
How?
 Pollination by alien species
 Anther/Microspore Culture
 Haploid inducing genes
Doubled Haploidy Time Line
1921: Natural production of haploids in Datura stramonium
observed. Followed by Nicotiana tabacum (1924)
1952: Doubled haploid, inbred maize lines produced.
Selected parthenogenic haploids and chromosome doubling
1964: Haploid plants from Datura innoxia by anther culture
1970: Haploid production in barley via wide crossing
1978/79: First doubled haploid cultivar: “Mingo” barley
Currently: Routine technique in breeding many cereal and
vegetable crops
Events in Androgenesis
maturation
stress
bi-cellular
pollen
mature pollen
male
gametophyte
uni-cellular
microspore:
cell with
restricted
developmental
potential
embryogenesis
embryogenic microspore
totipotent cell
embryo, sporophyte
Androgenesis Induction
Reprogramming of microspores
towards
sporophytic development
Sucrose and
nitrogen starvation
Heat shock
Ethanol
Gamma
irradiation
Cold stress
Colchicine
treatment
pH
Separate or in combination
Osmotic
stress
Hordeum bulbosum wide crosses
Produce F1 from Desired Cross
Emasculate 2-3 days before pollen shed
Ensure plentiful supply of pollen from wild
relative
Dust alien pollen onto open
emasculated flowers
Apply hormonal spray to pollinated
spike (can repeat 2-3 days later)
Bag pollinated spike and leave for 1012 days
Hordeum bulbosum wide crosses
Rescue developing embryos from spike
pollinated with alien pollen
Grow on in special rooting medium
Once plants established, trim roots and
treat with colchicine
Grow out plants and harvest seed from fertile plants
Anther Culture
Healthy Donor Plants
Harvest spikes
Apply stress conditions
Plate out anthers on induction medium
Sub-culture steps
Spontaneous doubling
Transfer to greenhouse
Field
http://barleyworld.org/sites/default/files/2014_dh_poster.pdf
Numbers of DH Cultivars
Species
Numbers
Method
Rice
>100
Anther culture
Anther, micospore culture &
wide crossing
Microspore culture,
spontaneous DH lines
Barley
>100
Rapeseed
>50
Wheat
>20
Anther culture, wide crossing
Pepper
>10
F1 from DH parent(s)
Asparagus
>10
Female x DH supermale
Tobacco
>10
Microspore culture, anther
culture
Also: mustard, eggplant, melon, triticale
Doubled haploids & inbred line development
in maize
• Hybrid maize (corn) is a major crop worldwide
• Hybrids derived from intermating inbred lines
 Inbred line development key to hybrid breeding
 Accelerate inbred line development means hybrid
development also accelerated
 In vitro production of doubled haploids
• Anther or microspore culture
 In vivo production of doubled haploids
• Haploid inducer lines either as male or female
• Induction at >1% haploid lines; morphological marker for
identification
• Possibly arise through defective sperm cell enabling
fertilization but chromosomes eliminated