Polyploids and haploids

Polyploids and haploids:
of n’s and x’s
www.dothegreenthing.com
barleyworld.org
manbir-online.com
oregonstate.edu
In humans, changes in chromosome number and structure
have major impacts on health. e.g. trisomy 21
In plants – both wild and domesticated – variation in
chromosome number is widespread and of evolutionary and
economic importance
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Polyploidy
(genetic flexibility, if not
always horsepower)
1 pair homologous chromosomes
0 sets of homoeologous chromosomes
HH
VV
2n = 2x = 14
30,000 genes
2 pairs of homologous chromosomes
2 sets of homoeologous chromosomes
AABB
VVVV
2n = 4x = 28
60,000 genes
3 pairs of homologous chromosomes
3 sets of homoeologous chromosomes
AABBDD
VVVVVV
2n = 6x = 42
90,000 genes
Euploid
An organism (diploid or polyploid) with exact multiples of a
basic ("x") chromosome number
Example:
Barley in the sporophytic generation is 2n = 14 and n = 7 in
the gametophytic generation
Since 7 x 2 = 14, barley is a diploid (euploid)
The "base number (x)" is 7
In the case of a diploid, "x" is the same as the "n" number.
Aneuploid
More or fewer chromosomes than an exact multiple of the x
number, e.g. 2x + 1
Extreme sorts of aneuploids are found more often in
polyploids than diploids
Aneuploids usually have lower fitness than euploids, but they
do have some interesting uses for genetics and agriculture
Aneuploid vocabulary:
•Monosomic: 2n –1
•Trisomic: 2n+1
Polyploid
More than two basic sets of chromosomes
The two types of polyploidy are
• autopolyploidy (three or more copies of the same genome)
• allopolyploidy (three or more copies of different genomes)
Example: Ploidy levels in the Triticeae (an allopolyploid
series) where "x" = 7
Note: Based on a genome formula alone, you cannot tell if an
organism is allopolyploid or autopolyploid
Sporophytic
generation
Gametophytic
generation
Ploidy level
Full formula
2n = 14
n=7
2x = Diploid
2n = 2x = 14
2n = 28
n = 14
4x = Tetraploid
2n = 4x = 28
2n = 42
n = 21
6x = Hexaploid
2n = 6x = 42
Autopolyploid: A cell or individual whose several chromosome
sets, three or more, are all homologous. These sets arise
within a species via a process of genome multiplication
Autopolyploids can be fully fertile (potato (2n = 4x = 48),
alfalfa (2n = 4x = 32))
Even though there are multiple homologous chromosomes,
there are even numbers of homologs, and pairs of homologs
show bivalent pairing
Meiosis is normal and balanced gametes are produced
Autopolyploid: A cell or individual whose several chromosome
sets, three or more, are all homologous. These sets arise
within a species via a process of genome multiplication
Newly synthesized autopolyploids are usually sterile due to
meiotic irregularities
Sterility in autotetraploids due to uneven numbers of
homologous chromosomes can be useful for producing
seedless fruits
Considerations with autopolyploids:
Genetics and breeding: Can be quite complex, since
potentially every allele at each locus can be different
– e.g. an autotetraploid could have up to four different
alleles at a locus
V1 V2 V3 V4
Sources: Autopolyploids can arise spontaneously
from a somatic doubling of chromosome number
and/or the formation of unreduced gametes, or they
can be induced
Inducing an autopolyploid:
Colchicine is a chemical used to “double” chromosome
numbers
Colchicine interferes with spindle fiber formation
Consider a 2n plant cell. It undergoes a normal S phase prior
to mitosis, leading to two sister chromatids per cell. The failure
of spindle fibers to form at Anaphase leaves the replicated
chromosomes in the same cell, resulting in a nucleus with
twice the number of chromosomes
Since the two chromosomes are identical, the resulting cell
contains two copies of the genome and is completely
homozygous
Allopolyploid: A cell or individual with genetically different
chromosome sets derived from two or more species
Allopolyploids arise through interspecific hybridization and
spontaneous chromosome doubling
Allopolyploids can also be created by crossing two
different species and doubling the chromosome number of
the F1
In general, allopolyploids behave like diploids due to
bivalent pairing – each of the genomes behaves
autonomously, although the multiple homoeologous
genomes may be collinear
Hordeum vulgare subsp. spontaneum Triticum urartu; Aegilops speltoides;
(2n = 2X = 14)
Triticum tauschii
(2n = 2x = 14)
Triticum monococcum
(Einkorn)
(2n = 2X = 14) Hulled
Triticum turgidum subsp. dicoccum
(Emmer)
(2n = 4x= 28) Hulled
Triticum aestivum subsp. spelta
(2n = 6x = 42) Hulled
Triticum turgidum subsp. durum
(durum)
(2n = 4x= 28) free threshing
Hordeum vulgare subsp. vulgare
(2n = 2x = 14)
Hulled, free threshing
(hull-less, naked)
Triticum aestivum
(2n = 6x = 42) free threshing
Example: Bread wheat - a natural allopolyploid
Wild einkorn
2n=2x=14; AA
X
Wild diploid
2n=2x=14; BB
F1 Hybrid = AB = sterile
spontaneous chromosome doubling
Wild emmer wheat
2n = 4x = 28; AABB
Domestication
Cultivated emmer wheat
2n = 4x = 28; AABB
X
Wild diploid
2n = 2x= 14; DD
F1 Hybrid = ABD = sterile
spontaneous chromosome doubling
Bread wheat
2n = 6x = 42; AABBDD
Manipulating ploidy
Meiosis issues and solutions: polyploids and haploids
Why sterility can occur in polyploids - and the solutions
www.jic.ac.uk
Meiosis issues with polyploids and
haploids
•Issue 1:
• Non-bivalent pairing
• Homologous chromosomes in the case of autoplyploids
• Homoeologous chromosomes in the case of
allopolyploids
• Gametes will not all get the same numbers of
chromosomes: this imbalance will cause sterility
Solution: Bivalent pairing
Meiosis issues with polyploids and
haploids
• Issue 2:
• Interspecific hybrid with one copy of each
of two genomes
• Haploid number of chromosomes
• Gametes will not get the same numbers of
chromosomes: this imbalance will cause
sterility
Solution: Chromosome (genome) doubling
Triticale: a human-made allopolyploid
Rye
2n=2x=14; RR
X
Durum wheat
2n=4x=28; AABB
F1 = ABR = sterile
colchicine chromosome doubling
Hexaploid triticale
AABBRR = 2n=6x=42
The banana: 2n = 3x = 33… Don’t be mislead by 4n!
Bananas and genetic vulnerability
The seedless watermelon: 2n = 3x = 33
• Breeding seedless watermelons
• The seedless watermelon is an
autopolyploid that is produced by
• creating 4x lines from 2x by chromosome
doubling
• crossing 4x by 2x to give 3x
The parting shot on polyploids: "With the advent of
genome sequencing and the availability of extensive EST data
and high-density, molecular marker-based maps, it became
clear that all plant genomes harbor evidence of cyclical,
recurrent episodes of genome doubling."
Haploids
A haploid has a single basic set of chromosomes: e.g.
maize is n=10; barley is n=7; and hexaploid wheat is n =
21
A haploid plant will grow and look quite like a normal plant
Since it has only the "n" number of chromosomes, it is
sterile
Sterility is due to the fact that there is only one
homologous chromosome per nucleus
Meiosis will lead to the production of imbalanced gametes
Doubled haploids:
When the chromosome complement of a haploid is doubled,
you create an “instant homozygote”
If these instant homozygotes are produced from F2 gametes
(i.e. pollen or eggs of F1 plants), then one can sample the
range of segregation and independent assortment possible
within the cross
Population size is critical in terms of recovering as many
unique configurations of alleles at multiple loci as possible
The key point with a doubled haploid population is that there
are no heterozygotes.
Doubled haploids:
The two most common mechanisms for doubled haploid
production can be classified as androgenetic (i.e. male-based
systems) or gynogenetic (i.e. female-based systems)
Within the androgenetic systems, there are anther and
microspore culture
Within the gynogenetic systems, there are chromosome
elimination and ovule culture
Anther and microspore culture:
Techniques have been perfected and commercialized in a
number of dicot and monocot species
Male gametes (either isolated microspores or microspores
within anthers) are used to establish embryogenic callus
and plants are regenerated from these embryoids
In some species, rates of spontaneous doubling are high
enough that colchicine is not required
On the downside, the callus phase may lead to unwanted
culture-induced (gametoclonal) variation
Chromosome elimination:
Hordeum bulbosum is a diploid, wild, perennial species of
barley
When H. vulgare is crosses with H. bulbosum, there is
fertilization of both egg and polar nuclei, and the interspecific
cross will show up to 90% fertility
Within 24 hours, the H. bulbosum chromosomes will be
preferentially eliminated, leaving a maternal haploid
The haploid embryos, having no endosperm to support them,
would normally die
With embryo rescue, haploid plantlets can be regenerated and
artificially doubled to give homozygous lines
Chromosome elimination:
The same procedure can be (and is) employed to produce
haploids from wheat, using maize and/or millet pollen
Doubled haploids and corn breeding
Hybrid corn is a huge commercial industry
F1 hybrids are made from the crosses of inbred lines
Inbred lines are of tremendous value
Therefore, accelerated development of inbred lines is a priority
Most companies are using a doubled haploid system to produce
inbreds, at considerable expense