1. Polyploidy overview
Polyploidy is common in plants – not because polyploids are always bigger, better and stronger but because with polyploidy there are potentially more possible alleles with similar functions. These alleles may be expressed differentially in different tissues, at different growth stages, or in response to different environmental signals.
Polyploids can be allopolyploids or autopolyploids
In both types, fertility results after genome doubling
In autopolyploids, which have multiple homologous chromosomes, bivalent pairing is essential for fertility.
In allopolyploids, bivalent pairing is achieved only between homologous chromosomes; homoeologous chromosomes do not pair.
Homologous chromosomes pair at meiosis (e.g. 1A and 1A) because they are nearly alike in terms of structure and function (e.g. ~ same loci in usually the same order ) with either different or identical alleles at each locus.
Homoeologous chromosomes (e.g. 1A and 1B) are similar in terms of their structure and function (e.g. similar loci in similar orders) but sufficiently different that they usually do not pair at meiosis. Over evolutionary time, alleles at homoeologous loci may preserve similar functions but diverge in terms of sequence and function.

2. Polyploidy: The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x)
Autopolyploid : Arises from hybridization genome doubling within a species.
1 pair homologous chromosomes shown. There will be 7 pairs (sets)
of homologous chromosomes, total.
This species has the “A genome”. Genomes are given at the n level. At the 2n level it is AA.
2 pairs of homologous chromosomes shown (1 pair of 1A, one pair of 1A’). There will be 7 pairs of A genome chromosomes and 7 pairs of A’ genome chromosomes. The 1A and 1A’ chromosomes are homologous. The “’” is used in this case to remind that the A’ genome arose from a duplication of the A genome. This species has the A and A’ genomes. At the 2n level it is AAA’A’.
3 pairs of homologous chromosomes shown
(1 pair of 1A, one pair of 1A’, one pair of 1A’’).
There will be 7 pairs of A genome
chromosomes, 7 pairs of A’ genome
chromosomes, and 7 pairs of A’’ genome
chromosomes. The “’” is used in this case to
remind that the A’ and A’’ genomes arose from
duplication of the A genome. This species has
the A, A’, and A’’ A’ genomes. At the 2n level
it is AAA’A’A’’A’’
1A 1A 1A’ 1A’
1A 1A
1A1A 1A’1A’ 1A’’1A’’
VAVA
VAVAVA’VA’
VAVAVA’VA’VA’’VA’’
2n = 2x = 14
30,000 genes
Diploid
2n = 4x = 28
60,000 genes
Autotetraploid
2n = 6x = 42
90,000 genes
Autohexaploid

3. Autopolyploidy
2n = 2x = 14 (AA)
Meiosis
N = 7, A egg
Or can spontaneously double to
AA egg
2n = 2x = 14 (AA)
Meiosis
N = 7, A pollen
Or can spontaneously double to AA pollen
A egg fertilized by A pollen = AA
Subsequent spontaneous doubling = AAAA Or
AA egg fertilized by AA pollen = AAAA
For full fertility, need bivalent pairing….

4. Polyploidy: The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x)
Allopolyploid: Arises from hybridization of one or more different species, with genome doubling after each interspecific hybridization to restore fertility (see next slide)
VAVA
VAVAVBVB
VAVAVBVBVDVD
2n = 2x = 14
30,000 genes
Diploid
2n = 4x = 28
60,000 genes
Allotetraploid
2n = 6x = 42
90,000 genes
Allohexaploid
1 pair homologous chromosomes shown. There will be 7 pairs of homologous chromosomes. There are no homoeologous chromosomes.
This species has the “A genome”. Genomes are given at the n level. At the 2n level it is AA.
2 pairs of homologous chromosomes shown (1 pair of 1A, one pair of 1B). There will be 7 pairs of A genome chromosomes and 7 pairs of B genome chromosomes. The 1A and 1B chromosomes are homoeologous, as are 2A and 2B, etc. This species has the A and B genomes. At the 2n level it is AABB.
3 pairs of homologous chromosomes shown
(1 pair of 1A, one pair of 1B, one pair of 1D).
There will be 7 pairs of A genome
chromosomes, 7 pairs of B genome
chromosomes, and 7 pairs of D genome
chromosomes. The 1A, 1B , and 1D
chromosomes are homoeologous, as are 2A, 2B,
and 2D, etc. This species has the A, B, and D
genomes. At the 2n level it is AABBDD.
1A 1A 1B 1B
1A 1A
1A1A 1B1B 1D1D

5. Allopolypoidy
2n = 2x = 14 (AA)
Meiosis
N = 7, A egg
2n = 2x = 14 (BB)
Meiosis
N = 7, B pollen
A egg fertilized by B pollen
Can grow as AB (2n = 2x = 14) but will be sterile since there can be no
no homologous pairing at meiosis (e.g. 1A can’t pair with 1B, etc.)
BUT if there is spontaneous doubling to give AABB (2n = 4x= 28), then the plant will be
fertile (e.g. 1A can pair with 1A; 1B with 1B, etc.)