1. Ploidy – Part 1: An Overview
PBG 430

2. Ploidy – so many options
Ploidy = number of chromosome copies in a genome
Haploid
Diploid
Polyploid
Haploid
Triploid
Autotetraploid
Allohexaploid
Allopolyploid with variable chromosome number

3. Terms to Remember: Euploidy
Total chromosome number is an exact multiple of a basic chromosome number (x)
Can be diploid (2x), triploid (3x), tetraploid (4x), …..
Examples
Barley (diploid): 2n = 2x = 14
Base number (x) = 7
Gametophytic generation (n) = 7
Sporophytic generation (2n) = 14
Durum wheat (tetraploid): 2n = 4x = 28
Gametophytic generation (n) = 14
Sporophytic generation (2n) = 28

4. Terms to Remember: Aneuploidy
Not euploid  more or fewer chromosomes than a multiple of the basic number
Classifications
“Monosomic” = loss of a chromosome, (2n-1)
“Trisomic” = addition of a chromosome, (2n+1)
Aneuploids have great value in genetic studies (e.g. locating genes in chromosomes)

5. Haploid barley growing in the lab
Haploidy
Typically, haploid angiosperm plants will be sterile
Diploid (fertile)
Haploid barley growing in the lab
Haploid (sterile)

6. Polyploidy
Polyploids have more than two basic sets of chromosomes (3X, 4X, 6X, etc.)
Autopolyploidy
Chromosome sets originate from the same species
Result of genome doubling within a species
Allopolyploidy
Chromosome sets originate from different ancestral genomes
Result of interspecific hybridization and genome doubling

7. Polyploids – Mitosis and Meiosis
Mitosis = ok
Meiosis ?
Bivalent pairing: essential for the formation of reduced (n) and balanced gametes

8. Autopolyploids Even number of chromosomes (4x, 6x, etc.)
Plants can be fertile…if there is bivalent pairing & normal meiosis
Examples:
Potato  2n = 4x = 48
Alfalfa  2n = 4x = 32
Odd number of chromosomes (3x, 5x, etc.)
Plants are usually sterile or abnormal
Because the complete chromosome complement cannot form into pairs, which causes normal meiosis to be disrupted
Aneuploidy
Example: Banana  2n = 3x = 33

9. Autopolyploidy – Comparison Example
Characteristic
Diploid Example
Tetraploid Example
Genome Quantity
1 genome = Genome A
2 genomes = Genome A duplicated Genome A
Locus Quantity
1 locus = V locus
2 loci = V locus duplicated V locus
Gene Quantity
1 gene = V gene
2 genes = V gene duplicated V gene
Allele Quantity
2 alleles = AA
4 alleles = AAAA
Homologous Chromosomes Quantity
1 pair of homologous chromosomes = AA
2 pairs of homologous chromosomes = AAAA
versus

10. Allopolyploids
An individual with chromosome sets from two or more different, but related, species
Interspecific hybridization + genome doubling:
Unreduced gametes
Reduced gametes (normal fertilization) but abnormal mitosis
Genome doubling can be either spontaneous (natural forms – bread wheat) or artificial (colchicine – doubled haploids)
Usually fertile  bivalent pairing (behave like diploids)
Homologous chromosomes within each ancestral species pair

11. Allopolyploidy – Comparison Example
Homologous vs. Homoeologous chromosomes?
Characteristic
Diploid Example
Tetraploid Example
Hexaploid Example
Genome Quantity
1 genome = Genome A
2 genomes = Genome A + Genome B
3 genomes = Genome A + Genome B + Genome C
Locus Quantity
1 locus = V locus
2 loci = V locus (A) + V locus (B)
3 loci = V locus (A) + V locus (B) + V locus (C)
Gene Quantity
1 gene = V gene
2 genes = V gene (A) + V gene (B)
3 genes = V gene (A) + V gene (B) + V gene (C)
Allele Quantity
2 alleles = AA
4 alleles = AABB
6 alleles = AABBCC
Homologous Chromosomes Quantity
1 pair of homologous chromosomes = AA
2 pairs of homologous chromosomes = AA, BB
3 pairs of homologous chromosomes = AA, BB, CC
Homoeologous Chromosome Quantity
None
2 sets of homoeologous chromosomes = AA/BB
3 sets of homoeologous chromosomes = AA/BB/CC

12. Polyploidy – Pros and Cons
Advantages
Disadvantages
Hybrid vigor:
More alleles, more loci, more genes = potential for heterozygosity
Allopolyploids = fixing of divergent parental genomes
Redundancy – masking of recessive alleles
Buffering capacity – differential gene expression
Changes in cell structure & shape – doubling genome content may increase cell volume
Sterility (assuming that’s a goal)
Changes in cell structure & shape – doubling genome content increases cell volume
Example: brittle wood in timber species
Sterility – if fertility is the goal

13. Genetic tables for example species – Autopolyploids
Plant
Banana
(Musa spp.)
Formula
2n = 3x = 33
Genome size
472 Mb
Approximate number of genes
36,000
Genome sequence
Sequenced! (See Canvas Supplemental materials for more info)
Pollination biology
Asexual (stalks), monoecious imperfect flowers (infertile)
Center of origin
Malaysia
Plant
Potato
(Solanum tuberosum)
Formula
2n = 4x = 48
Genome size
844 Mb
Approximate number of genes
39,000
Genome sequence
Sequenced! (See Canvas Supplemental materials for more info)
Pollination biology
Asexual (tubers), hermaphroditic selfing
Center of origin
South America (Andes)

14. Genetic tables for example species – Allopolyploids
Plant
Wheat
(Triticum aestivum)
Formula
2n = 6x = 42
Genome size
13,916 Mb
Approximate number of genes
40,000
Genome sequence
Sequenced! (See Canvas Supplemental materials)
Pollination biology
Hermaphroditic selfing
Center of origin
Mediterranean and Southeast Asia

15. By now you should be able to…
Define the many terms we learned in this lesson (remember “n” is used for generation cycle and “x” is used for ploidy level):
Euploidy (2n+0)
Aneuploidy (2n-1, 2n+1, …)
Polyploidy (3x, 4x, 5x, …)
Allopolyploid vs. autopolyploid
Hypothesize if a plant is fertile or sterile, based on its ploidy level and type (allopolyploid vs. autopolyploid).
Why are bananas sterile while wheat is not? What is the genetic mechanism (in meiosis) that is responsible for this difference?
Differentiate between homologous chromosomes and homoeologous chromosomes in an allopolyploid.
Estimate the number of genes, loci, and alleles in an induced autopolyploid as compared to the diploid from which it was induced.
Discuss advantages and disadvantages of polyploidy in natural ecosystems and agroecosystems.