1. Ploidy – Part 2: Autopolyploids
PBG 430

2. Recap – Autopolyploid chromosome numbers
Even numbers (4x, 6x, etc.) can be fertile
Example: Potato 2n = 4x = 48
Pairing of homologous chromosomes  bivalents and normal meiosis
Odd numbers (3x, 5x, etc.) = sterile or abnormal
Examples: Banana 2n = 3x = 33; watermelon 2n = 3x = 33
The complete chromosome complement cannot form into pairs and normal meiosis is disrupted

3. Naturally occurring autotetraploids
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
Usually self-pollinated; perfect flower
Center of origin
South America
Example: Potato (Solanum tuberosum)
Commercial potato production relies on asexual propagation
Although production of potatoes occurs with what are termed “seed potatoes,” these are actually pieces of tubers containing a set number of eyes, from which new plants will form.
Potatoes can set seed though!
“True potato seed”

4. Genetics and breeding of potato
Potentially very complex as up to four different alleles can be present.
Considering just two alleles (Aa), each possible genotype can be identified as:
Nulliplex (aaaa)
Simplex (Aaaa)
Duplex (AAaa)
Triplex (AAAa)
Quadriplex (AAAA)
4 alleles
6 possible gametes

5. Autopolyploids and bivalent pairing
The potato is an example of a naturally occurring autopolyploid with full fertility, due to bivalent pairing.
Parisod et al., 2009:
High frequency of bivalent pairing in autopolyploids
Increase in fertility associated with increased bivalent pairing in newly synthesized autopolyploids
However, there are also reports of fertility in the presence of multivalent formation

6. Inducing autopolyploids
Colchicine (and other chemicals) double the chromosome complement by interfering with spindle formation.
Consider a 2n = 2x = 14 plant
Gametogenesis: if no spindle fiber function at Anaphase 1 of meiosis, all homologous chromosomes migrate to the same cell, which then gives rise to a diploid gamete (2n = 2x = 14)
Fertilization with another diploid gamete gives rise to an autotetraploid zygote (2n = 4x = 28)
Somatic cells descended from the tetraploid cell via mitosis will be tetraploid (2n = 4x = 28)

7. Induced autopolyploids
The newly synthesized tetraploid will likely show high levels of sterility, but selection for fertility may be effective.
The selection for and increase in fertility is associated with bivalent pairing.
An example of the application of chromosome doubling is tetraploid perennial ryegrass.
The tetraploids are claimed to produce superior forage (due to changes in cell size) and to be more productive (an advantage of polyploidy)
Once tetraploids are developed and stable, tetraploid x tetraploid crosses will produce tetraploid progeny

8. Triploid bananas
The ancestors of cultivated bananas are the diploid species Musa acuminata (A) and Musa balbisiana (B)
Naturally occurring intraspecific and interspecific hybridizations led to various combinations of the A and B genomes
Most edible bananas are triploids with genomes of AAA (dessert), AAB (plantains), and ABB (cooking)
Multivalent formation means bananas are seedless
Seedless-ness is good for the consumer but problematic for developing new varieties – the only propagation method is asexual

9. Banana ancestors
The ancestors of modern, cultivated bananas are both diploid species. Both have the same floral structure.
Floral structure persists in the triploid banana (2n = 3x = 33)
Flowers are functional in diploids, but triploids do not experience successful fertilization
Plant
Musa acuminata (A)
Musa balbisiana (B)
Formula
2n = 2x = 22
Genome size
523 Mb
402 MB
Approximate number of genes
~36,000
Genome sequence
Sequenced! (See Canvas Supplemental materials for more info)
Unknown
Pollination biology
Cross-pollinated; monoecious, imperfect
Center of origin
Southeast Asia

10. Origin of the triploid bananas
A possible model for the origin of triploid banana:
Musa acuminata (AA) spontaneously doubles (AAAA) and crosses with a diploid M. acuminata (AA) or a diploid Musa balbisiana (BB)
The triploid bananas then would be propagated asexually

11. Triploids and sterility - The danger of genetic uniformity
90% of the dessert bananas (AAA) in the world are genetically identical – the variety “Cavendish”
Strictly asexual propagation
Cavendish is highly susceptible to Sigatoka disease (a fungal pathogen)
Multiple fungicide applications are required to control the disease
The search for genes conferring resistance to Sigatoka have not been successful to date
Induced mutations are not successful in “creating” resistance alleles

12. Expanding the genetic base of 3x banana
Genome sequencing efforts are underway in multiple Musa species to identify useful genetic variation.
The hope is this application of contemporary genetics tools can lead to more sustainable banana production.

13. Manipulating autopolyploidy
Triploid seedless watermelon (Citrullus lanatus)
Plant
Watermelon
(Citrullus lanatus)
Formula
2n = 2x = 22
Genome size
425 Mb
Approximate number of genes
~23,000
Genome sequence
Sequenced! (See Canvas Supplemental materials for more info)
Pollination biology
Usually cross-pollinated; monoecious, imperfect
Center of origin
South Africa

14. Manipulating autopolyploidy Triploid seedless watermelon
Triploid, seedless watermelons have really only been in the marketplace for the past 50 years!

15. The cost of seedless watermelon
Triploid-related sterility makes breeding and seed production more difficult and expensive
Development of suitable tetraploid parents
Selection against sterility
Selection against fruit abnormalities
Selection for ideal fruit characteristics
Cross the tetraploid and diploid parents to produce triploid offspring
Reduced triploid watermelon seed yield for seed companies
Triploids are less vigorous than diploid plants, which may require the triploids to be transplanted in the field (rather than direct seeding)
Pollination of triploid plants by a diploid is necessary for stimulating parthenocarpy
Growers devote ~30% of field to be planted with a pollenizer watermelon variety

16. Triploid seedless watermelon
What about the “seeds” in seedless watermelons?
The white, soft “seeds” sometimes found in seedless watermelons are not fertile
More rarely, black and fertile seeds do occur
These are most likely fertile diploids  the result of fertilization of a rare n = 11 female gamete with a full haploid complement of chromosomes by a n = 11 male gamete produced by the diploid pollenizer
Do you see the black seed?

17. Inducing autopolyploids in ornamentals
Polyploidy can be explored with the goal of sterility in order to:
Reduce likelihood of invasiveness
Prevent formation of unwanted fruit
Polyploids may also have desirable phenotypes for ornamentals, including larger, fleshier plants and flowers.
“Of primary interest is the development of sterile forms of nonnative species that are of economic importance to Oregon growers to prevent escape from cultivation. Primary techniques to achieve sterility/reduced fertility include development of triploids using ploidy manipulation, as well as mutagenesis by exposing seeds or meristems to physical and chemical mutagens.”
~Dr. Ryan Contreras, Dept. of Horticulture, Oregon State University

18. By now you should be able to…
For each of the three main autopolyploid examples (potato, banana, and seedless watermelon), describe the following:
Genome formula (?n = ?x = ?)
Origin of polyploidy: naturally occurring or induced?
Fertile or sterile? Explain why.
Explain how cultivated bananas and seedless watermelons originated or are produced.
Summarize why polyploidy can be a goal in ornamental variety development.
Describe the function of colchicine (and other chemicals) in inducing polyploidy.