Questions: Polyploidy and Autopolyploidy: Origins and Consequences

Question 1

A triploid (3n) banana plant is nearly sterile, while a tetraploid (4n) potato plant can reproduce sexually. What best explains this difference?

• A. Triploids have more chromosomes than tetraploids, creating greater meiotic disruption
• B. In a triploid, chromosomes cannot be divided evenly into two haploid sets — three copies per chromosome group produce multivalents and massively aneuploid gametes; a tetraploid has an even number of homologs that can potentially form balanced bivalent pairs
• C. Tetraploids undergo mitosis instead of meiosis, bypassing the chromosome pairing problem
• D. Triploids lack the colchicine needed to complete meiosis correctly

Answer: (B)

The key is divisibility of chromosome sets. In a triploid (3 copies of each chromosome), meiosis must divide three copies into two gametes — there is no way to do this evenly. The three copies form trivalents or a bivalent + univalent, leading to grossly aneuploid gametes that are not viable. In a tetraploid (4 copies), balanced segregation is at least possible: if all four copies pair as two bivalents, each gamete receives exactly two copies of each chromosome (a balanced 2n gamete). Over evolutionary time, mechanisms that promote bivalent pairing in tetraploids are selected, improving fertility further.

Question 2

How does colchicine produce polyploidy experimentally?

• A. Colchicine accelerates DNA replication, producing extra chromosome copies before the cell divides
• B. Colchicine prevents cytokinesis while allowing DNA replication, directly doubling chromosome number
• C. Colchicine disrupts spindle microtubule polymerization, preventing chromosome separation in mitosis — the cell completes DNA replication but cannot segregate chromosomes, producing a cell with double the normal chromosome number
• D. Colchicine induces unreduced gamete formation by blocking meiosis II

Answer: (C)

Colchicine binds to tubulin and prevents microtubule polymerization, which disassembles the mitotic spindle. Without a spindle, chromosomes cannot be pulled to opposite poles during anaphase. The cell completes DNA replication (chromosomes are duplicated) but cannot segregate them, so cytokinesis either fails to occur or produces a single cell with twice the normal chromosome complement. If this happens in cells that give rise to gametes, or early in development, the resulting organism or its offspring may be polyploid. This technique is widely used to create new polyploid crop varieties.

Question 3

Most polyploid organisms are sterile because the presence of extra chromosome sets generally disrupts meiosis too severely to produce viable gametes.

• T. True
• F. False

Answer: False

This is the most common misconception about polyploidy. While odd-ploidy polyploids (3n, 5n) are typically sterile because their chromosome sets cannot be divided evenly, even-ploidy polyploids (4n, 6n, 8n) can achieve normal or near-normal fertility. Over evolutionary time, selection favors mechanisms that promote orderly bivalent pairing in even-ploidy polyploids, and many successful crop plants (wheat 6n, potato 4n, strawberry 8n) are polyploids that reproduce sexually. Polyploidy has been one of the most important mechanisms of speciation in plants.

Question 4

Bread wheat (Triticum aestivum) is a hexaploid (6n) whose genome contains contributions from three ancestral diploid species, making it an example of how whole-genome duplication can drive speciation and crop domestication.

• T. True
• F. False

Answer: True

Bread wheat is an allohexaploid — its genome contains three distinct diploid genomes (A, B, and D, contributed by hybridization between three related grass species followed by chromosome doubling). This history of whole-genome duplication and hybridization gave bread wheat dramatically expanded genetic material and contributed to traits like grain size, yield, and stress tolerance that humans selected during domestication. It is one of the best examples of polyploidy as both an evolutionary and agricultural force.

Question 5

Explain why polyploidy is more common and evolutionarily significant in plants than in animals, using what you know about how polyploids arise and what determines their fertility.

Model answer: Several factors make plants more hospitable to polyploidy. First, many plants can reproduce asexually (vegetatively), allowing a new polyploid to persist and spread even if its initial sexual fertility is low. Animals require sexual reproduction for population establishment, so sterile or low-fertility polyploids cannot propagate. Second, plants with self-fertilization can form new species from a single polyploid individual without needing a mate of the same ploidy. Third, plant cells tolerate additional chromosome sets better than animal cells — the gene dosage imbalances that cause lethality in polyploid animals (where dosage-sensitive developmental pathways are more tightly regulated) are better buffered in plants. Finally, polyploidy provides such large fitness advantages in plants (larger cells, greater vigor, gene redundancy for neofunctionalization) that selection strongly favors its persistence.

The key biological asymmetry is that plants have multiple routes to bypass the immediate fertility problem that kills most new polyploids in animals. Vegetative reproduction, selfing, and greater developmental tolerance to ploidy changes give plant polyploids a foothold that animal polyploids rarely achieve. This is why ~70% of flowering plant species have polyploid ancestry, while vertebrate polyploidy is rare and mostly ancient.