Questions: Polyploidy and Instant Reproductive Isolation

Question 1

A tetraploid (4n) plant spontaneously arises in a field of diploid (2n) plants of the same species. The tetraploid is healthy and fertile. When it mates with a neighboring diploid plant, what happens to the offspring, and what does this imply about speciation?

• A. The offspring are diploid (2n) and fully fertile, because the tetraploid parent contributes only n gametes
• B. The offspring are triploid (3n), cannot undergo normal meiosis, and are typically sterile — the tetraploid is reproductively isolated from its diploid population in the same generation it arose
• C. The offspring are tetraploid (4n) because the diploid egg is fertilized by a tetraploid pollen grain
• D. The offspring are diploid but carry supernumerary chromosomes that are eliminated in subsequent generations

Answer: (B)

A tetraploid (4n) produces 2n gametes through normal meiosis. A diploid (2n) produces n gametes. Their cross yields a 3n triploid. Triploids cannot undergo proper meiosis because every chromosome set has an odd number of homologs — chromosomes cannot pair evenly. The result is aneuploidy in gametes, causing sterility or inviability. This means the tetraploid is immediately reproductively isolated from its diploid ancestors without any geographic separation — speciation in a single generation.

Question 2

Bread wheat (Triticum aestivum) is hexaploid (6n = 42 chromosomes). Why is allopolyploidy, rather than autopolyploidy, the mechanism responsible for this?

• A. Autopolyploidy only occurs in animals; allopolyploidy is the mechanism by which plants double their genomes
• B. Allopolyploidy combines genomes from two or more different species, and wheat's hexaploid genome contains three distinct ancestral genomes (A, B, and D) from three separate wild grass species
• C. Allopolyploidy produces a hexaploid in a single event, while autopolyploidy requires multiple separate doubling events
• D. Autopolyploidy would produce identical chromosome pairs in wheat, making the plant unable to produce seeds

Answer: (B)

Allopolyploidy involves hybridization between different species followed by genome duplication, incorporating two (or more) distinct genomes. Wheat's hexaploid genome contains three distinct subgenomes (A, B, D) derived from three different wild grass species (Triticum urartu, an Aegilops species, and Aegilops tauschii) through two successive hybridization-duplication events. Autopolyploidy would have doubled a single species' genome, producing multiple identical chromosome sets rather than the genetically diverse combination wheat has. The allopolyploid origin explains why modern wheat has exceptional genetic diversity and adaptability.

Question 3

Polyploidy is a valid mechanism for sympatric speciation — producing a new species within the same geographic area as the parent species, without physical separation.

• T. True
• F. False

Answer: True

Polyploidy is in fact the clearest known mechanism for sympatric speciation. Because it creates immediate reproductive isolation through chromosome number incompatibility, geographic separation is completely unnecessary. A new polyploid individual arises within the range of its diploid ancestors and is instantly reproductively isolated from them. This contrasts with most sympatric speciation models, which require special ecological conditions or assortative mating to build reproductive barriers gradually. Polyploidy achieves it in one event.

Question 4

After a polyploidy event, duplicated gene copies in the new polyploid are subject to the same selective pressures as the original gene and are unlikely to evolve new functions.

• T. True
• F. False

Answer: False

Gene duplication is a major source of evolutionary novelty. When a gene is duplicated (whether by polyploidy or local duplication), one copy can maintain the original essential function while the other is 'freed' from selective constraint — mutations that would have been lethal when only one copy existed are now tolerated. Over time, the duplicate can accumulate mutations that give it a new function (neofunctionalization) or the two copies can subfunctionalize (each taking on a subset of the original function). Polyploidy thus contributes not just to instant speciation but to long-term adaptive evolution by creating raw genetic material for new functions.

Question 5

Explain why triploid organisms are typically sterile, using what you know about meiosis and chromosome pairing.

Model answer: Meiosis requires homologous chromosomes to pair and segregate evenly. In a diploid, every chromosome has exactly one homolog to pair with. In a tetraploid, every chromosome has three homologs and can form stable pairs. But in a triploid, every chromosome has two homologs — one too many for standard pairing. During meiosis I, chromosomes cannot segregate evenly to the poles; some gametes end up with one copy of a chromosome, others with two. The resulting gametes have random, unbalanced chromosome numbers (aneuploid), and most are non-functional. The rare viable gametes are genetically unbalanced and typically produce inviable offspring when fertilized.

This steric incompatibility is the mechanical basis of polyploidy-based reproductive isolation. The tetraploid is fertile because it can form 2n gametes (each chromosome still has a pairing partner), but the triploid hybrid cannot recover from the 3-copy problem. Understanding this clarifies why even-numbered polyploids (tetraploid, hexaploid) can be fertile while triploids are not — and why seedless watermelons, which are triploid, require planting near a diploid pollinator to set fruit.