Questions: Polyploidy and Instant Speciation in Plants

Question 1

A tetraploid plant (4n) arises spontaneously in a field of diploids (2n). It can self-fertilize and produce viable seeds. A researcher attempts to cross it with the original diploid parent. What happens, and why?

• A. The cross produces fertile diploid offspring because the tetraploid contributes a normal n gamete through meiosis
• B. The cross produces fertile tetraploid offspring because the tetraploid's extra chromosomes compensate for the diploid's reduced set
• C. The cross produces triploid offspring that are sterile because three chromosome sets cannot pair evenly during meiosis
• D. The cross fails entirely because polyploid plants cannot fertilize diploid plants at the gamete level

Answer: (C)

The tetraploid (4n) produces 2n gametes via normal meiosis; the diploid produces n gametes. Their fusion creates a 3n triploid. Triploids are almost always sterile because during meiosis I, each group of three homologous chromosomes cannot pair evenly — one chromosome is always left without a partner, producing unbalanced gametes with incorrect chromosome numbers. This is how polyploidy creates instant reproductive isolation: the tetraploid can mate with other tetraploids (producing 4n offspring) but not successfully with diploids. No geographic separation, no gradual accumulation of incompatibilities — just arithmetic.

Question 2

Why is allopolyploidy especially effective at producing fertile new species, compared to simple hybridization between two species without chromosome doubling?

• A. Allopolyploidy doubles the number of genes, giving the new organism a growth advantage that allows it to outcompete the parental species
• B. The initial interspecific hybrid is typically sterile because divergent chromosomes cannot pair in meiosis; genome doubling gives every chromosome its own pairing partner, instantly restoring fertility
• C. Allopolyploidy creates entirely new gene combinations through recombination that neither parental genome could produce
• D. Allopolyploids are more reproductively isolated from parental species because they bloom at a different time of year

Answer: (B)

The key insight is the two-step mechanism. Step 1: two species hybridize, but because their chromosomes are too divergent to pair properly in meiosis, the hybrid is sterile — it cannot produce balanced gametes. Step 2: whole-genome duplication gives every chromosome a partner (the duplicate of itself), restoring the ability to undergo regular meiosis. Now the organism is fertile — but only with other allopolyploids carrying the same doubled genome. It cannot successfully cross back with either parent, making it a reproductively isolated species from the moment it arises. This is why allopolyploidy is called 'instant speciation.'

Question 3

Polyploid speciation requires geographic isolation because the new polyploid population should be separated from its parent species to accumulate enough genetic differences to become reproductively isolated.

• T. True
• F. False

Answer: False

This is precisely what makes polyploidy unique among speciation mechanisms. Reproductive isolation is not accumulated gradually — it is immediate. A newly formed tetraploid cannot produce fertile offspring by crossing with diploids (the cross produces sterile triploids), regardless of whether the two populations are geographically separated. Polyploidy is one of the clearest exceptions to the general rule that speciation requires geographic isolation, and it is a major reason why sympatric speciation — speciation within a shared geographic range — is common in plants.

Question 4

Triploid organisms are typically sterile because three sets of homologous chromosomes cannot pair evenly during meiosis I.

• T. True
• F. False

Answer: True

During meiosis I, homologous chromosomes must form bivalents (pairs of two) that then segregate to opposite poles. In a triploid, each chromosome type is present in three copies. When these try to pair, one chromosome from each trio is left without a partner, and the segregation process produces gametes with unbalanced chromosome numbers. These unbalanced gametes are unable to develop into viable offspring after fertilization. This is why seedless watermelons and bananas are triploids — they cannot undergo normal meiosis and therefore cannot produce seeds.

Question 5

Explain why a newly formed tetraploid plant is immediately reproductively isolated from its diploid parent population, even if they grow side by side.

Model answer: Reproductive isolation arises from arithmetic, not from accumulated genetic divergence. The tetraploid produces 2n gametes through normal meiosis; the diploid produces n gametes. When they cross, the offspring are triploid (3n). Triploid organisms cannot complete meiosis normally because their three chromosome sets cannot pair evenly, producing unbalanced gametes and infertile offspring. Meanwhile, two tetraploids can cross successfully, producing fertile 4n offspring. The tetraploid is therefore immediately isolated from diploids (crosses produce sterile offspring) while being reproductively compatible with other tetraploids. No geographic barrier or time is needed — the ploidy mismatch is the barrier.

This is the mechanistic heart of polyploid speciation and what distinguishes it from all gradual speciation models. The new species arises in a single generation. This is why polyploidy has been such a powerful driver of plant diversification — it provides a reproducible mechanism for instant speciation that does not depend on the rare coincidence of geographic isolation followed by divergence followed by secondary contact.