Speciation in Plants (HL) (HL IB Biology)

• In most situations speciation is a slow process; this is due to the slow rate at which allele changes accumulate
• In some plant species speciation can happen within a single generation; this is known as abrupt, or instant, speciation
• Abrupt speciation in plants can occur because plant cells are able to remain viable even when they are polyploid
  • Polyploid cells have more than two sets of chromosomes, e.g.
    • 3n = triploid
    • 4n = tetraploid
  • This is in contrast to normal body cells which are diploid (2n), and gametes which are haploid (n)

Polyploidy in plants

• Polyploidy can arise within a species; this is known as autopolyploidy, e.g.
  • Meiosis may occur incorrectly
    • During normal meiosis homologous pairs are divided equally into new daughter cells
    • If this separation fails, one daughter cell may contain two sets of chromosomes
      • The failure of chromosomes to separate fully is known as chromosome nondisjunction
    • The resulting diploid (2n) gamete can then fuse with a normal gamete to produce a 3n zygote, or with another diploid gamete to produce a 4n zygote
      • 3n zygotes are likely to be sterile, but a 4n zygote would be able to undergo meiosis
• Polyploidy can arise between different species; this is known as allopolyploidy
  • The diploid gametes from individuals of different species fuse together to produce a polyploid zygote
  • The resulting zygote is a polyploid hybrid
• Polyploid offspring may be so different to their parents that they are unable to breed with them to produce fertile offspring; this results in sympatric speciation
  • When this occurs as the result of allopolyploidy it is known as allopolyploid speciation
• Polyploidy in animals is very rare, but it occurs frequently in plants
• Polyploid varieties of plant appear to be successful, and it is thought that this could be due to advantages such as:
  • Being polyploid may allow hybrids that would otherwise be infertile to carry out meiosis due to their additional chromosomes
  • Polyploid plants are often larger and more vigorous than their diploid parents
  • Having more copies of each gene reduces the impact of any negative mutations that may arise as harmful alleles are masked

Polyploidy diagram

Separate meiosis events within an individual may produce either a 1n gamete OR a 2n gamete, which can result in a 3n zygote after self fertilisation (top)

Meiosis with nondisjunction in individuals from two different species can result in 2n gametes, which can lead to a 4n zygote if fertilisation occurs (bottom)

Examples of polyploidy: Persicaria

• The plant genus Persicaria, commonly known as smartweeds, contains a range of ploidy types
  • Persicaria foliosa is diploid (2n)
  • Persicaria japonica is tetraploid (4n)
  • Persicaria puritanorum is hexaploid (6n)
• It is thought that tetraploid species could have arisen by allopolyploidy between two diploid species, and that hexaploid species could have arisen by a hybridisation event between a diploid and a tetraploid species

Examples of polyploidy: Fallopia

• The genus Fallopia, commonly known as knotweeds, also contain polyploid species
  • Fallopia japonica (japanese knotweed) is octoploid (8n)
  • Fallopia sachalinensis (giant knotweed) is tetraploid (4n)
  • Fallopia xbohemica (bohemian knotweed) is hexaploid (6n)
• Bohemian knotweed is a polyploid hybrid of japanese and giant knotweed
  • Japanese and giant knotweed would have undergone normal meiosis in this instance to produce 4n and 2n gametes
• Japanese knotweed is a famously invasive species, and its polyploid nature is thought to aid its vigorous growth
  • Bohemian knotweed is thought to be even more vigorous

W. Carter, CC0, via Wikimedia Commons

Japanese knotweed is highly invasive. It is an example of a polyploid species.