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 can arise when an individual gains more than two sets of chromosomes from:
  • within a single species; this is autopolyploidy
  • two different species; this is allopolyploidy
• Polyploid varieties of plant appear to be successful, and it is thought that this could be due to advantages such as:
  • polyploidy 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

Autopolyploidy

• Autopolyploid 4n individuals can arise within a 2n plant population: 
  • During meiosis the separation of homologous pairs does not occur correctly, meaning that one daughter cell may contain two sets of chromosomes
    • The failure of chromosomes to separate during meiosis 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

Autopolyploidy diagram

4n individuals can arise within a plant population of 2n individuals; this is autopolyploidy

Allopolyploidy

• To generate allopolyploidy the diploid gametes from individuals of different species fuse together to produce a polyploid zygote
  • Individuals from two different species breeding together is known as hybridisation
  • The resulting zygote is a polyploid hybrid

Allopolyploidy diagram

Meiosis with nondisjunction in individuals from two different species can result in 2n gametes, which can result in a 4n hybrid zygote if fertilisation occurs

Speciation due to polyploidy

• Any 4n individuals in a population will produce 2n gametes, so will be unable to breed with 2n individuals in the original population to produce fertile offspring:
  • A 2n gamete fusing with a normal n gamete will result in a 3n zygote
  • An individual developing from a 3n zygote will be infertile
• A population that is unable to breed with its parent population to produce fertile offspring can be said to be a new species, meaning that speciation has taken place

Speciation due to polyploidy diagram

Polyploid individuals cannot breed with individuals from the parent population to produce fertile offspring, so can be said to be a new species

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.