10.2 Applications and skills

10.2.3 Speciation by polyploidy

  • In a diploid cell there are two sets of homologous chromosomes (2n). For example, there are 23 pairs of chromosomes in a human somatic cell (2n = 46).
  • Polyploidy is the condition of having more than two sets of homologous chromosomes. For example, triploid cells have three sets of homologous chromosomes (3n) and tetraploid cells have four (4n).

Figure 10.2.3a – Diploidy, triploidy and tetraploidyFigure 10.2.3a – Diploidy, triploidy and tetraploidy

Figure 10.2.3b – Polyploidy through meiotic errorFigure 10.2.3b – Polyploidy through meiotic error

  • Two different species mate (hybridisation). The hybrid offspring can then reproduce sexually or asexually to form a population of polyploids.

Figure 10.2.3c – Polyploidy by hybridisation of two speciesFigure 10.2.3c – Polyploidy by hybridisation of two species

  • Polyploid populations may become reproductively isolated from diploid populations – the gametes may not fuse successfully, or the embryo may not develop.
  • When the two populations are reproductively isolated, it means that speciation has occurred abruptly. This could happen with no geographic barrier – a form of sympatric speciation.
  • Polyploidy is more common in plants than in animals because it is more likely to be fatal to organisms that have chromosomes for sex determination.

Hybrid vigour

  • Many polyploids are crop species, such as the crop potato, Solanum tuberosum, which is a tetraploid.
  • In general, polyploids make good crops because they are larger and more robust than their diploid counterparts.
  • Many species of crop plants have been manipulated to increase phenotypic variation. The increased diversity makes crops more resistant to changes in the environment. The effect is called hybrid vigour. For example, hexaploid hybrids of Solanum tuberosum and Solanum brevidens are resistant to blight.

Polyploidy in the genus Allium

  • The genus Allium includes onions, leeks, garlic and chives. There are 800 species in this genus, and a huge natural variation in morphology and chromosome number due to polyploidy.
  • Chromosome number can be manipulated to increase allelic diversity and phenotypic variation. For example, researchers have used chemical methods to induce polyploidy in garlic, Allium sativum (2n = 16).
  • Examples of phenotypic and chromosome variation in the genus Allium are listed below: 

Ploidy

Number of chromosomes

Species name

Morphology

2n

24

A. cepa

Figure 10.2.3d – OnionFigure 10.2.3d – Onion

4n

32

A. porrum

Figure 10.2.3e –LeekFigure 10.2.3e – Leek

4n

32

A. tuberosum

Figure 10.2.3f – Chinese chiveFigure 10.2.3f – Chinese chive

8n

66

A. nutans

Figure 10.2.3g –Blue chiveFigure 10.2.3g – Blue chive

 

Figure 10.2.3h – Allium porrumFigure 10.2.3h – Allium porrum
Variation in size due to hybrid vigour: Allium sativum (garlic).

Key concept

Speciation by polyploidy can occur abruptly.

Concept help

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Figure 10.2.3i – S. tuberosumFigure 10.2.3i – S. tuberosum
Most potatoes cultivated around the world are tetraploid: Solanum tuberosum.

Nature of Science

Looking for patterns, trends and discrepancies: Patterns of chromosome number in some genera are explained by speciation due to polyploidy. How can patterns of chromosome number be explained without polyploidy?