Heterostyle Breeding Systems


Heterostyly, which is also known as heteromorphic self‐incompatibility, is one of a variety of breeding systems that have evolved to promote out‐crossing in flowering plants. Unusually, heterostyly combines both morphological and physiological traits to prevent self‐fertilisation. The multiple polymorphisms that comprise heterostyly are controlled by multiple genes, which necessarily segregate as a single genetic unit, often termed a ‘supergene’. Though heteromorphic self‐incompatibility is not particularly common in the angiosperms, it has evolved independently many times with surprising phenotypic consistency and is a remarkable example of convergent evolution. As a result, these breeding systems have received considerable research attention over the last 150 years, but substantial advances in our understanding of these systems have been made recently, largely as a result of reduced deoxyribonucleic acid (DNA) sequencing costs. These advances include identification of some genes that mediate heterostyly and have necessitated re‐evaluation of long‐standing dogma concerning the genetics and operation of these systems.

Key Concepts

  • Heterostyle breeding systems promote outbreeding in flowering plants.
  • Many independent evolutionary gains of heterostyly occurred in the angiosperms.
  • Heterostyly is encoded by ‘supergenes’ S‐loci that contain multiple linked genes.
  • The polyphyletic nature of heterostyly suggests that a different genetic basis underlies heterostyly in different genera.
  • S‐Loci in heterostyle species are hemizygous, being present as large insertions/duplications specific to the dominant alleles.
  • Self‐incompatibility in heterostyle species results from broad physiological differences rather than lock and key recognition systems.
  • Morphological characters may function to provide disassortative pollination, but at least some are likely to be inherent byproducts of pathways generating SI.

Keywords: heterostyle; heteromorphic self‐incompatibility; homostyle; convergent evolution; outbreeding; pollination

Figure 1. Diagram illustrating the reciprocal herkogamy seen in self‐incompatible wild type heterostyle flowers (a) and organ arrangements seen in the two forms of self‐compatible homostyle found (b).
Figure 2. (a) Half flowers of L‐ (left) and S‐morph (right) flowers of Primula vulgaris illustrating the stigma located in the mouth of a corolla tube and recessed anthers in the L‐morph and reciprocal positioning in the S‐morph. (b) View of L‐ (left) and S‐morph (right) flowers of Primula vulgaris, again illustrating the stigma at the corolla tube mouth of the L‐morph and anthers in the tube mouth in the S‐morph, which lead to the terms ‘pin’ and ‘thrum’ being used on this genus.
Figure 3. S‐ (a) and L‐ (b) flowers of Turnera subulata, illustrating open, bowl‐shaped flowers exhibiting heterostyly (for which the descriptive terms ‘pin’ and ‘thrum’ are inappropriate).


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Further Reading

Barrett SCH (1992) Evolution and Function of Heterostyly. Berlin: Springer‐Verlag.

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Franklin‐Tong VE (2008) Self‐Incompatibility in Flowering Plants. Berlin: Springer‐Verlag.

Richards AJ (1997) Plant Breeding Systems, 2nd edn. London: Chapman and Hall.

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McCubbin, Andrew(Mar 2019) Heterostyle Breeding Systems. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0027953]