Evolutionary Potential of Herkogamy

Abstract

Herkogamy, the spatial separation of male and female sexual functions in flowers, is an important floral trait mediating variation in plant mating systems. Understanding the evolutionary potential of herkogamy may, therefore, yield insights into the evolutionary potential of the mating system itself. Herkogamy differs from typical floral traits in being a composite trait determined by the relative positions of anthers and stigmas. This feature creates challenges for measuring the evolvability of herkogamy but may also contribute to comparatively high evolvability. Empirical studies have demonstrated substantial evolvability of herkogamy, and that selection on herkogamy can be strong in the event of rapid environmental change such as the decline of pollinators. These results suggest that the evolutionary potential of herkogamy is high, and that detectable evolutionary change can be expected within a few generations following changes in the reproductive environment.

Key Concepts

  • Herkogamy is a key floral trait associated with variation in plant mating systems and often varies widely among populations and species.
  • Herkogamy is a composite trait determined by the relative positions of anthers and stigmas within flowers, and the evolution of herkogamy may involve changes in the positions of anthers and stigmas, the variances in anther and stigma positions or both.
  • Evolution by natural selection is a two‐step process, the rate of which depends on the strength and direction of natural selection and the potential of traits to evolve (i.e. evolvability).
  • Compared to other floral traits, and morphological traits in general, herkogamy can evolve rapidly in percentage of the trait mean.
  • Natural selection on herkogamy is often weak when cross‐pollination is reliable, but strong when cross‐pollination is unreliable.
  • Strong selection combined with high evolvability means that herkogamy can evolve rapidly in response to environmental change.

Keywords: evolvability; plant mating systems; plant‐pollinator interactions; natural selection; quantitative genetics

Figure 1. Flowers of Clarkia xantiana visited by a specialist bee pollinator. Reproduced with permission from Moeller et al.,. © John Wiley and Sons.
Figure 2. Simple floral architecture (a) with a positive correlation (b) between pistil length (x) and stamen length (x). Here, herkogamy can be measured as x – x (c, d), which corresponds to the phenotypic direction perpendicular to the one‐to‐one slope between the lengths of the pistil and the stamens (along the black arrow in b). Average x – x corresponds to the distance between the population mean and the one‐to‐one slope. Vertical dashed lines indicate mean herkogamy on a signed ratio scale (d), and on a ratio scale (i.e. the mean of the absolute values; e). Reproduced with permission from Opedal et al. . © John Wiley and Sons.
Figure 3. Evolvabilities of flower size, male structures, female structures and herkogamy (on ratio and signed‐ratio scales), given as percentages (i.e. multiplied by 100). Thick lines within boxes indicate median values for each trait category. Boxes extend from the first to third quartile, range bars extend to 1.5× the inter‐quartile range, and data points outside this range are shown as open circles. Source: Data from Opedal et al. .
Figure 4. (a) The strength and direction of directional phenotypic selection on herkogamy can be estimated as the regression slope of relative fitness on herkogamy. (b) Expected relationship between pollination reliability and selection on herkogamy, where pollination reliability represents the abundance and efficiency of pollinators.
Figure 5. Outcome of experimental evolution of Mimulus guttatus flowers in the presence (B1, B2) or absence (A1, A2) of pollinators. Populations excluded from pollinators evolved reduced herkogamy and increased capacities of autonomous selfing. Reproduced with permission from Bodbyl Roels and Kelly . © John Wiley and Sons.
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Further Reading

Harder LD and Barrett SCH (2006) Ecology and Evolution of Flowers. Oxford: Oxford University Press.

Herlihy CR and Eckert CG (2007) Evolutionary analysis of a key floral trait in Aquilegia canadensis (Ranunculaceae): genetic variation in herkogamy and its effect on the mating system. Evolution 61: 1661–1674.

Lynch M and Walsh B (1998) Genetics and Analysis of Quantitative Traits. Sunderland, MA, United States: Sinauer Associates.

Opedal ØH (2018a) The evolvability of animal‐pollinated flowers: towards predicting adaptation to novel pollinator communities. The New Phytologist. DOI: 10.1111/nph.15403.

Opedal ØH (2018b) Herkogamy, a principal functional trait of plant reproductive biology. International Journal of Plant Sciences 179: 677–687. DOI: 10.1086/700314.

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How to Cite close
Opedal, Øystein H(Jan 2019) Evolutionary Potential of Herkogamy. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028258]