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.

The Diversity of Plant Sex Chromosomes Highlighted through Advances in Genome Sequencing

“…Across land plants exists an amazing variety of strategies for sexual reproduction [1]. Species have independently evolved self-incompatibility loci [2], temporal variation in flower development [3,4], and spatial distancing of male and female organs on the same plant [5][6][7], among many others [1]. Perhaps the most extreme case is dioecy, where sex-specific structures develop on separate plants.…”

The diversity of plant sex chromosomes highlighted through advances in genome sequencing

“…Across land plants exists an amazing variety of strategies for sexual reproduction [1]. Species have independently evolved self-incompatibility loci [2], temporal variation in flower development [3,4], and spatial distancing of male and female organs on the same plant [5][6][7], among many others [1]. Perhaps the most extreme case, however, is dioecy, where sex-specific structures develop on separate plants.…”

Genetics of quantitative traits with dominance under stabilizing and directional selection in partially selfing species

“…2017) and additive genetic variance (e.g., Hansen et al. 2011; Opedal 2019). Importantly, this model rests on the assumption that populations mate randomly.…”

“…The rationale for focusing on additive models is that the response to selection can be conveniently predicted as the product of the additive genetic variance and the selection gradient acting on a trait (Lande 1979). This simple model has facilitated the highly successful research programme in evolutionary quantitative genetics, which has yielded countless empirical estimates of natural selection (e.g., Siepielski et al 2017) and additive genetic variance (e.g., Hansen et al 2011;Opedal 2019). Importantly, this model rests on the assumption that populations mate randomly.…”