Manipulation of trait expression and pollination regime reveals the adaptive significance of spur length

Judith Trunschke,Nina Sletvold,Jon Ågren

doi:10.1111/evo.13927

Judith Trunschke, Nina Sletvold + Show 1 more

Open Access

https://doi.org/10.1111/evo.13927

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Abstract

Understanding the mechanisms of adaptive population differentiation requires that both the functional and adaptive significance of divergent traits are characterized in contrasting environments. Here, we (a) determined the effects of floral spur length on pollen removal and receipt using plants with artificial spurs representing the species-wide variation in length, and (b) quantified pollinator-mediated selection on spur length and three traits contributing to floral display in two populations each of the short-spurred and the long-spurred ecotype of the orchid Platanthera bifolia. Both pollen receipt and removal reached a maximum at 28-29mm long spurs in a short-spurred population visited by short-tongued moths. In contrast, pollen receipt increased linearly across the tested range (4-52mm) and pollen removal was unrelated to spur length in a long-spurred population predominantly visited by a long-tongued moth. The experimentally documented effects on pollen transfer were not reflected in pollinator-mediated selection through female fitness or pollen removal indicating that the natural within-population variation in spur length was insufficient to result in detectable variation in pollen limitation. Our study illustrates how combining trait manipulation with analysis of causes and strength of phenotypic selection can illuminate the functional and adaptive significance of trait expression when trait variation is limited.

Highlights

Divergent selection is a key process shaping and maintaining population differentiation and phenotypic diversity (Mitchell-Olds et al 2007; Siepielski et al 2013)
Understanding the mechanisms behind adaptive population differentiation requires identification of traits contributing to local adaptation and an analysis of their relationship to fitness in contrasting environments
Floral depth has been identified as a key trait in adaptive radiations (e.g., Whittall and Hodges 2007; Fernandes-Mazuecos et al 2019), and among-population variation in floral depth has been correlated with the morphology of pollinators in several systems (e.g., Anderson and Johnson 2008; Pauw et al 2009; Boberg et al 2014)

Summary

Introduction

Divergent selection is a key process shaping and maintaining population differentiation and phenotypic diversity (Mitchell-Olds et al 2007; Siepielski et al 2013). In flowering plants that depend on animal pollinators for successful pollen transfer, spatial variation in the composition of pollinator communities is expected to cause variation in selection on floral traits (Grant 1949; Stebbins 1970; Kay and Sargent 2009; Van der Niet et al 2014) This is because pollinators differ in their responses to visual and olfactory signals (Huber et al 2005; Streisfeld and Kohn 2007; Waelti et al 2008) and in their morphological fit with the reproductive parts of the flower (Anderson and Johnson 2009; Newman et al 2014). The effect of morphological fit can differ between components of pollination success: The morphological fit can be less important for pollen removal than for pollen receipt if pollen can attach to several locations on the pollinator’s body, whereas pollen deposition on a stigma requires attachment to a specific area (Cresswell 2000; Ellis and Johnson 2010)

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