Abstract
Genome duplication in plants is thought to be a route to speciation due to cytotype incompatibility. However, to reduce cross-pollination between cytotypes in animal-pollinated species, distinctive floral phenotypes, which would allow pollinator-mediated assortative mating between flowers, are also expected. Chamerion angustifolium is a Holarctic species that forms a hybrid zone between diploid and tetraploid populations in the North American Rocky Mountains. Extensive research has shown that these cytotypes differ in many ways, including some floral traits, and that pollinators can discriminate between cytotypes, leading to assortative mating. However, two signals commonly used by insect pollinators have not been measured for this species, namely petal colour and floral scent. Using greenhouse-grown diploids and tetraploids of C. angustifolium from the ploidy hybrid-zone in the North American Rocky Mountains, we show that both floral scent signals and petal reflectance differ between cytotypes. These differences, along with differences in flower size shown previously, could help explain pollinator-mediated assortative mating observed in previous studies. However, these differences in floral phenotypes may vary in importance to pollinators. While the differences in scent included common floral volatiles readily detected by bumblebees, the differences in petal reflectance may not be perceived by bees based on their visual sensitivity across the spectra. Thus, our results suggest that differences in floral volatile emissions are more likely to contribute to pollinator discrimination between cytotypes and highlight the importance of understanding the sensory systems of pollinators when examining floral signals.
Highlights
Genome duplication is an important mechanism of speciation in plants (Rieseberg and Willis, 2007; Soltis et al, 2009; Barker et al, 2016) and is notable as one of the few roads to speciation that can occur in sympatry (e.g., Vallejo-Marín et al, 2016)
Floral Signals Vary With Cytotype may face even larger challenges because they are often regarded as being morphologically similar to their diploid progenitors and as such have historically been lumped as the same species (Soltis et al, 2007; Spoelhof et al, 2017)
We found significant differences in petal reflectance between diploids and tetraploids (Table 1), in the 400–500 nm and 600–700 nm range (Figure 2)
Summary
Genome duplication is an important mechanism of speciation in plants (Rieseberg and Willis, 2007; Soltis et al, 2009; Barker et al, 2016) and is notable as one of the few roads to speciation that can occur in sympatry (e.g., Vallejo-Marín et al, 2016). Pollinator-mediated reproductive isolation between cytotypes has strong theorical support and empirical evidence for some species where it has been measured (e.g., Kennedy et al, 2006; Thompson and Merg, 2008; Roccaforte et al, 2015; Sutherland et al, 2020; Laport et al, 2021), few studies have examined differences in multifaceted floral signals between cytotypes making generalisations challenging. Signaling differences are likely for C. angustifolium cytotypes from mixed populations because pollinators do contribute to reproductive isolation in the species, which traits aid those decisions are largely unknown
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