Pollinator divergence and pollination isolation between hybrids with different floral color and morphology in two sympatric Penstemon species

Abstract

Differential visitation of pollinators due to divergent floral traits can lead to reproductive isolation via assortative pollen flow, which may ultimately be a driving force in plant speciation, particularly in areas of overlap. We evaluate the effects of pollinator behavioral responses to variation of intraspecific floral color and nectar rewards, on reproductive isolation between two hybrid flower color morphs (fuchsia and blue) and their parental species Penstemon roseus and P. gentianoides with a mixed-pollination system. We show that pollinators (bumblebees and hummingbirds) exhibit different behavioral responses to fuchsia and blue morphs, which could result from differential attraction or deterrence. In addition to differences in color (spectral reflectance), we found that plants with fuchsia flowers produced more and larger flowers, produced more nectar and were more visited by pollinators than those with blue flowers. These differences influenced the foraging behavior and effectiveness as pollinators of both bumblebees and hummingbirds, which contributed to reproductive isolation between the two hybrid flower color morphs and parental species. This study demonstrates how differentiation of pollination traits promotes the formation of hybrid zones leading to pollinator shifts and reproductive isolation. While phenotypic traits of fuchsia and red flowers might encourage more efficient hummingbird pollination in a mixed-pollination system, the costs of bumblebee pollination on plant reproduction could be the drivers for the repeated shifts from bumblebee- to hummingbird-mediated pollination.

Figure 1

Visitation rate of bumblebees and hummingbirds on Penstemon blue and fuchsia flowers. (A) visitation frequency (%), (B) number of visits per plant (foraging bouts), (C) visit duration to focal plant, (D) number of flowers probed per foraging bout. The figure was generated using ggplot2 in R (https://cran.r-project.org/web/packages/ggplot2/index.html) and edited in Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.).

Figure 2

Receiver-centric variables (chroma) of a tetrahedral color space analysis of blue, fuchsia, Penstemon gentianoides, and P. roseus. (A) red, (B) blue, (C) ultraviolet, (D) green. Plot of variables using a sensory based analysis indicating the location of each point in a tetrahedral color space for avian vision (E) and bi-dimensional for bee vision (F) of blue and fuchsia flowers, with black dots corresponding to individual observations and grey dot represents the mean value. (G) Plot showing chromatic contrasts against the background of fuchsia, blue, P. gentianoides, and P. roseus flowers for hummingbirds (closed circles) and bumblebees (open circles) color perception. Data are color distances (in units of just noticeable differences, JNDs) by color contrast (y-axis) for six contrasts. The dotted horizontal lines indicates JND = 1 (Weber), above which the pair of color contrast is considered to be distinguished by bumblebees and hummingbirds. Points and error bars indicate mean ± standard error chromatic distances between different species and hybrid flower color morphs. *Significance at the 0.05 level, t-test. The plots in figure were generated using ggplot2 in R (https://cran.r-project.org/web/packages/ggplot2/index.html) and edited in Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.).

Figure 3

Main pollinators of Penstemon blue and fuchsia flowers. (A) Selasphorus platycercus (by C. Lara); (B) Bombus ephippiatus (by J. Cardona); (C) fruit production and effectiveness of bumblebees and hummingbirds as pollinators of flowers within each color morph; (D) fruit production after reciprocal crosses between species, Penstemon gentianoides and P. roseus, and between hybrid flower color morphs. Data (mean ± SE) with the same superscript letters are not significantly different between groups (P < 0.05). Tukey post hoc tests: P < 0.05 for all pairwise comparisons. The plots in figure were generated using ggplot2 in R (https://cran.r-project.org/web/packages/ggplot2/index.html) and edited in Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.).

Figure 4

Study site, study species and spectral color reflectance measures of the blue and fuchsia morphs. (A) La Malinche National Park (LMNP), Tlaxcala, Mexico. The geographic map was generated with the software ArcGIS Desktop 10.1 (ESRI 2001, Redlands, CA) based on satellite imagery source (https://earth.google.com/web/@19.22446138,-98.03311368,4072.84173853a,26147.38187905d,35y,0h,0t,0r) from Google (Google Maps satellite La Malinche National Park, Tlaxcaka, Mexico; retrieved February 17, 2019). ArcGIS Desktop 10.1 software was downloaded from https://arcgis-sp1-for-desktop-quality-improveme.software.informer.com/. Dots indicate the exact locations of floral patches of the blue and fuchsia morphs. The figure was drawn using Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.). (B) Penstemon gentianoides (by C. Lara). (C) blue (left) and fuchsia (right) morphs (by C. Lara). (D) Penstemon roseus (by C. Lara). (E) spectral color reflectance measures of the parental species and the blue and fuchsia morphs (±SE). The plot was generated using ggplot2 in R (https://cran.r-project.org/web/packages/ggplot2/index.html) and the figure drawn and edited in Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.).

Figure 5

Experimental design of a hand-pollination experiment. The pollination treatments included intraspecific crosses, interspecific crosses, intermorph or intramorph crosses, and backcrosses among Penstemon gentianoides, P. roseus, and the ‘blue’ and ‘fuchsia’ hybrid flower color morphs. J. F. Ornelas drew the flower image in figure. The figure was drawn and edited in Adobe Illustrator CS6 v16.0.0 (Adobe Systems, Inc.).