OVERVIEW

Bumblebees and flowers have long been suspected to exhibit a form of positive coadaptation. While flowers use bumblebees in pollination, bumblebees seek flowers for nectar production. The relationship was known to involve natural selection and has recently been accepted as a strong form of sexual selection. Due to the relationship’s high relevance in both organisms’ life cycles, patterns of coevolution are seen both in generalist and specific plants and bumblebees, forming a classic example of mutualism. In this paper, we discuss this bond in terms of symmetry, vividity, and organ accessibility in flowers, and streamlined morphology, hair presence, and memory in bumblebees. We also discuss the relevance of patterned flight.

Flower Bilateral Symmetry and Bumblebee Behavior

Bees are known to positively perceive bilateral symmetry (Moller 1995.) To negate the possibility of conditioning as an explanation, this relationship was found in “flower-naive” bees, or bees with no exposure to floral patterns, concluding the preference to be “innate” (Rodriquez et al. 2004.) Additionally, the form of symmetry which is most preferred - di-symmetry - is known to specifically be an “evolutionary intermediate” (Culbert 2016.) This research proposes two forms of selection towards the trait: a directional natural selection from the pollinator, and an “interaction-independent” sexual selection on the flower side (Murphy 1998.) Although this is not perfectly in line with the Darwinian “special case” of sexual selection since it does not involve a direct interaction, reproductive success in flowers has a lot to do with pollination frequency, and so sexual selection is a valid theory.

I. Flower Vividity and Bumblebee Behavior

The trait of vividity - expressed as high contrast petal colors - makes flowers more detectable for bumblebees (Spaethe et al. 2001.) This is a mutualistic advantage, reducing bumblebee flight, and therefore energy expenditure. Similarly, the trait of detecting this vividity in bees is selected for (Moller and Eriksson 1995.) Colors prove to have a unique effect on the pollinator-flower relationship - specific vivid colors select for specific pollinators. Therefore, vividity could help generalist flowers attract a larger number of bumblebees at appropriate evolutionary stages (Yan et al. 2016.)

Some of these variations particularly affect bumblebees. A good example of this would be color contrasts affecting bumblebee memory. High throughput sequencing was used on bumblebees experiencing “extreme” memories, such as very new colors. There was a notable change in gene expression, even at small levels of cognition. This was also one of the breakthrough examples of gene identification in coevolutionary patterns. Amongst the 55+ genes that were affected post-acclimatization, several candidate genes could be identified in the study, which helps concentrate efforts, and increases candidate gene approach possibility, and solidification of natural selection as a legitimate theory for memory resolution (Li et al. 2018.)

II. The Link Between Bumblebee And Flower Morphology

While we have talked about vividity adding to pollinator specificity. A similar specificity is exhibited by bumblebees. To increase efficiency in collecting pollen and nectar, bees have developed “fine adjustments” which could only truly be explained by a slow, complex form of adaptation. Similarly, the flower attracts almost solely the kind of bumblebees with these adjustments and produces nectar for these bees. This is called a “fine-tuned, one-to-one coevolutionary mechanism” (Shimizu et al. 2014.)

The idea of using fine adjustments and exhibits like vividity to assert specificity is an important finding, since some of the earlier work on the coevolution of bees and flowers was focused on plants like Delphinium nelsoni, Delphinium barbeyi, Aconitum columbianum and Epilobium angustifolium, where the relationship was entirely mutually exclusive (Pyke 1978.) Therefore, these findings provided some of the first breakthroughs on coevolutionary links between more generalist bumblebees and flowers, and how evolution often end ups driving which organisms they interact with.

Similarly, bumblebees have undergone trade-offs between body sizes and the environment they grow in. Plasticity in bumblebees has been shown to be more affected by food availability - and therefore, flower nectar - than by thermoregulation. In fact, even where it is impacted by temperatures and climates, bumblebees change morphologically in warmer and wetter periods of the year due to the increased possibility of nectar production by flowers at these times (Peat et al. 2005.) These are all signs of bumblebee morphology being adapted to plants.

III. Why This Research Matters

This research matters due to two reasons: understanding this mode of coevolution, and understanding the threat bees and bee memory currently faces. Coevolution between flowers and bumblebees is a classic form of coevolution in plants, and plant sexual selection is very heavily focussed on interactions with pollinators like bees. However, our idea of both of these evolutionary terms often is limited to animals exclusively and plants exclusively. Understanding this idea would leap our idea of the applicability of evolution. Lastly, bee learning could be negatively affected by agricultural pesticides like neonicotinoids (Stanley et al. 2015.) Therefore learning needs to be understood holistically, and action needs to be taken to prevent bee elimination.

Overall, this relationship has caused significantly different traits in both bumblebees and flowers, which would not be considered evolutionarily “optimal” if mutualism was not considered. This example shows why the consideration of interactions between organisms is so important to understanding both natural and sexual selection patterns within the organism, and why the Darwinian model and other successful evolutionary models have always succeeded due to observing an organism in an ecosystem, and not isolated from its habitat.

Future Directions

There is a lot of research on kin selection and the generation formation of kins in bees. This can serve as a starting point into this coevolutionary link, and how it differs between bees at different “levels” of a kin.

Outline of Referenced Works

Flower Bilateral Symmetry and Bumblebee Behavior: Moller 1995, Rodriquez et al. 2004, Culbert 2016, Murphy 1998.

Flower Vividity and Bumblebee Behavior: Spaethe et al. 2001, Moller and Eriksson 1995, Yan et al. 2016, Li et al. 2018.

The Link Between Bumblebee And Flower Morphology: Shimizu et al. 2014, Pyke 1978, Peat et al. 2005.

Why This Research Matters: Stanley et al., 2015.

References

Møller AP, Eriksson M (1995.) “Pollinator preference for symmetrical flowers and sexual selection in plants.” Oikos 73:15-22.

Moller, AP (1995.) "Bumblebee Preference for Symmetrical Flowers." Proceedings of the National Academy of Sciences 92, 6.

Rodriguez, Ivana, Andreas Gumbert, Natalie Hempel De Ibarra, Jan Kunze, and Martin Giurfa (2004.) "Symmetry Is in the Eye of the Beeholder: Innate Preference for Bilateral Symmetry in Flower-naive Bumblebees." Naturwissenschaften 91, 8.

Culbert, B M (2016.) “Floral Symmetry Affects Bumblebee Approach Consistency in Artificial Flowers.” Journal of Pollination Ecology 18, 1.

Murphy, Christopher G (1998.) "Interaction-Independent Sexual Selection and the Mechanisms of Sexual Selection." Evolution 52, 1 :8-18.

Spaethe J, Tautz J, Chittka L (2001.) “Visual constraints in foraging bumblebees: Flower size and color affect search time and flight behavior.” Proceedings of the National Academy of Sciences of the United States of America. 98, 7:3898-3903.

Yan J, Wang G, Sui Y, Wang M, Zhang L (2016.) “Pollinator responses to floral colour change, nectar, and scent promote reproductive fitness in Quisqualis indica (Combretaceae).” Scientific Reports. 6:24408.

Li, Li, Songkun Su, Clint J. Perry, Maurice R. Elphick, Lars Chittka, and Eirik Søvik (2018.) "Large-scale Transcriptome Changes in the Process of Long-term Visual Memory Formation in the Bumblebee, Bombus Terrestris." Scientific Reports 8, 1.

Shimizu, Akira, Ikumi Dohzono, Masayoshi Nakaji, Derek A. Roff, Donald G. Miller III, Sara Osato, Takuya Yajima, Shûhei Niitsu, Nozomu Utsugi, Takashi Sugawara, and Jin Yoshimura (2014.) “Fine-tuned Bee-Flower Coevolutionary State Hidden within Multiple Pollination Interactions.” Scientific Reports 4:3988.

Pyke, GH (1978.) “Optimal foraging in bumblebees and coevolution with their plants.” Oecologia 36,281-293.

Peat J., Darvill B., Ellis J., and Goulson D.. (2005.) “Effects of climate on intra‐ and interspecific size variation in bumble‐bees.” Funct. Ecol. 19:145–151.

Stanley DA, Smith KE, Raine NE (2015.) “Bumblebee learning and memory is impaired by chronic exposure to a neonicotinoid pesticide.” Scientific Reports 5, 1:16508.