Plants and insect pollinators: the adventures of a great couple!

Stories, photos, accounts…

Coevolution is an evolution which takes place in a joint and synchronous way. If we keep in mind that Evolution acts by selecting organisms throughout a historic process, we can thus believe that Evolution may imply coevolution. That is why biologists interested in Evolution usually define coevolution as follows: coevolution can be used when two different populations, each subject to a specific set of constraints, have their evolutionary histories which interact. This relation could lead to specialization, mutualism, and sometimes cospeciation (when two species emerge mutually, one thanks to the other one). Therefore we shall characterize these relations according to their intensity and nature. But you should not forget that coevolution has a strong genetic basis: “in a plant-insect relation, it is necessary to demonstrate that there is a causal relation mutually between a selective action exerted by the insect and a characteristic of the plant”.^[1]

A lot of insects have developed particular relationships with flowers, becoming integrated into their mode of reproduction thanks to pollination. Numerous plant families became entomophiles (i.e. pollinated by insects) during their evolutionary history. An insect carries grains of pollen from a stamen onto a stigma. In exchange for this transport, the insect can take some nectar, a very nourishing substance secreted by the nectar glands of the flower. We can notice that a lot of Angiosperms are pollinated by numerous orders of insects among which Lepidoptera (butterflies), Hymenopterans (bees and wasps), Dipterans (flies) and Beetles are represented.

The concept of coevolution was introduced by Ehrlich and Raven in 1964^[1] to model the intimate relationship developed between some Lepidoptera and plants. This coevolution has taken various forms, techniques and tricks on behalf of plants and insects to take full advantage of this relation, supervised by the evolution and the selection of the most adapted organisms.

About the seduction between plants and insects: some words of love do not hurt

First of all, it is necessary to know that flowers and insects can speak to each other! And their languages are also complex and diversified that ours. To attract insects, flowers are able to turn themselves into real “advertising hoardings”. They indicate to the insects that they own some nectar and how to get it.

For example, numerous flowers have evolved to form inflorescences in capitulum or still sport bright colours to distinguish themselves. Insects perceive the ultraviolet but not the red, and certain flowers could become red after fertilization, making themselves invisible to insects which will then concentrate on other flowers. Of course insects are not aware of their pollination act, and they only look for nourishing juice. As a consequence, flowers have to develop strategies if they want that one insect land on their stamina and takes the precious pollen, guarantee of the survival of their genes. Vincent Albouy, in the laboratory of entomology of the MNHN in Paris, tells about the flower of evening primrose in an article entitled Flowers Speak to Insects^[2] : «[this flower] seems to be yellow for us ; no traces of nectar signal. In reality, it reflects the ultraviolet ray. Yellow + ultraviolet give a visible colour for insects, which the scientists call “purple of the bees”». Furthermore, the flower shows an increasing gradient of ultraviolet ray of the periphery up to its center, as a consequence insects are naturally guided towards the nectar glands.

But plants can also communicate and seduce insects thanks to chemical ways, by secreting nice-smelling compounds which insects can perceive at important distances. Moreover a study recently published in Annals of Botany^[3] investigated this approach, to explain how two sympatric species of fig tree (two species stemming from the same initial population in the same area of geographical distribution) could develop a specific pollination with two different wasps. The compositions of the perfumes of these two flowers are different, therefore they would allow to preserve a different attractiveness according to the insect. To conclude, the specificity of the plant-insect relationship could have a role in speciation (creation of new species), and may generate biodiversity.

Another mode of communication is the most recently discovered electromagnetism. Flying insects are positively charged while the flowers are negatively charged. A team of biologists from Bristol showed that a bumblebee, Bombus terrestris,^[4][5] identifies some flowers in this way: when it approaches flower or lands on them, the electrical potential of the flower is changed for a short time by electrostatic induction. These changes could be perceived by others to identify the flowers already visited.

Who told you that the physical did not count?

And better to be original! A very famous case was presented in 1862 to Charles Darwin. They brought him a very special orchid Agraecum sesquipedale, which has honey glands at the bottom of a long tube 30cm below the flower. Being aware of pollination processes, Darwin hypothesized that there must be a butterfly with a long proboscis. This hypothesis was long criticized by his peers, until the hypothetical butterfly was discovered in 1903, with a trunk 22cm long; it was called Xanthopan morgani praedicta in homage to the famous prediction. This proboscis has been probably acquired by gradual changes in the anatomy of the two species in order to coevolve together towards a specific pollination system.^[6] This reminds us the specific case of the pollination of A. cadetii, in the Mascarene Islands, specifically controlled by a grasshopper.⁷

As one seduces, one can also deceive, and the flowers do not deny!

Flowers have developed a series of tips to be effectively visited by pollinators without having to produce nectar, which costs much energy. But these tips, resulting from evolutionary changes, are constantly threatened by the adaptation of insects, by selecting and learning which can eventually discover the lie. So these recently discovered mimetic orchids^[7] adapted their morphology to resemble another popular species of insects filled with nectar they do not produce. Mimicry is one discovered by Darwin (again and again …) and has been observed in numerous occasions. Other flowers rely on seduction: they took the form of female individuals of a species of insect. Then the male insect attempts to mate with the flower, of course without great success, which has the effect of shaking the anthers which can shed pollen that will be attached to the body of the duped animal and will try his luck elsewhere. However, this technique is only for young insects, researchers having noticed that a form of learning takes place: older already amazed insects will do not make it twice!

So here are exposed to you the adventures of a remarkable relationship developed by pollinating insects and flowering plants. As you will discover in our next article the intimate bond that is created during the evolution and led to situations in the least original… To be continued!

Images

Bombus terrestris. Photo by Gennaro Pascale Caicedo. [cc]by-nc-nd/[/cc] Butterfly. Photo by Amy Lloyd. [cc]by-nc-nd/[/cc] Evening Primrose. Photo by Dean Gugler. [cc]by-nc[/cc] Monarch Butterfly. Photo by Kevin Cole. [cc]by[/cc]

Bibliography

1. Harry M. 2008. Génétique moléculaire et évolutive 2e édition. Maloine. pp 379-380
2. Albouy V., Les fleurs parlent aux insectes , http://www7.inra.fr/opie-insectes/pdf/i133albouy.pdf
3. Wang G., G. Compton S-G & Chen J., 2013. The mechanism of pollinator specificity between two sympatric fig varieties : a combination of olfactory signals and contact cues, Annals of Botany 111: 173-181. DOI: 10.1093/aob/mcs250
4. Morin H., Le bourdon électrifié par les fleurs http://www.lemonde.fr/sciences/article/2013/02/21/le-bourdon-electrifie-par-les-fleurs_1836608_1650684.html
5. Clarke D., Whitney H., Sutton G. & Robert D. , Detection and Learning of Floral Electric Fields by Bumblebees, Science 340 (6128). pp. 66-69. DOI: 10.1126/science.1230883
6. Le sphinx et l’orchidée (l’évolution prédictible), http://www.docsciences.fr/Le-sphinx-et-l-orchidee
7. Vale A., Rojas D., Acanda Y., Sanchez-Abad N-L. & Navarro L., 2012 A New Species of Tetramicra (Orchidaceae: Laeliinae) from Baracoa, Eastern Cuba. Systematic Botany 37(4): 883-892. DOI: 10.1600/036364412X656491