The life of a high-powered duo! (2)

Stories, photos, accounts…

Here, finally, our second article about coevolution in plant-pollinator relationships ! We can remind you that coevolution is a mechanism which gathers two distinct populations (here a plant species and a pollinator) that will be able to evolve together by influencing each other during the course of their evolution. Morphological adaptations, trickeries… everything is good to perpetuate its own genes ! No memories? Then a short video to summarize it all : Jonathan Drori talks about communication in plants ^[1] (which is not necessarily sound-based…).

But the connection is even stronger!

In some cases, mutualism is so strong that the respective life cycles of plants and insects are completely interlinked. A major example is the fig tree. Fig trees can be monoecious or dioecious (in the last case, male and female flowers are on different plant trees). Pollination can be only done thanks to a chaldicien wasp of the Agaonides family. Flowers are totally confined in figs, so it is impossible to allow transport of pollen if the insect does not enter the fruits. The insect can then reproduce in figs, and its descendants feed nutrients from the fruit. The life cycles of insects and figs are thus synchronized, according to the species of fig tree : if it is the monoecious or dioecious species.

In the case of dioecious fig trees, some plant trees are male and the others are female. Male fig trees are special, as they contain two types of flowers: male flowers but also female flowers. During the Spring, the male plant attracts a female wasp (from the genus Blastophaga), which has been fertilized and loaded with pollen from another fig tree. The insect can spawn in the figs and then die. During a few weeks, the larvae of this wasp will grow in the fig, consuming the necessary nutrients. The males are the first to hatch and can fertilize females, prisoners in the galls (the pistil of female flowers present in the male fig). This fertilization is the signal of maturity of the male fig, which can then open. Female wasps, fertilized, can therefore leave the fig passing by male flowers of the male fig and as a consequence to be covered by pollen. Several cycles may occur in male figs until mid-July when the attraction of insects for them is finished, replaced by the female fig attractiveness. Female insects fertilized and loaded with pollen, can enter the female figs to spawn. In doing so, they pollinate the female fig: however, their eggs cannot develop. In some species of fig trees, an intersexual chemical mimicry ^[3] takes place : female fig trees, harmful for pollinators because of the anatomy of the flowers which not allow the development of eggs, emit an attractive fragrance similar to the one produced by male figs. That fragrance prevents insects from distinguishing the sex of the tree they visit. Thus the majority of insects of an entire population is deceived, and female figs work as traps with 95% of individuals which die without leaving any offspring. ^[2]&[4] How the insect population succeed in surviving from one year to the next ? Approximately 5% of individuals of a population experience a late development ; therefore they can leave the male fig (in August for the last ones) whereas they become attractive again, and as a consequence maintain the perpetuation of pollinators. As the insect population is largely decimated every year, mutualism has been described as unstable.

Nevertheless there is a second form of mutualism ; it concerns the monoecious fig tree for which mutualism has been described as stable. These trees bear figs containing both pollen-producing male flowers and female flowers, unlike dioecious. Like for dioecious, a female insect fertilized and loaded with pollen enters the fig to lay its eggs and, at the same time, pollinate the female flowers. These flowers can give seeds, and in parallel, a new generation of insects can develop. Male insects are the first to hatch and can help females to emerge, fertilize them and dig the closed fig to lay out an exit tunnel. Female insects can then escape from the fig by charging turn pollen, while the males die soon after they leave the fig. This mutualism is characterized as stable, because both partners have equal reproductive advantage.^[2].

So examples of coevolution between a plant and a pollinator insect are numerous and take diverse forms.

But are we sure we are still dealing with coevolution? Have we necessarily to think about coevolution when it seems that a plant and an insect have developed communication means during their evolution? One study published in 2012 ^[5] asks this complex question. The Aracae plants have developed a system of pollination involving beetles ; these beetles are attracted by the plant which produces chemical molecules usually used by the insects to communicate between each other. Can we talk about coevolution or just about adaptation ? Developing a phylogenetic approach, the research team (Switzerland) at the origins of this study, showed that the ancestors of these insects already used such volatile compounds for communicating during the Jurassic, i.e. 40 million years before the first Aracae plants developed a mode of reproduction based on pollination by beetles. There may be no coevolution here, but a great example of adaptation of plants to a system of communication which was preexisting in the insect population. We should not forget that coevolution relies on a strong genetic basis which allows selection of a behaviour which may provide the plant with a reproductive advantage and a better fitness.

But even if coevolution does not always take place, cross-pollination by insects remains a crucial process for our human societies. Early in the 2000s, the notion of «ecosystem service» has emerged to report the positive action of insects for pollination, in particular in agriculture.^[6]. The loss of this essential ecosystem service, as a dramatic consequence of biodiversity erosion triggered by the human activity, has begun to be calculated and appears to be inestimable. The incurred risk is huge : indeed it concerns all our food supply, besides other economic branches. This awareness led to the elaboration of programmes of conservation and risk assessment for biodiversity, like the European ALARM ^[7] programme (Assessing large environment Risks for biodiversity with tested Methods) ; this project corresponds to the biggest European programme for assessment of biodiversity loss, and it specifically looked into the question of pollinators.

References:

1. Conference of Jonathan Drori for Ted Talks http://www.ted.com/talks/lang/en/jonathan_drori_the_beautiful_tricks_of_flowers.html
2. Harry M., Génétique moléculaire et évolutive 2e édition, Maloine, 2008, pp 382-383
3. C.L.Soler C., Proffit M., Bessière J., Hossaert-McKey M. & Schatz B., Evidence for intersexual chemical mimicry in a dioecious plant, Ecology Letters Volume 15, Issue 9, pages 978-985, September 2012. DOI: 10.1111/j.1461-0248.2012.01818.x
4. Anstett M-C., Kjellberg F.(Directeur de thèse), Contraintes et libertés dans l’évolution des mutualismes figuiers/pollinisateurs, Travaux Universitaires – Thèse nouveau doctorat
   1994 [Note(s) : [73 p.]](bibl.: 137 ref.) (Année de soutenance : 1994) (No : 94 MON2 0184)
5. Schiestl F.P., Dötterl S., The evolution of floral scent and olfactory preferences in pollinators: coevolution or pre-existing bias ?, Evolution Volume 66, Issue 7, pages 2042-2055, July 2012. DOI: 10.1111/j.1558-5646.2012.01593.x
6. La biodiversité des pollisateurs est indispensable, http://www.futura-sciences.com/fr/news/t/developpement-durable-1/d/la-biodiversite-des-pollinisateurs-est-indispensable_5425/
7. Website of the project ALARM http://www.alarmproject.net/alarm/