Some plants host aphids, which in turn carry bacteria. The fact that these intricately involved organisms evolve together is what fascinates Emmanuelle Jousselin. Since beginning her research, she has endeavoured to understand the mechanisms of co-evolution, that is, how species that live together mutually influence each other’s evolution.
Since 2004, she has explored plant-aphid-bacteria relationships from a food perspective. Intrigued by the wide variety of aphids’ food choices, she strives to understand how this diversity came about and what role is played by the bacteria that live in symbiosis with the aphids, as well as the adaptation process of aphids to their plant hosts. As a model, she chose aphids of the Brachycaudus genus, which includes several Prunus (plum, apricot, peach and cherry trees) pests and species with very diverse food strategies. In 2006, Jousselin was awarded “Young Researcher” funding for this project by the French national research agency (ANR).
The first step in her research was establishing the evolutionary tree of her model aphid, which she did with the assistance of skilled aphid systematists. Gradually, 28 species of Brachycaudus, of 40 reported worldwide, were collected. Jousselin found many Brachycaudus in France, but also several in Scotland, Sicily and even Kazakhstan, where she conducted her research without any local assistance other than a driver and an interpreter. She tracked the insects by following the sources she had noted and collected the insects in both natural environments and orchards.
The sequencing of a few marker genes enabled Jousselin to sort all 28 aphid species according to degree of genetic relation and to create the evolutionary (or phylogenetic) tree for Brachycaudus, with notes on corrections or questions with regard to classification.
Jousselin then began studying a type of bacteria that lives in the aphids’ gastrointestinal tract and showed that the 28 aphid species were linked to 28 different bacterial strains. Molecular research similar to that conducted on the aphids was carried out on the bacteria, allowing Jousselin to establish the phylogenetic tree of the bacteria. This tree can be superimposed on that of the aphids, proving not only a co-evolution, but a co-speciation.
Finally, she sought to understand the wide-ranging specific food preferences of aphids. Some species feed exclusively off one plant species, while others have no preference and yet others change hosts during their development. She pieced together the entire history of these traits by studying the role of different species in the evolutionary tree and that of associated bacteria, and by looking for morphological and physiological changes that accompany diet modification.
It was a very ambitious approach; as she says herself, “establishing a phylogeny is difficult and involves skills in systematics and sometimes complex data analysis.”
It was very early on that Jousselin developed a hunch that evolutionary history might shed light on the biological functioning of organisms and their interactions. In fact, she studied it as part of her PhD thesis at the Ecole Nationale Supérieure d’Agronomie in Montpellier and her post-doctorate fellowship at the University of Pretoria in South Africa. Her study model at the time was the extraordinary association between figs and a small wasp that reproduces in the fig and pollinates it. With regard to evolution, Jousselin demonstrated that the wasp went from a passive to an active pollination mode through the acquisition of a thoracic pollen basket, which it fills with a specialised behaviour. With regard to functional mechanisms, she showed the adaptation advantage of the wasp’s evolution: pollinated flowers offer the wasp larvae better quality food. To prove this, Jousselin conducted experiments comparing the development of wasps reared on normal figs with wasps reared on non-pollinated figs.
Her original approach compels her to combine a wide range of disciplines and scientific practices: exploration in the field, systematics, molecular biology, behavioural studies and experiments.
Jousselin is part of several groups in which she works towards bringing together evolutionary ecology specialists with so-called “functional” ecology experts. She is involved in the French ComEvol working group and the organisation of its annual conference. She co-manages the local group Community Ecology within her research unit, where she has increased contacts and discussions. In 2007, she was elected to serve on her research unit’s board. Her goal is this: to help researchers “co-evolve” with the various disciplines of ecology!
In September 2007, Jousselin received the INRA Young Researcher Award, which acknowledges exceptional work by a newly recruited researcher.
Emmanuelle Jousselin with her team, © Inra C. Slagmulder
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