Insects are ecologically dominant in terrestrial ecosystems in terms of animal biomass and biodiversity. Our lab investigates some of the factors that helped insects to conquer the world using advanced techniques of molecular biology and next generation sequencing methods. We use termites and cockroaches as models, and particularly focus our research on three factors that contributed to the success of termites and cockroaches: (1) the evolution of symbiotic association with microorganisms, helping insects to thrive on new food sources; (2) the historical biogeography of insects, and the factors at the origin of their cosmopolitan distribution; (3) the molecular mechanisms used by termites to synthesize their defensive compounds.
(1) Evolution of symbiosis between insects and microorganisms
Termites, with the help of bacteria, protists and/or fungi, are among the rare organisms that feed on wood and soil. Bacteria provide enzymes and synthesize essential nutrients from nitrogen wastes. Similarly, cockroaches are associated with Blattabacterium, a bacteria participating to the recycling of uric acid and providing essential amino acids to their host cockroach. In both cases, the insect host and the bacteria/protist/fungus symbionts cannot survive without each other.
The symbiosis between insects and bacteria involves partners that act conjointly, and have biosynthetic and enzymatic pathways that complement each other. We investigate the molecular mechanisms of insect-bacteria association using genome and transcriptome sequencing and intend to answer these questions: How wood- and soil-feeding termites obtain essential nutrients? What is the role of gut fauna and endosymbionts in termite digestion and nutrient recycling? Why some cockroaches lost their Blattabacterium? How genomes of symbiotic microbes evolve over time?
(2) Historical biogeography of insects
Dispersal events, and the timing of those dispersal events, can be inferred using molecular phylogenies. We have used full mitochondrial genomes to reconstruct termite phylogenies, and resolve the origin of their worldwide distribution patterns. Our results show that termites became the dominant decomposers of tropical rainforests during the global cooling which took place at the end of the Eocene, 34 million years ago, affecting forest ecosystems worldwide. Although these results show that mitochondrial genome-based phylogenetic trees provide excellent resolution for most nodes, with support largely exceeding that of the trees obtained by a few nuclear and mitochondrial genes, some nodes are still weakly resolved, which prevents a full understanding of the historical biogeography of this important insect group.
Thanks to the quick progress of sequencing technologies large dataset can now be sequenced on a regular basis. Our unit uses transcriptomes, from which hundreds of orthologous genes can be extracted, to compute robust phylogenetic trees resolving the topology of yet questionable nodes. We use transcriptome-based phylogenies to study termites and related insects.
(3) Molecular mechanisms of termite defence
Termites form large colonies attracting numerous predators and harshly competing with neighbours. To protect against enemies, termites have evolved various defensive mechanisms, many of which are based on the secretion of toxic compounds. Some defensive compounds are ubiquitous and used by many termites and other insects, such as quinones, some highly reactive molecules derived from aromatic compounds. Our lab studies the biochemical pathways used to synthesize these defensive compounds. Insects can acquire defensive compounds or their substrates from the environment, synthesize the compounds with their own set of enzymes, or acquire them from symbiotic microorganisms. We use a transcriptomic approach to identify the genes involved in the biochemical pathways.