Comparative transcriptomics of phenotypic and reproductive castes in termites

Insect societies are characterized by a reproductive division of labour. Only a few individuals reproduce, while the large majority of individuals forgo their own reproduction and perform all the tasks necessary for the maintenance of the colony.

This project aims to identify in the termite Cavitermes tuberosus the genes involved in caste and sex determination, as well as in parthenogenetic reproduction. Indeed, queens of C. tuberosus are replaced by daughters produced by automictic parthenogenesis. These parthenogenetic females go through a developmental stage never described in termites, a caste of «aspirants» that have the features and altruistic behaviour of the workers until they differentiate into secondary reproductive and supersede the queen to produce offspring.

  1. 1.We sequence and de novo assemble the genome of C. tuberosus.

  2. 2.We perform de novo transcriptome assembly and compare gene expression profiles between sterile and reproductive individuals, and between reproductives produced through sexual and asexual reproduction.

  3. 3.We compare the expression pattern of target genes using RT-qPCR. The genes are selected on two criteria: (i) genes known to play an important role in caste determination and parthenogenesis and (ii) target genes selected during the previous transcriptomic study.

Keywords: genome, transcriptome, castes, parthenogenesis, gene family, gene regulation

Wolbachia, a master manipulator or a bacteriocyte-associated nutritional mutualist of the termite Cavitermes tuberosus?
Genome sequence and functional genomic analysis

The neotropical termite Cavitermes tuberosus combines sexual reproduction and thelytokous parthenogenesis with gamete duplication, thereby benefiting from both strategies. One way of realizing thelytoky is through induction by maternally inherited bacteria such as Wolbachia, Cardinium or Rickettsia.
These reproductive parasites, recognized as major actors in Evolution, enhance their own transmission by manipulating the reproduction of their hosts.

In the termite C. tuberosus, all reproductive and altruistic individuals, whether they are male or female, sexually or parthenogenetically produced, are infected with a single Wolbachia strain (wCtub) which belongs to the supergroup F. wCtub is not currently associated with parthenogenesis induction in the Cavitermes lineage. However, wCtub may share a mutualistic relationship with the termite C. tuberosus.

Our project aims to elucidate this symbiotic association by conducting a functional analysis of the Wolbachia genome to identify genes involved in interactions with the host. Whole genome sequencing and transcriptome analysis will shed light on the actual nature of the symbiosis between the bacteria Wolbachia and the termite C. tuberosus, and/or will reveal signatures of past reproductive manipulations.

Keywords: genome, transcriptome, gut symbiosis, nutritional mutualism, fitness of host

Evolutionary ecology of biological invasions - The case of the social insects

Biological invasions have major ecological and economical consequences, especially by disrupting trophic cascades and by homogenising biodiversity. Each invasion process is characterised by three stages: the introduction, the establishment of viable populations, and their proliferation in the new environment. Demographic, ecological and genetic mechanisms occur in each stage.

My researches aim at studying biological invasions in social insects and lie on a combinatory approach including genetic, behavioural, chemical and ecological data. It focuses on demographic, ecological and evolutionary mechanisms that limit or facilitate invasions. Three complementary ways are considered:

  1. 1.The first is based on the study of the ecological and genetical characteristics of invaders;

  2. 2.The second aims at determining the invaded ecosystem's characteristics allowing the invasion by alien populations;

  3. 3.The third way relates to the association between the invading species and the invaded ecosystem. This evolutionary approach investigates two paradoxes:

  4. Why some exotic species succeed in an environment different from the one they are adapted to, sometimes displacing native species?

  5. Why, despite founder effects, invading populations adapt so rapidly to new environmental conditions?

Keywords: evolution, ecology, bioinvasions, adaptation, bottleneck, genetic diversity, endangered species

Ecology of the invasive ant Pheidole megacephala

The big-headed ant Pheidole megacephala, apparently native to southern Africa, is now widespread throughout the temperate and tropical zones of the world. It is favoured by shaded and moist environments, but can exist wherever there is anthropogenic disturbance.

This ant is a serious threat to biodiversity and is one of the 100 of the world's worst invasive alien species. Despite its ecological and economical importance, P. megacephala has received remarkably little scientific attention.

I aim to investigate the micro- and macrogeographic genetic structure of this species in its native range in West Africa, as well as in its introduced range in Australia.

Three aspects are addressed in this study:

  1. 1.The characterisation of the population genetic structure, mating system and dispersal strategies of P. megacephala, by using both microsatellite and mitochondrial DNA markers;

  2. 2.The investigation of the genetic differentiation between native and introduced populations of the species;

  3. 3.The analysis of the relationship between the level of genetic diversity, the variations in cuticular hydrocarbons (i.e. chemicals involved in colonial and kin recognition) and the level of intra-specific aggression.

Keywords: biological invasions, invasion history, population genetic structure, reproductive system, chemical recognition, behaviour, social insects

Population genetics and social regulation of reproduction in humivorous termites

Humus-feeding termites are abundant and ecologically important in soils of neotropical rainforests.
However, their reproductive and dispersal strategies remain poorly known. In nests of Cavitermes species, multiple neotenic reproductives, mostly female, are frequently encountered. This is unusual in Termitidae, especially among humus feeders, but commonplace in invasive xylophagous termites with a high breeding rate.

We study the life-history traits of a guyanese Cavitermes species. Our interests are directed towards investigating population genetic structure and reproductive and dispersal strategies.

Keywords: termites, molecular ecology, microsatellites, population genetics, caste determination, breeding structure

Sex and clonality in the little fire ant

Making sex is costly: sexual females potentially transmit twice less genes per offspring than asexual females (the «two-fold cost of sex» as called by Maynard Smith) and suffer some additional costs (e.g. time and energy to find and attract a mate, risk of predation, exposure to sexually transmitted diseases). On the other hand, asexual lineages are confronted to the short term (e.g. accumulation of deleterious mutations, low level of genetic diversity) and long term (e.g. evolutionary fate of the species) disadvantages.

In some populations of the ant Wasmannia auropunctata, each sex (i.e. male versus female) and each caste (i.e. queen versus worker) are the outcome of a peculiar reproductive system. Queens use alternate modes of reproduction for the production of reproductive and non-reproductive females. While workers are produced by sexual reproduction, new queens are invariably clonally produced. This mode of reproduction increases queens’ transmission rate of their genes to their reproductive female offspring while maintaining genetic diversity in the worker force. However, while clonal production of queens increases the queen’s relatedness to their reproductive daughters, it potentially reduces male reproductive success to zero. This is because in ants and other haplodiploid species, males achieve direct fitness only through diploid female offspring since they normally develop from unfertilized maternal eggs. In an apparent response to this conflict between sexes, males thwart queens by also clonally transmitting their genomes to sons.

These findings demonstrate that haplodiploidy and caste-determination system provide grounds for the evolution of extraordinary genetic systems and unleashed modes of expression of sexual conflicts. They also reveal a complex interaction between genetic and environmental factors in the widespread success of the invasive ant, W. auropunctata.

Keywords: sexual reproduction, parthenogenesis, caste determination, ant, Wasmannia auropunctata

Population genetic structure, mating system and conflicts in ants and termites

A key feature of eusocial insects is the division of labour in reproduction between one or a few fertile individuals – the queen(s) – and many sterile nestmates that function as helpers – the workers. The reproductive altruism of workers has long been considered as one of the most important paradox of Evolution. Today, kin selection is recognised as a prime selective force for the evolution of reproductive altruism. Hamilton’s kin selection theory states that workers may benefit helping relatives reproduce as long as the relatives they aid share a higher than average proportion of their genes with the workers and effectively pass on copies of the workers' genes to the next generation.

Despite its central role in the evolution of cooperation and reproductive altruism in animals, kin selection also predicts conflicts between colony members. Because the individuals from a colony are not genetically identical, their reproductive interests may be different. These conflicts are diversified, both regarding their actors and their causes.

I aim to investigate how several factors of the breeding system (i.e. the number of breeders, their genetic relationships and their relative contribution to the reproduction) affect the colony genetic structure, the reproductive conflicts, the dispersal strategies, the level of inter-nest aggression or the task preferences in several ant (Linepithema humile, Pheidole pallidula, P. xerophylla, Cataglyphis cursor, C. sabulosa, Lasius niger...) and termites species (Cavitermes, Subulitermes...).

Keywords: Hymenoptera, Isoptera, kin selection, ants, polygyny, polyandry, queen replacement, reproductive skew, sex ratio, queen-worker conflict, nestmate recognition, intraspecific competition, task preference, size polymorphism