In colonies of social Hymenoptera (which include all ants, as well as some wasp and
bee species), only queens reproduce whereas workers generally perform other tasks.
The evolution of worker's reproductive altruism can be explained by kin selection,
which states that workers can indirectly transmit copies of their genes by helping the
reproduction of relatives. The relatedness between queens and workers may however
be low, particularly when there are multiple queens per colony, which limits the
transmission of copies of workers genes and increases potential conflicts between
colony members. In this thesis, we investigated the link between social structure
variations and conflicts, and explored the mechanisms involved in variation of colony
queen number in ants.
According to kin selection, workers should rear the brood they are most related to.
In social Hymenoptera, males are haploid whereas females (workers and queens) are
diploid. As a result, workers can be up to three times more related to females than
males in some colonies, where they should consequently favour the production of
females. In contrast, queens are equally related to daughters and sons in all types of
colonies and therefore should favour a balanced sex ratio. In a meta-analysis across
all studies of social Hymenoptera, we showed that colony sex ratio is generally
largely influenced by workers. Hence, the evolution of social structures where queens
and workers are equally related to males and females may contribute to decrease the
conflict between the two castes over colony sex ratio.
Another conflict between queens and workers can occur over male production.
Many species contain workers that still have the ability to lay haploid eggs. In some
social structures, workers are on average more related to sons of queens than to sons
of other workers. As a result, workers should eliminate worker-laid eggs to favour
queen-laid eggs. We showed that in the ant Formica selysi, workers eliminate more
worker-laid than queen-laid eggs, independently of colony social structure. These
results therefore suggest that worker policing can evolve independently from
relatedness, potentially because of costs of worker reproduction at the colony-level.
Colony queen number is a key parameter that influences relatedness between
group members. Queen body size is generally linked to the success of independent
colony foundation by single queens and may influence the number of queens in the
new colony. In the ant F. selysi, single-queen colonies produce larger queens than
multiple-queen colonies. We showed that this association results from genes or
maternal effects transmitted to the eggs. However, we also found that queens
produced in colonies of the two social forms did not differ in their general ability to
found new colonies independently. Queen body size may also influence queen
dispersal ability and constrain small queens to be re-adopted in their original nest
after mating at proximity. We tested the acceptance of new queens in another ant
species, Formica paralugubris, which has numerous queens per colony. Our results
show that workers do not discriminate between nestmate and foreign queens, and
more generally accept new queens at a limited rate.
To conclude, this thesis shows that mechanisms influencing variation in colony
queen number and the influence of these changes on conflict resolution are complex.
Data gathered in this thesis therefore constitute a solid background for further
research on the evolution and the maintenance of complex organisations in insect
societies.