Queen acceptance in a socially polymorphic ant

A central question in social evolution is what processes regulate the number of breeders in each social group. Here, we tested whether differences in the rate of acceptance of new queens by resident workers could be a proximate cause explaining the coexistence of single- and multiple-queen colonies in an ant population. We found that Formica selysi workers discriminated against foreign (non-nestmate) queens, which contributes to maintaining the genetic integrity of the social group essential to kin selection. All the young and newly mated foreign queens introduced into experimental groups of workers died within 48 h. In contrast, workers frequently accepted young newly mated nestmate queens. The survival of nestmate queens was not significantly lower in groups of workers originating from single- queen colonies than in groups of workers originating from multiple-queen colonies. Finally, virgin queens had significantly higher survival than mated queens. Together, these results show that the maintenance of single-queen and multiple-queen colonies in the same population is unlikely to be caused by strong differences between the two types of colonies in their rate of acceptance of new queens by workers. They also suggest that the discrimination of queens by resident workers restricts the dispersal of foreign queens among colonies, but not the acceptance of additional nestmate queens.

Egg-Laying "Intermorphs" in the Ant Crematogaster smithi neither Affect Sexual Production nor Male Parentage.

We study male parentage and between-colony variation in sex allocation and sexual production in the desert ant Crematogaster smithi, which usually has only one singly-mated queen per nest. Colonies of this species are known to temporarily store nutrients in the large fat body of intermorphs, a specialized female caste intermediate in morphology between queens and workers. Intermorphs repackage at least part of this fat into consumable but viable male-destined eggs. If these eggs sometimes develop instead of being eaten, intermorphs will be reproductive competitors of the queen but-due to relatedness asymmetries-allies of their sister worker. Using genetic markers we found a considerable proportion of non-queen sons in some, but not all, colonies. Even though intermorphs produce ∼1.7× more eggs than workers, their share in the parentage of adult males is estimated to be negligible due to their small number compared to workers. Furthermore, neither colony-level sex allocation nor overall sexual production was correlated with intermorph occurrence or number. We conclude that intermorph-laid eggs typically do not survive and that the storage of nutrients and their redistribution as eggs by intermorphs is effectively altruistic.

Nestmate recognition in the unicolonial ant Formica paralugubris

In unicolonial populations of ants, individuals can mix freely within large networks of nests that contain many queens. It has been proposed that the absence of aggression in unicolonial populations stems from a loss of nest mate recognition, but few studies have tested this hypothesis. We investigated patterns of aggression and nest mate recognition in the unicolonial wood ant, Formica paralugubris. Little aggression occurred, even between workers from nests separated by up to 5 km. However, when aggression took place, it was directed toward non-nest mates rather than nest mates. Trophallaxis (exchange of liquid food) occurred very frequently, and surprisingly, workers performed significantly more trophallaxis with non-nest mates than with nest mates (bias 2.4:1). Hence, workers are able to discriminate nest mates from non-nest mates. Higher rates of trophallaxis between non-nest mates may serve to homogenize the colony odor or may be an appeasement mechanism. Trophallaxis rate and aggression level were not correlated with geographical distance and did not differ within and between two populations separated by several kilometers. Hence, these populations do not represent differentiated supercolonies with clear-cut behavioral boundaries. Overall, the data demonstrate that unicoloniality can evolve despite well-developed nest mate recognition. Reduced levels of aggression might have been favored by the low rate of interactions with foreign workers, high cost of erroneously rejecting nest mates, and low cost of accepting foreign workers.

Reliure de : La Farce de maistre Pierre Pathelin, avec son Testament à quatre personnages. Nouvelle edition

[Farce de maître Pierre Pathelin (français). 1723]

Patterns of positive selection in seven ant genomes.

The evolution of ants is marked by remarkable adaptations that allowed the development of very complex social systems. To identify how ant-specific adaptations are associated with patterns of molecular evolution, we searched for signs of positive selection on amino-acid changes in proteins. We identified 24 functional categories of genes which were enriched for positively selected genes in the ant lineage. We also reanalyzed genome-wide data sets in bees and flies with the same methodology to check whether positive selection was specific to ants or also present in other insects. Notably, genes implicated in immunity were enriched for positively selected genes in the three lineages, ruling out the hypothesis that the evolution of hygienic behaviors in social insects caused a major relaxation of selective pressure on immune genes. Our scan also indicated that genes implicated in neurogenesis and olfaction started to undergo increased positive selection before the evolution of sociality in Hymenoptera. Finally, the comparison between these three lineages allowed us to pinpoint molecular evolution patterns that were specific to the ant lineage. In particular, there was ant-specific recurrent positive selection on genes with mitochondrial functions, suggesting that mitochondrial activity was improved during the evolution of this lineage. This might have been an important step toward the evolution of extreme lifespan that is a hallmark of ants.