Evolutionary biology of social insects
The emergence of social behavior occurred several times during animal evolution. Among the known taxa which include social organisms, insects constitute one of the most outstanding examples. The order Hymenoptera, in fact, includes ants, bees and social wasps (beside to solitary bees and wasp species) and the epifamily Termitoidae (order Blattodea) is entirely made by social species. Despite the notable biological divergence between hymenopteran and termites, they all evolved social behavior. The extreme expression of social behavior is the eusociality: in this condition all individuals live in a colony and there is strict division of labor (nest defense, brood caring, etc). In particular, only a small fraction of colony mates reproduces (often a single pair) while the others forego their reproduction in order to take care of the nest and of reproducers’ offspring. The kin selection explains, at least in part, the evolution of the social behavior: individuals gain fitness indirectly by helping relatives to produce offspring. Other explanations include ecological factors that may guarantee indirect fitness.
Termites live in colonies that are structured in different ways, also depending on the reproductive strategies adopted by founders. Generally speaking, a colony is founded through sexual reproduction between winged males and females (primary reproductives; future kings and queens) swarmed out from their natal nests. As the colony grows up and/or after a founder’s death, neotenics (secondary reproductives) develop and contribute to the production of more offspring. In some other instances, two colonies may fuse in a single colony.
The breeding structure has an important impact on the colony genetics; for example, the emergence of secondary reproductive unavoidably leads to inbreeding because they are offsprings of the primary reproductives and, therefore, all genetically related.
In some species of the genus Reticulitermes, as the European taxon R. lucifugus, the primary queen, after founding the colony by mating with the primary king, produces secondary queens through parthenogenesis. Upon primary queen's death, secondary queens will, then, mate with the primary king extending the contribution of the primary queen through time (Asexual Queen Succession, AQS). This help to preserve the original genetic variability that, otherwise, would be progressively reduced. AQS system was not detected in all Reticulitermes species and the distribution of this strategy within the termite phylogeny suggests it emerged several times during termite evolution.
At variance of Reticulitermes termites, dry-wood termites of the genus Kalotermes are known to nest and feed in a single wood log. In some European populations, we observed frequent colony fusion events with mixed families composed by more than two original colonies. Moreover, fused colonies may belong to highly divergent genetic lineages and/or different taxa. Colony fusions lead to the death of queens and kings allowing false workers (pseudoergates) to develop into reproducers and to inherit the nest (Accelerated Nest Inheritance). In Kalotermes species, colony fusions appear to occur more frequently than expected on the basis of eusociality models.
Research efforts, therefore, focus our studies on understanding the genetic and environmental factors that may promote such behavior.
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