A blog post highlighting the article by T. Chouvenc, M. Basille and N.-Y. Su in Insectes Sociaux

By Thomas Chouvenc

One of the reasons for the success of social insects is that their nest provides a homeostatic fortress for the colony, protecting it from external environmental changes and external threat. This is particularly true in large, mature colonies of ants, termites and bees, where a large worker cohort can provide optimal care for the developing brood and the modification of the nest structure itself provides a safe “home”.

However, like any other organisms, social insects may still be susceptible to developmental stress. Embryos first develop in the womb (or egg), and after birth continue to develop until it reaches maturity (adulthood). During this development phase, an individual is subjected to environmental and epigenetic stressors throughout its growth phase. Fluctuating asymmetry has historically been used as an indirect measure of exposure to developmental stress, and the relationship is that, the more stressful the conditions are for a developing organism, the more it will display asymmetrical traits at the end its development.

In the Asian subterranean termite, Coptotermes gestroi, soldiers sampled from mature colonies display highly symmetrical traits, suggesting that conditions for a developing termite in a large and healthy colony are optimal, and very little stress is imposed on the developing brood (Chouvenc et al. 2014). This is because there is an army of workers taking care of them in the most dedicated nursing behavior. However, in newly started colonies, the king and queen are alone to take care of their initial brood, and for many months, all the young termites hatching and developing in this stressful environment are subjected to limited resources and less than optimal parental care. As a result, the first few termites produced in a new colony are highly deformed and display highly asymmetrical traits. However, as the colony grows and additional workers are produced, the brood receives additional care and the individuals produced are progressively looking more symmetrical. I sent a few termite samples from my incipient colonies to a colleague for identification, without telling him the origin of the samples. His response was: “Tom, your samples are all messed up! You didn’t do a good job conserving the samples.” The fact was I preserved them in the same way that I preserved my other samples but the source of the deformed samples was from a young colony.

A: C. gestroi soldier from an incipient colony, B: Soldier from a mature colony.

In Chouvenc et al. 2017, we showed that the quality of termites produced in a colony improves over time and that, as the colony grows, termite eggs and larvae develop in better conditions, resulting in “better looking” termites. We were able to identify two independent origins of the stress imposed on very young termite colonies. First, the quality of brood care was found to be critical in producing highly symmetric individuals, and that the more workers present in a colony, the more symmetrical the newly produced termites looked. Second, in the first year of development, the termite colony produces “cheap” soldiers, as their development is accelerated.

These cheap soldiers are a way for the colony to quickly produce a few soldiers to defend the young colony and reach the optimal soldier ratio for the colony (Chouvenc et al. 2015). However, accelerated development imposes a heavy stress on developing soldiers, which display strong asymmetrical traits as a result. Later in the life of the colony, soldiers are then produced through a different developmental pathway, with additional time and resources invested in them, resulting in larger, better looking, and more functional soldiers.

Therefore, a newly established termite colony is extremely limited in its caring capacity, time and resources, and the initial investment in the first brood is very poor, resulting in termites exhibiting morphological evidence of their stress. When the colony grows, the care toward the brood improves and more time and resources are allocated to the new brood, providing stable developing conditions resulting in “good looking” termites.

One could say that the appearance of a termite may not say much about the quality of an individual, however these asymmetric individuals produced early in the life the colony have a short life span, confirming the cost of developmental stress on their individual physiology and metabolism. Workers and soldiers produced from the first initial egg batch laid by the queen usually die within the first year of the life of the colony (Chouvenc and Su 2014). In contrast, termites that developed in a mature colony in optimal conditions can live up to four years. Therefore, the initial parental and alloparental care toward the developing brood can directly be a measure of the initial investment in larvae, and the longevity and functionality of the resulting individual, a measure of the return on investment.

References

Chouvenc T and Su NY. 2014. Colony age-dependent pathway in caste development of Coptotermes formosanus Shiraki. Insectes Sociaux, 61: 171-182.

Chouvenc T, Basille M. Li H-F and Su N-Y. 2014. Developmental instability in incipient colonies of social insects. PloS one, 9: p.e113949.

Chouvenc T, Basille M and Su N-Y. 2015. The production of soldiers and the maintenance of caste proportions delay the growth of termite incipient colonies. Insectes Sociaux, 62: 23-29.

Chouvenc T, Basille M and Su N-Y. 2017. Role of accelerated developmental pathway and limited nurturing capacity on soldier developmental instability in subterranean termite incipient colonies. Insectes Sociaux. In press.