A Bug’s Life

December 8, 2020

Written by: Greer Prettyman

Maybe you’ve felt like you had a calling in life; an internal force that made it feel as if you were destined to be a scientist, a dancer, a teacher, or a baseball player. While our unique combinations of personalities, talents, and environments may make us more likely to pursue some paths over others, our trajectories are not predetermined. For some creatures, though, social roles are set in motion mostly by biology. Social insects, for example, have jobs that require special physical and behavioral development. Let’s take a look at how some ants are made to be queen.

An organized division of labor is a hallmark of a functioning society. In a colony of ants, just as in any social group, there are different jobs that must be done. For ants, a queen is responsible for laying eggs and reproducing the colony. Worker ants perform many jobs like foraging for food, caring for juveniles, and protecting the colony. All of the worker ants are females, as males are responsible only for mating with the queen, and thus have short lifespans and limited social roles. Each species of ant (there are over 10,000 different ant species!) has a slightly different caste system, but for the Florida harvester ant, there are three possible roles for females: queens, major workers, and minor workers¹.

With genotyping technology, the entire genetic codes of many species of insects have been mapped, allowing researchers in the field of insect sociobiology to gain insight into the specific biological differences between ants with different social roles². Although the roles in the colony are very different, in most species each ant actually starts out with the same set of genes. Therefore, it cannot be a factor inherent in the DNA itself that makes some ants queens and other workers. Instead, epigenetic regulation must explain the differentiation in ant phenotypes. Epigenetics is a broad term that defines processes that alter what genes will be turned on (expressed) or off that are not a part of the DNA code. A process called DNA methylation is one epigenetic change that can lead to increased expression of specific genes. Queens of many ant species have increased DNA methylation in several parts of the genome, including regions related to reproductive organs, giving rise to their egg-laying capabilities³.

Through epigenetic changes, transient environmental conditions can start a cascade of gene expression that continues for an insect’s lifetime. One of these environmental factors seems relatively simple— food. Ants that are fed more nutrient-rich food are more likely to become queens⁴. In an experiment with Florida harvester ants, dietary nutrients were found to correlate with social group¹. Queen ants had a higher composition of nitrogen and carbon, markers of a nutritious diet, compared to major and minor workers. Food intake also relates to body size, another factor that differentiates ant roles. Body size varies with social group and typically ants that are larger in size possess more queen-like reproductive systems⁴. 

Development is often regulated by hormones, chemical compounds that act as signals in the brain and body. One such compound called juvenile hormone (JH) helps to modulate caste differentiation in ants. In one experiment on Indian jumping ants, giving ants JH during a certain period of larval development was enough to cause them to become queens⁵. In honeybees, increased nutrition during early development led to greater production of JH during larval stages, and it is likely that food is also linked to hormone levels in ants. JH itself can interact directly with proteins that start and stop expression of specific genes^2,6. Therefore, environmental conditions affect the neuroendocrine system, which then leads to epigenetic changes that turn an ant into a queen.

Hormone signaling can also help to explain one special feature of queen ants: their long lifespans. Queens of the Lasius niger species, or black garden ants, can live up to 28 years⁷. In many animals, the hormones that are involved in reproduction can compromise the immune system. Queen ants, however, both reproduce copiously and have much longer lifespans than ants in all other groups. Since queen have lower JH levels as adults than ants in other castes, researchers conducted an experiment to determine if JH was related to long lifespans and reproductive capacities⁸. They found that queens that were given JH as adults produced fewer eggs and were more likely to die from a fungal infection than those that did not receive JH. This suggests that queens are typically not using JH for reproductive purposes and thus avoiding the accompanying immune suppression, allowing them to live longer. However, the exact mechanisms behind the impressively long lifespan of the queen are still somewhat of a mystery. 

For small creatures, there is a big range of diversity between species of social insects in exactly how castes are determined. By studying social insects and learning about the fascinating complexities in these tiny societies, we can also learn about epigenetic cascades that help us to understand pathways that determine human development, social behavior, and aging.

References:

1. Smith, C. R., Anderson, K. E., Tillberg, C. V, Gadau, J., & Suarez, A. V. (2008). Caste Determination in a Polymorphic Social Insect: Nutritional, Social, and Genetic Factors. Am. Nat, 172, 497–507. 
2. Yan, H., Simola, D. F., Bonasio, R., Liebig, J., Berger, S. L., & Reinberg, D. (2014, October 11). Eusocial insects as emerging models for behavioural epigenetics. Nature Reviews Genetics. Nature Publishing Group. 
3. Bonasio, R., Li, Q., Lian, J., Mutti, N. S., Jin, L., Zhao, H., … Reinberg, D. (2012). Genome-wide and caste-specific DNA methylomes of the ants camponotus floridanus and harpegnathos saltator. Current Biology, 22(19), 1755–1764. 
4. Trible, W., & Kronauer, D. J. C. (2017, January 1). Caste development and evolution in ants: It’s all about size. Journal of Experimental Biology. Company of Biologists Ltd. 
5. Penick, C. A., Prager, S. S., & Liebig, J. (2012). Juvenile hormone induces queen development in late-stage larvae of the ant Harpegnathos saltator. Journal of Insect Physiology, 58(12), 1643–1649. 
6. Jindra, M., Palli, S. R., & Riddiford, L. M. (2013). The juvenile hormone signaling pathway in insect development. Annual Review of Entomology. 
7. Hölldobler B, Wilson EO. (1990).The ants. Cambridge, MA: Harvard University Press.
8. Pamminger, T., Treanor, D., & Hughes, W. O. H. (2016). Pleiotropic effects of juvenile hormone in ant queens and the escape from the reproduction–Immunocompetence trade-off. Proceedings of the Royal Society B: Biological Sciences, 283(1822).