Epigenetics and Animal Caste Behavior

Many behaviorally social animals depend on a certain individuals to complete certain tasks. This specialized organization is referred to as a caste system in biology. Two examples of animals that thrive utilizing a caste system are ants and bees. Typical caste groups could include a queen, workers, and soldiers. These castes are anatomically and morphologically distinct. Although in bees, subcastes of worker bees exist such as foragers which gather food and nursers which feed and care for other bees. Both groups are a part of the worker caste but each carry out different duties. Typically there is no interchanging between castes, a queen bee's behavior is fixed and will never become a worker. However research has found that bees are able to change between the worker subcastes of foragers and nursers.



In Nature Neuroscience, Herb et al. published their research on the epigenetics of honeybees Apis mellifera (Herb et al., 2012). The scientists found over 150 methylated regions that were different between nurses' and foragers' DNA. Following this discovery the researchers designed an experiment to observe the two subcastes. All forager bees were sent out of the hive to gather food, and the researchers removed all nurse bees from the hive so that all which remained was larvae and the queen. Once the foragers returned half of the group changed to become new nurses and the other half remained foragers. The results showed that this drastic phenotypic "U-Turn" was the result of epigenetic switches in the methylation of the epigenome. In a follow-up experiment 45 particular reversible methylation sites were identified when the researchers looked a region in the genome when a bee changed from nurse to forager and back again from forager to nurse.
(Herb et al., 2012)

This study exemplifies that phenotypic behavior is able to be regulated by epigenetic changes, it is not only the DNA that solely modulates behavior but epigenetic changes as well.

This type of notion is not solitary to bees. Epigenetics also plays a role in the caste system in ants as well. The full genomic sequence of two types of ant species, Jerdon's jumping ant Camponotus floridanus and the Florida carpenter ant Harpegnathos saltator, proved to unveil epigenetic differences in castes as well. Bonasio et al. published their research on this concept in Science Magazine  in 2010 (Bonasio et al., 2010). The two types of ants are socially different, carpenter ants rely on the queen and once she dies the colony will likely perish as well although in Jerdon's jumping ant worker ants can battle to become queen. (Harmon, 2010). The Jerdon's jumping worker ant that becomes queen is called a gamergate queen and it will change physically and behaviorally to maintain its duties. Importantly though, in both species the queen ant lives longer than workers.

Camponotus floridanus (left) & Harpegnathos saltator (right)

According to Bonasio et al., "Telomere shortening is a hallmark of cellular senescence in multicellular eukaryotes, and the enzyme telomerase (TERT), which counteracts telomere shortening, prolongs life span upon overexpression. TERT RNA levels were highest in eggs and lower in adults in both C. floridanus and H. saltator, but they were up-regulated in H. saltator gamergates" (Bonasio et al., 2010). This is a particular finding that proves in gamergate ants epigenetic changes occur to increase the lifespan of the ant as it switches from worker to queen. It is epigenetic changes that allow a worker ant to behaviorally and physically change its characteristics within its lifetime. This epigenetic ability is an evolutionary important advantage because it allows the ant colony to continue quickly and easily once a queen dies. 

Both bees and ants rely on epigenetic changes to thrive and change behavioral castes for the good of their population. 

References:

Bonasio, R., Zhang, G., Ye, C., Mutti, N. S., Fang, X., Qin, N., . . . Liebig, J. (2010). Genomic Comparison of the Ants Camponotus floridanus and Harpegnathos saltator. Science, 329(5995), 1068-1071. doi: 10.1126/science.1192428

Harmon, K. (2010). First Ant Genomes Promise Insight into Epigenetics and Longevity. Scientific American. http://www.scientificamerican.com/article/first-ant-genomes-epigenetics/

Herb, B. R., Wolschin, F., Hansen, K. D., Aryee, M. J., Langmead, B., Irizarry, R., . . . Feinberg, A. P. (2012). Reversible switching between epigenetic states in honeybee behavioral subcastes. Nat Neurosci, 15(10), 1371-1373. doi: http://www.nature.com/neuro/journal/v15/n10/abs/nn.3218.html#supplementary-information

All images accessed on 23/05/15 and are referenced from appearing top to bottom of blog:

http://antark.net/wp-content/uploads/2014/09/ants-pests-argentine1.jpg
http://content.mycutegraphics.com/graphics/science/boy-scientist-with-bee-in-jar.png
http://www.nature.com/neuro/journal/v15/n10/images/nn1012-1329-I1.jpg
http://holykaw.alltop.com/wp-content/uploads/2013/10/ant_bee_1170-770x460.jpg
http://ucanr.edu/blogs/bugsquad/blogfiles/1574.jpg