Original Publication: 04/18/2012
Edited for grammar/syntax: 07/24/2019
Epigenetics is a new field of biology that deals with an only recently discovered method of DNA inactivation called DNA methylation. DNA methylation is the process in which sections of DNA are “methylated” and primarily occurs on cytosines, although they could occur on any nucleotide. In this paper, the current state and understanding of DNA methylation and how it relates to the development and evolution of insect castes (particularly in the eusocial insect groups) is reviewed. Methylation is not the only possible epigenetic mechanism. DNA acetylation (the addition of acytel molecules) is also possible, as well as ubiquitination (the addition of the ubiquitin protein). However, DNA methylation is probably the most common. The end result of DNA methylation is the existence of a secondary language on top of the DNA language that can be modified by environmental factors, can be passed on to the next generation, and influence the development of offspring. DNA methylation can also have an evolutionary affect by increasing the rate of mutations in genes that are methylated for multiple generations, for genes that are inactivated can accumulate stop codons and other deleterious mutations. Based on this, the authors hypothesize that DNA methylation is potentially the primary method for caste selection in eusocial insects.
Epigenetics brings a whole new aspect to the table for understanding how castes evolved, and how castes are regulated (should a larva develop into a queen or worker?) in eusocial systems. In hymenopteran eusocial species, there is typically a vast amount of physical diversity amongst castes (workers, soldiers, queens, and male drones), and workers have found it hard to explain this diversity using only genetic methods. This is largely due to the fact that it is well known that the development and selection of what a larva will develop into is environmentally based, but scientists do not have a clear idea of exactly how that developmental “decision” is enforced. Epigenetics stands as a good explanation for how environmental factors can influence larval development, and the authors suggest this probably carried out by the presence of DNA methylation genes such as DNMT3, coincidentally which Drosophila is lacking and so was thought unimportant. The direct connection between the expression of DNMT3 and the genes that are methylated is a new, expanding area of research.
Another one of the difficulties in understanding the evolution of eusociality has been trying to explain its evolution in terms of kin selection; specifically, that haplodiploid species exhibit on average 75% more genetic relatedness of sisters than other species. The benefits of a haplodiploid system as an example of kin selection theory were that it provided a strict means for both the regulation of sexual dimorphism (males are made up of only the queen’s genome) and suggested some involvement in the development of castes. However, epigenetics and DNA methylation offers a much better explanation for the existence of both large amounts of sexual dimorphism and phenotypic plasticity. DNA methylation has been found in many eusocial hymenopteran species, as well as primitively social hymenopterans, suggesting that DNA methylation is both a heavily conserved trait and is correlated to sociality, phenotypic plasticity and sexual dimorphism. Better understanding the phylogenetic location of insect groups that make use of DNA methylation can probably elucidate the question as to whether or not DNA methylation is the sole (or primary) source of caste determination.
The authors also attempt to lay out a conceptual framework for future studies, however, I found their model unclear. What the authors seem to suggest is that eusociality is correlated with DNA methylation, but not a requirement. They do, however, do a good job outlining the specific areas of DNA methylation that need to be explored and understood to eliminate other possible explanations for the correlation between DNA methylation and eusociality, such as understanding the mechanistic effects that DNA methylation has on gene splicing and whether or not it is possible for eusocial insects to exhibit caste differentiation without DNA methylation genes.
Literature Cited Weiner, S.A. and Toth, A.L., 2012. Epigenetics in social insects: a new direction for understanding the evolution of castes. Genetics research international, 2012. doi: http://dx.doi.org/10.1155/2012/609810