By Jonathan Crowe
What links a queen honeybee to a particular group of four atoms (one carbon and three hydrogen atoms, to be precise)? The answer lies in the burgeoning field of epigenetics, which has revolutionized our understanding of how biological information is transmitted from one generation to the next.
The genetic information stored in our genome – the set of chromosomes that we inherit from our parents – directs the way in which we develop and behave. (We call the attributes and behaviours exhibited by an organism its ‘phenotype’.) Traditionally, the genetic information was thought to be encoded solely in the sequence of the four different chemical building blocks from which our DNA is constructed (that is, our genome sequence). If a DNA sequence changes, so the resulting phenotype changes too. (This is why identical twins, with genomes whose DNA sequences are identical, look the same, but other individuals, whose genomes comprise different DNA sequences, do not.) However, the field of epigenetics opens up a strong challenge to this traditional view of our DNA sequence being the sole dictator of phenotype.
So what actually is epigenetics? In broad terms, epigenetics refers to the way that the information carried in our genome – and the phenotype that results when this information is ‘deciphered’– can be modified not by changes in DNA sequence, but by chemical modifications either to the DNA itself, or to the special group of proteins called histones that associate with DNA in the cell. (It’s a bit like taking a book, with a story told in the author’s words, and adding notes on the page that alter how the story is interpreted by the next person to read it.)
But what has epigenetics to do with the group of four atoms, the one carbon and three hydrogen atoms mentioned at the start of this blog post? These four atoms can combine to form a methyl group – a central carbon atom, with three hydrogen atoms attached; the addition of methyl groups to both DNA and histone proteins in a process called methylation is a primary way in which epigenetic modification occurs. For example, the addition of a methyl group to one of the four chemical building blocks of DNA (called cytosine, C) either when it appears in the sequence CG (where G is the building block called guanine) or the sequence CNG (where N represents any of the four chemical building blocks of DNA) appears to result in that stretch of DNA being ‘switched off’. Consequently, the information stored in that stretch of DNA is not actively used by the cell; that stretch of DNA falls silent.
But what of our queen honeybee? Where does she fit into our story? A queen honeybee has an identical DNA sequence to her workers. Yet she bears some striking differences to them in terms of physical appearance and behavior (amongst other attributes). These differences are more than just skin-deep, however: the pattern of methylation between queen and worker larvae differs. Their genomes may be the same at the level of DNA sequence, but their different patterns of methylation direct different fates: the queen honeybee and her workers develop into quite distinct organisms.
Things take an interesting turn when we consider the cause of these different methylation patterns: the diets that the queen and workers experience during their development. The queen is fed on large quantities of royal jelly into adulthood, while worker larvae face a more meager feast, being switched to a diet of pollen and nectar early on. It is these diets that influence the way in which the queen and worker bees’ genes are switched on and off.
It is not just the queen honeybee whose genome is affected by the environment (in her case, diet). Mice exposed to certain chemicals during pregnancy have been found to produce offspring who became obese more often than would be expected. In these offspring, the methylation of a particular gene associated with the onset of obesity (and other conditions) was seen to decrease, causing the gene to be switched on when it would normally be switched off.
These findings bring a new twist to the classic nature/nurture debate: while it still holds that our phenotype – the physical attributes and behaviours we display – is dictated by our DNA sequence, it happens in a way that can be modified by factors in the world around us, operating through epigenetic mechanisms. This should give us food for thought when we recognize that our own genome is susceptible to epigenetic modification too. How could the environment we experience in early life – in the womb, even – be determining our phenotype in later life – our health and wellbeing? Food for thought indeed.
Jonathan Crowe is Editor in Chief for Natural, Health & Clinical Sciences in the Higher Education Department at Oxford University Press. A biochemistry graduate, he manages OUP’s undergraduate textbook publishing programme across a range of science and science-related disciplines. He is also an author of Chemistry for the Biosciences, now in its second edition, and was a runner-up in the Daily Telegraph/BASF Young Science Writer Awards (in 2001, when he was still classed as being ‘young’).