Female honey bees all have the same genetic makeup. When they are in the immature stage called a larva, however, one female in the hive eats a protein-rich substance called royal jelly that dramatically alters the expression of many genes. The queen grows much larger than her peers, stays in the hive, and is tended to by the other bees. Above all, the queen lives up to several years and is fertile, laying the eggs that will produce more bees. The multitudes of her female compatriot larvae express a different set of genes, becoming workers who build the honeycomb, forage for food, and tend to the queen for their short lives of a few weeks. All of these differences ultimately come from the environment, specifically the royal jelly diet. Recent investigations have shown that the differences in gene expression between these very different yet genetically identical bees are due to differences in DNA methylation (Investigating Life: Gene Expression and Behavior).

Although they are reversible, many epigenetic changes such as DNA methylation and histone modification can permanently alter gene expression patterns in a cell. In a germ line cell that forms gametes, epigenetic changes can be passed on to the next generation. But what determines these epigenetic changes? A clue comes from a recent study of monozygotic (identical) twins. Monozygotic twins come from a single fertilized egg that divides to produce two separate cells; each of these goes on to develop a separate individual. Monozygotic twins thus have identical genomes. But are they identical in their epigenomes? A comparison of DNA in hundreds of such twin pairs shows that in tissues of 3-year-olds, the DNA methylation patterns are virtually the same. But by age 50, by which time the twins have usually been living apart in different environments for decades, the patterns are quite different. This indicates that the environment plays an important role in epigenetic modifications, and thus in the regulation of genes that these modifications affect.

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