A beehive consists of at least one queen bee who lays the eggs, a larger number of worker bees who tend the eggs, and developing larvae. The queen and worker bees look very different. The most noticeable difference is the size of the queen: she's much larger than the workers. Yet the size difference isn't the result of different genes.
Whether an egg develops into a queen or into a worker bee depends on its environment within the hive, specifically whether it's fed exclusively royal jelly for the first days of its life. Royal jelly is a substance produced by special glands of worker bees and fed to all larvae. An all-royal-jelly diet equals a new queen.
The existence of different castes in bees (and in other social hymenoptera — a class of insects that includes ants and wasps) offers a nice example of how changes in the path of development can result in organisms with different body forms. We know that any female bee can become a worker or a queen — the final form isn't based on differences at the DNA sequence level but instead on differences in how those genes are expressed.
Researchers can look at patterns of gene expression in developing bees and figure out exactly which genes are regulated differently in queens versus workers. Queens develop from eggs that experience an increase in the production of metabolic enzymes and that regulate the genes associated with hormonal activity differently.
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