What's the link between enhanced calf nutrition, higher milk output and honeybees? If you said DNA methylation then you are no doubt a confirmed epigeneticist! As for the rest of us, to better understand the relationship let's start with a look at bee differentiation within the hive. All female bee larvae contain the same DNA—in fact they are genetic clones—and yet there is significant differentiation between the queen and worker bees. Because both castes are exclusively female, (the reproducing males are called drones), this difference is not accounted for by sex. So, how can two clones become different? The answer is found in epigenetics.
While two organisms may be genetically identical, the way the genes are expressed allows for very large differences. Epigenetics describes the mechanisms by which individual genes are either expressed or inhibited. It is as though each gene is associated with a dimmer switch that can either attenuate or amplify its importance, (i.e. its impact on the phenotype), according to changes in the organism's external and internal environment. For queen bees, the transformational environmental changes are linked to the ingestion of royal jelly.
Royal jelly is composed of 67% water, 12.5% crude protein including a variety of amino acids, 11% simple sugars and 5% fatty acids. It also contains multiple trace minerals, enzymes and vitamins. All larvae are fed royal jelly for the first three days, however the queen feasts on it for a lifetime – and in great quantities. But the catalyst behind queen differentiation is a specific component of the protein called 57-kDa, better known as Royalactin. Royalactin increases body size and ovary development and shortens developmental time in honeybees. The difference between queen and worker castes is one of the most striking examples found in nature of phenotypic polymorphism due to epigenetic factors.
|Epigenetic Phenotypic Polymorphism in honey bees (identical DNA)|
Fed royal jelly during lifetime (food quantities = 10x more than workers)
|Fed royal jelly for 3 days, then downgraded mixture of pollen and honey|
|Rapid growth: 200mg final weight||100 mg final weight|
Lifetime = 3 years
|Lifetime = 1 month|
|Maturation of reproductive organs (up to 2000 eggs per day)||Rudimentary, inactive ovaries|
On the cellular level, the phenotypic polymorphism of honeybees is mediated by a process called CpG methylation. CpG methylation is able to establish and maintain diverse patterns of gene expression from the same genome under specific temporal, spatial and environmental conditions—the internal and external environment pressures mentioned above. This non-reproductive modification of genomic DNA provides a link between genomes and environment and may result in a phenotypic change that is heritable, independently of DNA mutation.
Paradoxically, it was only relatively recently that the process of genomic methylation was established in invertebrates. This was because the technology was not yet available to detect modifications (methylomes) with very low methylation levels.
While honeybees offer the most striking example of epigenetic polymorphism, the field of epigenetics in insects provides a rich vein of data on mechanisms shared with vertebrates. It's helping scientists learn more about context-dependent gene expression.
So can the right calf milk replacer along with an enhanced feeding plan constitute the royal nectar that will turn calves in high-production queens of the dairy? Evidence shows this to be the case, with lifetime benefits including a significantly higher first lactation and enhanced health. Today, scientists in LifeStart research programs and at other research centres around the world are working to better understand this effect and the specific role that epigenetics play in linking nutrition and performance. Of course calves and bees are far from being the only organisms in which epigenetic mechanisms are active. All Eukaryotes (organisms composed of one or more cells containing visibly evident nuclei and organelles) are concerned. Already extensive research has focused on epigenetic mechanisms in humans, sheep and rats, to name just a few. Watch this space for more developments!