Review: Navajas et al. (2008). BMC Genomics, 9: 301.

Written: February 25, 2009

Posted: 03/02/09

Word count: 704

Question: What are the genetic consequences of varroa parasitism?

Answer: Many genes getting turned on and off, which can tell us how we might deal with them

When you really think about it, you have to be really glad that, as humans, we don’t have any parasites that are even remotely comparable to varroa mites. Oh, sure, we have ticks and lice, tapeworms and bot flies. But imagine having a vampire-like horseshoe crab clinging to your back all of the time, the size of a dinner plate and constantly draining you of your bodily fluids. I’m anemic just thinking about it!

Put in that perspective, how honey bees are able to cope with varroa at all is nothing short of a miracle. But yet somehow, many bees are able to tolerate and even thrive in face of such a daunting enemy. Some recent research, much of which has been reviewed in this column, has demonstrated how certain bees can tolerate varroa and live in spite of them.

Biologists are interested in mechanisms of how things work in nature. So when findings such as this come out, the next logical step is to determine how certain bees can put up with varroa while most others cannot. The question is neither trivial nor academic, as understanding the precise means by which bees can resist varroa will help us develop means to control them. This is where the recent publication of the honey bee genome can help.

Genes are something that we inherit from our parents, encoded in our DNA. Each gene codes for a protein, and the combination of all of these gene products is what makes us what we are. These genes can be highly regulated, each being turned on or off, or tuned up or down, depending on the needs of the organism. You can imagine, therefore, that the entire genome of any living thing is like a huge panel of toggle switches that get flicked on and off to deal with whatever challenges a critter might face.

It is this pattern of gene expression—this panel of toggle switches—that a trans-Atlantic research team has recently published with respect to what happens when varroa parasitizes honey bees. They employed the power of the honey bee genome to explore, gene by gene, which ones get tuned up or tuned down as a consequence of varroa. Moreover, they compared the changes in gene expression in bees that were either susceptible or tolerant of varroa mites. This two-by-two design (tolerant vs. susceptible stocks with or without varroa) enabled the researchers to see exactly which genes are affected by parasitism, host resistance, or both.

What they found was that 148 genes were significantly different among their test groups. Of these, 32 were different depending on whether or not the bees were infested with varroa, 116 were different depending on whether or not the bees were tolerant of varroa, and two were different with both. Those genes that were affected by parasitism tended to be genes that regulate development, metabolism, and immunity (not surprising—if you get infected by something, your immune genes kick into high gear as well). Those genes that were affected by bee stock (tolerant vs. susceptible), however, tended to be genes that regulate the nervous system and olfaction (the bees’ ability to smell). What this suggests, therefore, is that mite-tolerant bees are able to put up with varroa because they are better able to sense and behaviorally respond to the mites (i.e., hygienic behavior) rather than some internal physiological resistance.

These findings really highlight the value in knowing the honey bee genome, without which understanding the internal mechanisms of parasitism would be impossible. By concentrating on behavioral mechanisms, rather than chemical or physiological mechanisms, beekeepers might be able to develop a more sustainable relationship with varroa. Let’s just hope they don’t decide to start parasitizing us…

Reference

Navajas, M., A. Migeon, C. Alaux, M. L. Martin-Magniette, G. E. Robinson, J. D. Evans, S. Cros-Arteil, D. Crauser, and Y. Le Conte. (2008). Differential gene expression of the honey bee Apis mellifera associated with Varroa destructor infection. BMC Genomics, 9: 301.