The Plant Virus and the Bee

The past couple of years have not been good ones for honeybees. For reasons unknown, honeybee colonies around the globe have experienced Colony Collapse Disorder. With Colony Collapse Disorder 30 - 90% of hives are lost. Adding to the honeybees' growing list of problems is a plant virus, tobacco ringspot virus, which has been found in the bees. Astonishingly, the tobacco ringspot virus has accomplished a 1.6 billion year evolutionary hurdle from its original plant hosts to honeybees, according to a study published in the journal mBio in January.

Honeybees are crucial to human's food production. Roughly one out of every three mouthfuls in our diet is directly or indirectly connected to honeybee pollination. If you like almonds, cantaloupe, cucumbers, apples, berries, or honey the declining population of bees is not good for your palate. In addition to all the foods bees add to our diet, they also are responsible for an increased crop value of $15 billion dollars each year. Needless to say, honeybees are pretty important.

Tobacco ringspot virus was first recorded in the United States in 1941. The most devastating disease caused by the virus is bud blight of soybean, which can reduce yields 25 - 100%. The virus isn't exclusive to tobacco plants. It can infect 35 different families of plants including tomatoes, cucumbers, and beans. Stunted growth, discoloration of leaves in a ringspot pattern, and even death are symptoms of the virus.

Tobacco ringspot virus is an RNA virus, which may account for its evolutionary leap. Although RNA and DNA are both nucleic acids their functions differ. DNA is like a cookbook; it stores the recipes for making proteins. RNA is the messenger molecule that tells cells how to make the proteins. The enzyme that makes RNA by copying DNA, RNA polymerase, is not as thorough correcting errors as the enzyme DNA polymerase, which replicates DNA. Therefore, RNA tends to contain more mistakes, a.k.a. mutations, than DNA. In humans a single strand of RNA has a very short life span and any mutations are eliminated upon the RNA strand's destruction.

RNA viruses, on the the other hand, are a whole other ball park. Since the viruses' genetic information is derived from RNA, mutations can have a major evolutionary impact on the viruses. Mutations are the driving force behind diversity, which leads to natural selection. RNA viruses have the greatest potential for host-hopping says the study. Although it is rare for viruses to leap from host to host in different kingdoms it is not unheard of. The Rhabdoviridae family of viruses, which includes Rabies, contains a few viruses that can infect both plants and animals.

Tobacco ringspot virus can be spread many different ways between plants. A small soil worm called a dagger nematode can even

transmit the virus between plants. The worm has a small, sharp stylus that it uses to extract plant juices. Several types of leaf-eating insects, like grasshoppers, can also spread tobacco ringspot virus. Honeybees come into play in the transmission of the virus too. A bee carrying infected pollen can spread the virus from plant to plant. But how on earth did tobacco ringspot virus end up infecting the honeybees?

A team of American and Chinese scientists discovered tobacco ringspot virus in honeybees during a screening of all different types of viruses present in honey bees and pollen. If tobacco ringspot virus was merely passing through a bee it would replicate in the gut or salivary glands. Unfortunately for bees, the scientists found only a small amount of virus particles in the gut and salivary glands. The virus had instead replicated in the bees' wings, antennae, trachea, blood (called hemolymph), and especially in the nerves. Science writer Jennifer Frazer compares the intrusive virus to a robber making himself comfortable in someone else's home.

On top of all this bad news, there are more disheartening results in store for bees. Tobacco ringspot virus was also found in Varroa destructor mites, which are like parasites to bees. The mites may be responsible or partially responsible for Colony Collapse Disorder. Luckily for the mites, tobacco ringspot virus was only found in its guts. The possible capability of the virus to kill off Varroa destructor mites seems unlikely.

The group of scientists studied the similarities and differences between the tobacco ringspot virus genes from honeybees, Varroa destructor mites, and plants. The viral genes from the bees and mites were "closely related," suggesting that the mites were introduced to the virus through bees. Evidence of tobacco ringspot virus was also identified in pollen stored in beehives.

Tobacco ringspot virus's presence doesn't automatically mean the virus causes bees harm. Thus, the same team of scientists monitored six "strong" and four "weak" beehives for a year to determine if the virus is hazardous to bees. Other viruses thought to contribute to Colony Collapse Disorder were studied too. Higher concentrations of tobacco ringspot virus and the other viruses did, in fact, foretell colony collapse.

Still many questions remain with regards to tobacco ringspot virus. It is unknown whether the virus can survive in the bee population without periodical reintroductions. We now know that a plant virus can be spread to bees but can infected bees transmit the virus to plants? Most importantly, is tobacco ringspot virus one of the causes of Colony Collapse Disorder or merely brought on by a weakened population of honeybees more prone to viral infection?

Switching from host to host is no small feat, especially when the two hosts, plants and bees, are more than a billion evolutionary years apart. There are many different challenges a virus must face to make this jump. For example, a virus must first come into contact with its new host and figure out a mechanism of transmission between its new hosts. Tobacco ringspot virus is the first recorded plant virus to have been spread to honeybees through contaminated-pollen. For the bees' sake I sincerely hope scientists can determine what causes Colony Collapse Disorder and how to prevent it soon.

References:

Frazer, J. "Suspicious Virus Makes Rare Cross-Kingdom Leap From Plants to Honeybees." Scientific America. January 31, 2014

United States Department of Agriculture. Honey Bees and Colony Collapse Disorder (2013).

Rodgers, P. "Plant Virus Jumps 1.6-Billion-Year Species Barrier To Infect Honeybees." Forbes. January 31, 2014.

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Erik Hooymans (via Wikimedia).

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