Disease spreads from plant to bee

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By Song My Hoang

TRSV found in honeybees may explain colony collapses

The tobacco ringspot virus (TRSV), previously believed to be exclusive to plants,  has made a jump across 1.6 billion years of evolution to infect honeybees.

A team of scientists from the United States and China began screening bees and pollen for viruses and unexpectedly discovered that TRSV, a plant-pathogenic virus, had infested honeybees.

TRSV poses as a potential suspect for the ongoing mystery of the colony collapse disorder, a phenomenon characterized by the abrupt disappearance of honeybee colonies.

Colony collapse disorder was first reported in 2006 and has spread globally.  In the winter of 2012-2013, one-third of U.S. honeybee colonies disappeared.

TRSV is an RNA virus, which means that its heredity information is RNA rather than DNA.

RNA replication has an extremely high mutation rate because its replication process lacks a proofreading mechanism that the DNA replication process includes. The high mutation rate is a powerful tool of evolution, and it generates a source of genetic diversity that allows RNA viruses to adapt to new selective conditions.

RNA viruses remain the most likely source of emerging and reemerging infectious diseases. RNA viruses such as HIV, SARS and H1N1 have been a worldwide public health concern.

TRSV is thought to solely infect a wide range of plants, including tobacco, tomato, cucumber, beans and many woody plants. It can inhibit the growth of the plant or kill the plant.

TRSV produces a characteristic discolored ring spot pattern on the leaves of infected plants.

TRSV is spread between plants in a variety of ways. The virus can be transmitted directly to the next generation by an infected seed, it can be transferred from one plant to another plant by insect vectors or a honeybee can spread the virus to a new plant by introducing infected pollen.

Honeybees have electrically-charged bodies that allow pollen to adhere to their bodies, but they also store pollen in a specialized pollen basket located on their hind legs. The foraging behavior of honeybees increases the likelihood of moving virus-contaminated pollen to the flowers of healthy plants.

Honeybees exposed to virus-contaminated pollen could also be infected, and the infection is systematically spread throughout their entire bodies, researchers wrote in a study in the American Society of Microbiology’s online journal mBio.

A virus establishes an infection in a novel host through three processes.

First, it must come into direct contact with the host for the viral particles to gain entry.

Second, the virus must undergo genetic changes that allow it to gain entry into host cells.

Third, it must undergo genetic changes to defend itself from the host’s immune defense and replicate its genome using the host’s replication machinery.

The study’s detection of replicate intermediates of TRSV in different tissues of honeybees and the prevalence of TRSV in honeybee populations provides strong evidence that TRSV has overcome each of these processes.

Food-bourne transmission is the most important route for virus transmission in honeybees. However, the researchers observed that the virus was not concentrated in the honeybees’ gut and salivary glands. The virus mainly replicated in their wings, nerves, antennae, trachea and blood.

There was a more extensive infection detected in the nervous system than in other internal tissues, suggesting that severe TRSV can potentially cause impairment of the nerve and muscle in honeybees.

The researchers sampled six strong and four weak hives of bees over the course of the year, in which strong and weak colonies were based on the size of the adult population, amount of sealed brood and presence of food stores. They investigated the presence of TRSV and a variety of other viruses implicated in colony collapse disorder.

Colony collapse was dependent on higher concentrations of TRSV and these other viruses.

The researchers also discovered that TRSV was present in Varroa mites, an obligate parasite of honeybees.  TRSV isolates from honeybees, Varroa mites and bee pollen were phylogenetically similar, indicating that they descended from a common ancestor.

It was postulated that Varroa mites obtained the virus from honeybees during their blood feeding and the virus-infected bees contaminated bee pollen. It remains unclear whether TRSV is the primary cause for the decline in the honeybee population.

Further work is needed to investigate TRSV’s mechanism of pathogenicity in honeybees.

The consequences for the agricultural economy as well as the ecosystem are dire if the steep decline in the population of honeybees continues, regardless of whether the decline is a result of TRSV or some other cause.

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