Pathogenic Plant Virus Jumps to Honey Bee
A viral pathogen that typically infects plants has been found in honey bees and could help explain their decline. Researchers working in the U.S. and Beijing, China report their findings in mBio, the online open-access journal of the American Society for Microbiology.
The routine screening of bees for frequent and rare viruses “resulted in the serendipitous detection of Tobacco Ringspot Virus, or TRSV, and prompted an investigation into whether this plant-infecting virus could also cause systemic infection in the bees,” says Yan Ping Chen from the U.S. Department of Agriculture’s Agricultural Research Service (ARS) laboratory in Beltsville, Maryland, an author on the study.
“The results of our study provide the first evidence that honeybees exposed to virus-contaminated pollen can also be infected and that the infection becomes widespread in their bodies,” says lead author Ji Lian Li, at the Chinese Academy of Agricultural Science in Beijing.
“We already know that honey bees, Apis melllifera, can transmit TRSV when they move from flower to flower, likely spreading the virus from one plant to another,” Chen adds.
Notably, about 5% of known plant viruses are pollen-transmitted and thus potential sources of host-jumping viruses. RNA viruses tend to be particularly dangerous because they lack the 3’-5’ proofreading function which edits out errors in replicated genomes. As a result, viruses such as TRSV generate a flood of variant copies with differing infective properties.
One consequence of such high replication rates are populations of RNA viruses thought to exist as “quasispecies,” clouds of genetically related variants that appear to work together to determine the pathology of their hosts. These sources of genetic diversity, coupled with large population sizes, further facilitate the adaption of RNA viruses to new selective conditions such as those imposed by novel hosts. “Thus, RNA viruses are a likely source of emerging and reemerging infectious diseases,” explain these researchers.
Toxic viral cocktails appear to have a strong link with honey bee Colony Collapse Disorder (CCD), a mysterious malady that abruptly wiped out entire hives across the United States and was first reported in 2006. Israel Acute Paralysis Virus (IAPV), Acute Bee Paralysis Virus (ABPV), Chronic Paralysis Virus (CPV), Kashmir Bee Virus (KBV), Deformed Wing Bee Virus (DWV), Black Queen Cell Virus (BQCV) and Sacbrood Virus (SBV) are other known causes of honeybee viral disease.
When these researchers investigated bee colonies classified as “strong” or “weak,” TRSV and other viruses were more common in the weak colonies than they were in the strong ones. Bee populations with high levels of multiple viral infections began failing in late fall and perished before February, these researchers report. In contrast, those in colonies with fewer viral assaults survived the entire cold winter months.
TRSV was also detected inside the bodies of Varroa mites, a “vampire” parasite that transmits viruses between bees while feeding on their blood. However, unlike honeybees, the mite-associated TRSV was restricted to their gastric cecum indicating that the mites likely facilitate the horizontal spread of TRSV within the hive without becoming diseased themselves. The fact that infected queens lay infected eggs convinced these scientists that TRSV could also be transmitted vertically from the queen mother to her offspring.
“The increasing prevalence of TRSV in conjunction with other bee viruses is associated with a gradual decline of host populations and supports the view that viral infections have a significant negative impact on colony survival,” these researchers conclude. Thus, they call for increased surveillance of potential host-jumping events as an integrated part of insect pollinator management programs.
Solving of 200-Year-Old Bee Puzzle Began at UC Davis
Arizona State University Provost Robert E. Page, Jr., emeritus professor and former chair of the UC Davis Department of Entomology, and two other UC Davis-affiliated scientists are among the key members of a scientific team from the United States, Germany and France that cracked the 200-year secret of complementary sex determination in honey bees.
The research, “Gradual Molecular Evolution of a Sex Determination Switch in Honeybees through Incomplete Penetrance of Femaleness,” is published in the December edition of Current Biology. The research shows that five amino acid differences separate males from females.
The lead author, Martin Beye, an evolutionary geneticist at the University of Duesseldorf, Germany, was Page’s former UC Davis postdoctoral researcher. Bee breeder-geneticist Michael “Kim” Fondrk provided the genetic material from crosses using Page’s bees that he tends at the Harry H. Laidlaw Jr. Honey Bee Research Facility, UC Davis.
“The story goes back to Johann Dzierson in the mid 1800s through Mendel, through Harry Laidlaw to me and to my former postdoc at Davis, Martin Beye,” Page said.
“Much of the work was done at UC Davis beginning in 1990,” Page said. “From 1999-2000, Martin Beye was a Fyodor Lynen Fellow in my lab funded by the Alexander von Humboldt Foundation. During that year he began the sequencing and characterization of the csd gene; the paper was eventually published as a cover article in Cell.”
Said Fondrk: “This project was a long time in making; it began soon after our Cell paper was published in 2003. First we needed to assemble variation for alleles at the sex locus, by collecting drones from many different, presumably unrelated queens, and mating one drone each through an independently reared set of queens using instrumental insemination (which was Fondrk’s task). “Then a second set of crosses was made to identify and isolate individual sex alleles. The progeny that resulted from this cross were taken to Germany where Martin Beye’s team began the monumental task of sequencing the sex determination region in the collected samples.”
“It’s taken nearly 200 years, but scientists in Arizona and Europe have teased out how the molecular switch for sex gradually and adaptively evolved in the honey bee,” wrote ASU spokesperson Margaret Coulombe, director of academic communications for the ASU College of Liberal Arts and Sciences.
Silesian monk Johann Dzierson began studying the first genetic mechanism for sex determination in the mid-1800s. Dzierson knew that royal jelly determines whether the females will be queen bees or honey bee colonies, but he wondered about the males.
Dzierson believed that the males or drones were haploid – possessing one set of chromosomes, a belief confirmed in the 1900s with the advent of the microscope. In other words, the males, unlike the females, came from unfertilized eggs.
“However, how this system of haplodiploid sex determination ultimately evolved at a molecular level has remained one of the most important questions in developmental genetics,” Coulombe pointed out in her news release.
The collaborators resolved the last piece of the puzzle.
“Once again, the studies by Dr. Rob Page and his colleagues have unraveled another mystery of honey bee development,” commented Extension apiculturist Eric Mussen of the UC Davis Department of Entomology and Nematology, who was not involved in the study but knows the work of many of the collaborators. “It would be interesting if someone investigated the same type of sexual dimorphism in other hymenopterans to determine if they all use the same, ancient-based mechanism.”
The authors studied 14 natural sequence variants of the complementary sex determining switch (csd gene), for 76 genotypes of honey bees.
“While complex, the researchers had several tools at hand that their predecessors lacked to solve this sexual determination puzzle,” Coulombe wrote. “First, honey bees are ideal study subjects because they have one gene locus responsible for sex determination. Also, Page and former graduate student Greg Hunt identified genetic markers – well-characterized regions of DNA – close to the complementary sex determining locus to allow gene mapping. In addition, Hunt and Page found that the honey bees’ high recombination rate – the process by which genetic material is physically mixed during sexual reproduction – is the highest of any known animal studied, which helped Beye isolate, sequence and characterize the complementary sex determining locus. Page and Beye were also able to knock out an allele and show how one could get a male from a diploid genotype; work that was featured on the cover of the journal Cell in 2003.
“However, the questions of which alleles were key, how they worked together and in what combinations and why this system evolved were left unanswered, though tantalizing close. This compelled the current team of collaborators to step back to review what actually constitutes an allele.”
Page was quoted in the news release: “There has to be some segment of that gene that is responsible in this allelic series, where if you have two different coding sequences in that part of the gene you end up producing a female. So we asked how different do two alleles have to be? Can you be off one or two base pairs or does it always have to be the same set of sequences? We came up with a strategy to go in and look at these 18-20 alleles and find out what regions of these genes are responsible among these variants.”
“In this process,” Page said, “we also had to determine if there are intermediate kinds of alleles and discover how they might have evolved.”
“What the authors found,” wrote Coulombe, “was that at least five amino acid differences can control allelic differences to create femaleness through the complementary sex determiner (csd) gene – the control switch.”
Page explained: “We discovered that different amounts of arginine, serine and proline affect protein binding sites on the csd gene, which in turn lead to different conformational states, which then lead to functional changes in the bees – the switch that determines the shift from female to not female.”
The authors also discovered a natural evolutionary intermediate that showed only three amino acid differences spanned the balance between lethality and induced femaleness, Coulombe wrote. The findings suggest that that incomplete penetrance may be the mechanism by which new molecular switches can gradually and adaptively evolve.
Other co-authors included Christine Seelmann and Tanja Gempe of the University of Duesseldorf; Martin Hasslemann, Institute of Genetics at the University of Cologne, Germany; and Xavier Bekmans with Université Lille, n France. Grants from the Deutsche Forschungsgemeinschaft supported their work.
Page, who studies the evolution of complex social behavior in honey bees, from genes to societies, received his doctorate in entomology from UC Davis in 1980, and served as an assistant professor at Ohio State University before joining the UC Davis Department of Entomology in 1989. He chaired the department for five years, from 1999 to 2004 when ASU recruited him as the founding director and dean of the School of Life Sciences, an academic unit within College of Liberal Arts and Science (CLAS).
Recognized as one of the world’s foremost honey bee geneticists, Page is a highly cited entomologist who has authored more than 230 research papers and articles centered on Africanized bees, genetics and evolution of social organization, sex determination and division of labor in insect societies. His work on the self-organizing regulatory networks of honey bees is featured in his new book, The Spirit of the Hive: The Mechanisms of Social Evolution, published in June 2013 by Harvard University Press.
(by Kathy Keatley Garvey, UC-Davis Dept. of Entomology and Nematology)
Ancient Pheromones Keep Queens in Charge
Researchers have identified a particular class of structurally similar, queen-specific hydrocarbons that suppress the reproduction of ant, wasp and bumblebee workers alike -- and they suggest that these pheromones have been around, signaling fertility in social insects, for nearly 150 million years. Previous studies have shown that when it comes to such social insects, queens maintain their monopoly on reproduction by emitting chemical signals that render their loyal workers infertile. But, even though these signals, called pheromones, achieve the same end in various species, they are structurally diverse. Annette Van Oystaeyen and colleagues studied the chemical profiles of the outer skeleton, or cuticle, of the desert ant, the common wasp and the buff-tailed bumblebee and found several compounds that were specifically overproduced in the queens of each species. They tested those chemicals on workers and discovered that, even when their queens were gone, the presence of saturated hydrocarbons kept the workers infertile. (Meanwhile, however, control groups of the insect species rapidly developed ovaries in the absence of their queens.)
Van Oystaeyen and her colleagues compared their findings to those of 90 other published studies and investigated the chemicals that have been consistently overproduced by queens across 64 different species. Their findings reveal that saturated hydrocarbons are, by far, the most common class of chemicals overproduced by social insect queens. In fact, their study suggests that similar hydrocarbons were used by the solitary ancestors of ants, wasps and bumblebees to indicate their reproductive status millions of years ago. The study suggests that these chemicals have been evolutionarily stable, and that queen pheromones are honest signs of the queen’s fertility (not manipulative signals, variable over time, meant to actively suppress worker reproduction). A Perspective article by Michel Chapuisat explains this study in more detail and highlights its implications regarding the ancient origins of eusociality.
PROCEEDINGS OF THE AMERICAN BEE RESEARCH CONFERENCE NOW AVAILABLE!
The 2012 American Bee Research Conference was held February 7-8 at APHIS Headquarters in Greenbelt, MD in conjunction with the annual meeting of the Apiary Inspectors of America. The twenty-sixth American Bee Research Conference will be held in Hershey, PA in conjunction with the annual meeting of the American Honey Producers Association in January 2013. To access these abstracts now, click on the link below. These abstracts represent some of the latest bee research being conducted in the United States. Enjoy!