Evolution of the Sacbrood Virus

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The evolution of the Sacbrood virus (SBV) is characterized by the genomic changes that have occurred in SBV since its initial discovery in 1913, which have enabled the virus to continuously infect a wide array of honeybee colonies. [1] SBV is single stranded RNA virus (genus: Iflavirus) that most commonly infects honeybee larvae , and is known to wipe out entire honeybee colonies quickly. [2] Due to SBV, there has been sharp declines in honey bee populations in Europe, as well as a 30% decline each year in U.S. colonies. [3] [4] Studies on the evolution of SBV have arisen in hopes to stop these colony devastations. [4] SBV is one of the most widely studied honeybee viruses in terms of genomic analysis, leading to it having the highest number of complete genomes isolated compared to any other viruses known to honeybees. [3] Through these genome studies, it has been found that there are two distinct lineages of SBV, each characterized by a high mutation rate, leading to multiple subtypes in both lineages. [3] In studying how these lineages have evolved through time, new discoveries in their pathogenicity and different honeybee resistance mechanisms have been unveiled. [5]

Contents

General virology

SBV is one of the few viruses known to infect honeybees that researchers have been successful in sequencing the full genome for. SBV is a single stranded RNA virus. [2] Its genome consists of one large open reading frame with 8,832 nucleotides as well as a poly-A tail are at the 3' end. [6] Most of the structural genes are located at the 5' end of the genome and the non-structural genes at the 3' end. [6] The virus contains a region that encodes a polyprotein, which is the precursor for the main functional proteins that are cleaved to form the capsid. The main functional capsid proteins are termed VP1, VP2, and VP3. [6] Overall, the structure of this virus is similar to others in the Picornavirales order, but it has evolved a unique feature. [6] The difference in SBV comes from the functional protein responsible for the viral genome delivery in host cells. [2] In most Picornavirales, the capsid contains the functional protein VP4 for this. [2] SBV, instead of having VP4, has a small protein attached to the surface of the capsid, called MiCP. [2] Researchers believe it is this small protein that is responsible for forming pores on the capsid. The pores then enable the virus to inject its genome into the host cell. [2] These pores will form on the capsid when exposed to and acidic pH, which would happen upon entry to any host cells. [2]

Evolutionary lineages

A. mellifera Western honeybee.jpg
A. mellifera
A. cerana Apis cerana 08354.jpg
A. cerana

SBV has affected honeybees globally and is divided into two distinct lineages. The names of the lineages are derived from what species of honeybee they infect. The two types are termed AC genotype SBV and AM genotype SBV. [7] AC genotype will infect A. cerana and AM genotype will infect A. mellifera . [7] A. cerana is largely localized in many eastern countries, and there are different subgroups of the AC genotype depending on the region from which the virus was isolated. [8] For example, the Chinese Sacbrood virus, CSBV, is arguably one of the most studied subgroups and is a subgroup of the AC genotype. [8] The genomic sequence of the two lineages are slightly different, with the critical difference coming from genotypic changes in the region encoding the VP1 functional protein. [7] These structural differences can explain the pathogenic differences seen in the two types. [9] The differences in the subgroups of each lineage can be explained by SBV's high mutation rate. [3] The high mutation rate is due to the fact that SBV does not proofread during RNA replication, leading to multiple strains in both lineages. [3]

Apis cerana genotype

The AC genotype is more widely studied compared to the AM genotype. [10] There are numerous strains in this lineage, most coming from China. [10] The first major honeybee colony viral infestation of this genotype was in Guangdong Providence, China in 1972. [10] It has since been identified in other countries like South Korea, India, and Thailand. [11] [9] The strain from the first major SBV colony infestation of this genotype was fully sequenced in 2001. [12] Throughout the evolution of the AC lineage, there have been some major colony devastations due to SBV. [13] Most notably, there was a 100% wipeout of A. cerana in Thailand in 1976. [13] Now that two lineages have been identified, studies have found some differences between the AM genotype and AC genotype, in terms of pathogenicity, which have led to these colony devastations. [9] The AC genotype is slightly more pathogenic and it is known to cause more detrimental effects in honeybee larvae than the AM genotype. [9] Some of these detrimental effects include increased accumulation of ecdysial fluid, cuticle coloration, and possibly death. [2] Death of honeybee larvae is significantly more prevalent in AC infections as opposed to AM. [14] As for studies on the different subgroups in the AC genotype, they have revealed that the sequence changes between those do not cause significant changes in SBV's pathogenicity. [10]

Apis mellifera genotype

Strains of the AM genotype have been identified and sequenced mainly in the U.S, U.K, Australia, and South Korea. [3] Compared to the AC genotype, the AM genotype is less studied and has fewer complete genome sequences available. [3] Though less studied overall, it was this genotype that led to the initial discovery of the virus. [1] It was first identified in the U.S. in 1913. [1] Compared to the high levels of infection with the AC genotype, the AM genotype is known to only infect about 15% of the species. [15] Typically, the AM genotype does not cause lethal consequences in A. mellifera, which has peaked researchers interest on this species of honeybee and why this species of honeybee seem to be more resistant. [3]

New advances

Pathogenicity

Studies on the evolution of SBV have ramped up recently due to increased death in honeybees seen almost globally. [4] [3] The high mutations in SBV pose a constant threat to honeybee colonies, so understanding how the virus is evolving is a broadening topic in honeybee research. Recent studies have found that the AC genotype is capable of infecting A. mellifera, whereas it was thought previously that there was a species barrier between the two virus lineages. [9] Another recent study found evidence that the virus is more pathogenic in cold weather, which would explain why it is more prevalent in the early spring time. [3] The molecular mechanism behind this is currently unknown, but uncovering this in the future could help researchers and beekeepers with prevention protocols. [3]

Resistance mechanisms

Since A. mellifera do not typically experience the same detrimental effects from SBV as A. cerana, this has led to studies trying to uncover possible means of resistance in A. mellifera. It has been reported that honeybee colonies that have better hygiene are more resistant to SBV. [5] Since hygiene is a heritable behavioral trait in honeybees, a recent experimental evolution study selected colonies with better hygiene and examined their larvae survival rate over several generations. [5] They found that after multiple generations the resistance to SBV continues to increase, which offers a possible evolutionary path that honeybees will see in the future. [5] Honeybees with stronger immune systems that are able to resist infection from SBV will likely not only lead to positive selection in honeybees, but further drive the evolution of SBV to evade these mechanisms. [3]

Related Research Articles

<span class="mw-page-title-main">Honey bee</span> Colonial flying insect of genus Apis

A honey bee is a eusocial flying insect within the genus Apis of the bee clade, all native to mainland Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosmopolitan distribution of honey bees, introducing multiple subspecies into South America, North America, and Australia.

Nosema apis is a microsporidian, a small, unicellular parasite recently reclassified as a fungus that mainly affects honey bees. It causes nosemosis, also called nosema, which is the most common and widespread of adult honey bee diseases. The dormant stage of N. apis is a long-lived spore which is resistant to temperature extremes and dehydration, and cannot be killed by freezing the contaminated comb. Nosemosis is a listed disease with the Office International des Epizooties (OIE).

<i>Varroa jacobsoni</i> Species of mite

Varroa jacobsoni is a species of mite that parasitises Apis cerana. The more damaging Varroa destructor was previously included under the name V. jacobsoni, but the two species can be separated on the basis of the DNA sequence of the cytochrome oxidase I gene in the mitochondrial DNA.

<i>Apis florea</i> Species of bee

The dwarf honey bee, Apis florea, is one of two species of small, wild honey bees of southern and southeastern Asia. It has a much wider distribution than its sister species, Apis andreniformis. First identified in the late 18th century, Apis florea is unique for its morphology, foraging behavior and defensive mechanisms like making a piping noise. Apis florea have open nests and small colonies, which makes them more susceptible to predation than cavity nesters with large numbers of defensive workers. These honey bees are important pollinators and therefore commodified in countries like Cambodia.

<span class="mw-page-title-main">Cape honey bee</span> Subspecies of honey bee

The Cape honey bee or Cape bee is a southern South African subspecies of the western honey bee. They play a major role in South African agriculture and the economy of the Western Cape by pollinating crops and producing honey in the Western Cape region of South Africa. The species is endemic to the Western Cape region of South Africa on the coastal side of the Cape Fold mountain range.

Kakugo virus is a picorna-like virus most commonly found in the brains of worker bees. It is a subtype of the Deformed wing virus. The Kakugo virus, when resident in a bee's brain, can contribute to aggressive behaviors similar to those preeminent during a bee's guard phase in their life cycle. Kakugo is the first virus to have been found to cause aggressive behavior, although because the virus was only recently discovered to have such effects, research into the matter is limited.

<i>Apis koschevnikovi</i> Species of bee

Apis koschevnikovi, Koschevnikov's honey bee, is a species of honey bee which inhabits Malaysian and Indonesian Borneo, where it lives sympatrically with other honey bee species such as Apis cerana.

<i>Apis dorsata</i> Species of insect

Apis dorsata, the rock bee or giant honey bee, is a honey bee of South and Southeast Asia. They are typically around 17–20 mm (0.7–0.8 in) long and nests are mainly built in exposed places far off the ground, like on tree limbs, under cliff overhangs, and under buildings. These social bees are known for their aggressive defense strategies and vicious behavior when disturbed. Though not domesticated, indigenous peoples have traditionally used this species as a source of honey and beeswax, a practice known as honey hunting.

<i>Deformed wing virus</i> Species of virus

Deformed wing virus (DWV) is an RNA virus, one of 22 known viruses affecting honey bees. While most commonly infecting the honey bee, Apis mellifera, it has also been documented in other bee species, like Bombus terrestris, thus, indicating it may have a wider host specificity than previously anticipated. The virus was first isolated from a sample of symptomatic honeybees from Japan in the early 1980s and is currently distributed worldwide. It is found also in pollen baskets and commercially reared bumblebees. Its main vector in A. mellifera is the Varroa mite. It is named after what is usually the most obvious deformity it induces in the development of a honeybee pupa, which is shrunken and deformed wings, but other developmental deformities are often present.

Nosema ceranae is a microsporidian, a small, unicellular parasite that mainly affects Apis cerana, the Asiatic honey bee. Along with Nosema apis, it causes the disease nosemosis, the most widespread of the diseases of adult honey bees. N. ceranae can remain dormant as a long-lived spore which is resistant to temperature extremes and dehydration. This fungus has been shown to act in a synergistic fashion with diverse insecticides such as fipronil or neonicotinoids, by increasing the toxicity of pesticides for bees, leading to higher bee mortality. It may thus play an indirect role in colony collapse disorder. In addition, the interaction between fipronil and N. ceranae induces changes in male physiology leading to sterility.

<span class="mw-page-title-main">East African lowland honey bee</span> Subspecies of honey bee native to Africa

The East African lowland honey bee is a subspecies of the western honey bee. It is native to central, southern and eastern Africa, though at the southern extreme it is replaced by the Cape honey bee. This subspecies has been determined to constitute one part of the ancestry of the Africanized bees spreading through North and South America.

<span class="mw-page-title-main">Western honey bee</span> European honey bee

The western honey bee or European honey bee is the most common of the 7–12 species of honey bees worldwide. The genus name Apis is Latin for "bee", and mellifera is the Latin for "honey-bearing" or "honey carrying", referring to the species' production of honey.

<i>Apis cerana japonica</i> Subspecies of bee

Apis cerana japonica is a subspecies of the eastern honey bee native to Japan. It is commonly known as the Japanese honey bee. Analysis of mitochondrial DNA suggests that the ancestors of this subspecies came to Japan from the Korean Peninsula via Tsushima Island. Genetic differentiation between Japanese honeybees and Korean honeybees occurred about 20,000 years ago, which coincides with the separation of Japan's Tsushima Island from the Korean Peninsula due to sea level rise. They have been observed moving into urban areas in the absence of natural predators.

<span class="mw-page-title-main">Iflaviridae</span> Family of viruses

Iflaviridae is a family of positive sense RNA viruses insect-infecting viruses. Some of the insects commonly infected by iflaviruses include aphids, leafhoppers, flies, bees, ants, silkworms and wasps. The name "Ifla" is derived from the name "Infectious flacherie virus", a member species. There is one genus (Iflavirus) and 16 species in this family.

<i>Apis cerana</i> Species of insect

Apis cerana, the eastern honey bee, Asiatic honey bee or Asian honey bee, is a species of honey bee native to South, Southeast and East Asia. This species is the sister species of Apis koschevnikovi and both are in the same subgenus as the western (European) honey bee, Apis mellifera. A. cerana is known to live sympatrically along with Apis koschevnikovi within the same geographic location. Apis cerana colonies are known for building nests consisting of multiple combs in cavities containing a small entrance, presumably for defense against invasion by individuals of another nest. The diet of this honey bee species consists mostly of pollen and nectar, or honey. Moreover, Apis cerana is known for its highly social behavior, reflective of its classification as a type of honey bee.

<span class="mw-page-title-main">Chronic bee paralysis virus</span> Virus which infects bees

Chronic bee paralysis virus (CBPV) commonly affects adult Apis mellifera honey bees and causes a chronic paralysis that can easily spread to other members of a colony. Bees infected with CBPV begin to show symptoms after 5 days and die a few days after. Chronic bee paralysis virus infection is a factor that can contribute to or cause the sudden collapse of honeybee colonies. Since honeybees serve a vital role in ecological resilience, it is important to understand factors and diseases that threaten them.

<i>Slow bee paralysis virus</i> Species of virus

Slow bee paralysis virus (SBPV) is a virus discovered in England in 1974 that infects honeybees, bumblebees, and silkworms through Varroa destructor mite infestations. The virus causes paralysis in the front two pairs of legs of adult bees eventually killing its hosts. The virus is in the iflaviridae family of viruses. Infection by iflaviridae viruses is among the leading cause of death of honeybee colonies. As bees and silkworms are of great economic and biological importance, the virus is the subject of ongoing research.

<i>Black queen cell virus</i> Species of virus

The black queen cell virus (BQCV) is a virus that infects honey bees, specifically Apis mellifera, Apis florea, and Apis dorsata. Infection of the latter two species is more recent and can be attributed to genetic similarity and geographical closeness.

<i>Snodgrassella alvi</i> Species of bacterium

Snodgrassella alvi is a species of Gram-negative bacteria within the Neisseriaceae and was previously the only known species of the genus Snodgrassella. It was isolated and scientifically described in 2012 by Waldan K. Kwong and Nancy A. Moran, who named the bacteria after the American entomologist Robert Evans Snodgrass.

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