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Genetic Diversity Matters

Posted June 5, 2024

History

The honeybee originated in the Old World, where it diverged into more than two dozen recognized subspecies. Initial introduction of the honeybee (subspecies A. m. mellifera) to North America occurred in the 17^th century and records indicate that another seven subspecies were introduced by 1922, when further importations were restricted. With the notable exception of the introduction of African A. m. scutellata into Brazil in 1957 and subsequent expansion of descendant Africanized populations into parts of the southern U.S., no additional subspecies have been introduced into these existing New World honeybee populations.

The first recorded importation occurred in 1622, when the Virginia Company sent ships full of seeds, fruit trees and various animals, including bees. Most honeybee importations occurred between 1859 and 1922. In 1922, in response to the discovery of a parasitic honeybee mite in Europe, a law was passed that restricted the importation of adult honeybees into the United States. Between 1859 and 1891 seven additional subspecies were brought into the United States. However, only two subspecies were favored by the beekeeping community (A. m. carnica, and A. m. ligustica) and remain available as selected strains today in the United States.

When considering genetic diversity, it is helpful to realize that the honeybee populations originally introduced into North America were altered through two structural genetic “bottlenecks”. First, the initial “sampling” of each subspecies chosen for importation consisted of a few tens to hundreds of queens, representing only a small fraction of the genetic diversity within each subspecies. Secondly, only nine of the more than two dozen named Old World subspecies found within the species Apis mellifera were ever introduced into the Americas. Thus, overall “sampling” of the within-species diversity was only partial, with 2/3 of the named subspecies never having been introduced into the Americas. After the initial importations, additional losses of genetic diversity could have been expected due to “genetic drift.” Genetic drift can be thought of as changes in gene frequencies across generations due to chance or as the effect of inbreeding in small populations, both of which can lead to loss of gene diversity.

In addition to the bottlenecks mentioned above limited importations over the last 90 years of honeybees from areas of natural origin, coupled with a queen production system that annually produces most US commercial queens from a relatively small number of queen mothers, represent additional genetic bottlenecks. Such bottlenecks could reduce genetic diversity and may limit our ability to select strains of bees that can both tolerate Varroa mites and be commercially productive. For instance, in studies conducted in 1993-1994 and in 2004-2005, U.S. commercial queen producers self-reported the production of close to 1 million queens for sale from around 600 and 500 queen “mothers”, respectively.

The U.S. beekeeping industry is built upon the development of large-scale queen and package bee production without access to stocks of origin, standardized evaluation, or stock improvement programs. They are also based upon the selection of a few queen mothers from among thousands of colonies within commercial operations that are generally derived from only two European subspecies, Central Europe or Carniolan (A. m. carnica) and Italian (A. m. ligustica). Sadly, access to beneficial scientific bee-breeding programs have largely been dependent upon institution and government support. These are frequently subject to short-term funding and programs that have been turned over to the industry historically lack oversight and soon become unrecognizable. Without a long-term commitment and supporting resources, selection efforts are relaxed, and the gains are quickly lost.

Genetic diversity is crucial to any breeding program and has been correlated with increased longevity and overall genetic health of a species. So, what is a backyard beekeeper to do? How do they ensure they get the genetics they are paying for? Does the commercial breeder really know the physical and behavioral issues you will experience with the bees you buy? So where do you get healthy, genetically diverse, and local parasite and pathogen resistant honeybees? Maybe it’s simply in your backyard.

Genetic Migration “Bottleneck”

Across Europe, Africa and western and central Asia, the honeybee evolved and adapted to a large variety of climatic and ecological conditions. Currently, 26 different subspecies are recognized within this range, with classification based on their form and structure (morphology). From this large pool of honeybee subspecies, a small number of honeybees of a modest subset was exported to North America beginning their gene flow (genetic migration) from the Old World to the New World.

In the early 1990s the production queens advertised for sale in the United States were mainly Italian, Carniolan, or Caucasian except for several commercial strains of variable genetic origin, such as Buckfast, Starline, and Yugo. Based on interviews with beekeepers in 2004–2005, 16 of 36 queen producers obtained their breeder stock from universities or government programs that developed specific genetic lines. However, most of the queen mothers used by the industry were still from producer-maintained Italian and Carniolan stocks.

Since then, queen and package bee production has “bottlenecked” U.S. honeybee genetic migration to two geographically distinct regions, each contributing roughly equal numbers of queens for sale. The western queen-producing region is primarily located in central California, with some operations in southern California. The second major queen-producing area is in the southeastern United States, with most operations located from Florida through to Texas. In these regions commercial honeybee breeding populations are maintained by queen producers who typically select for a limited set of traits desirable to apiculture, including honey production, colony growth, colony survivorship and temperament. Coloration is also a major criterion used for the selection of different strains.

Queen Quality and Insemination Diversity

The primary perceived problem for beekeepers is a diminished quality of queens, and recent survey results from beekeeping operations in the U.S. confirms this view. 305 beekeeping operations in the U.S. were surveyed accounting for a total of 324,571 beehives. According to the interviewed beekeepers, their primary perceived problem was ‘poor queens’, with 31% of the dead colonies because of one or more issues with the mother queen. By contrast, starvation (28%), varroa mites (24%), and CCD (9%) were significant but less prevalent causes of mortality. “Poor” queens encompass many different problems, but most of these reports document premature supersedure (queen replacement), inconsistent brood patterns, early drone laying (indicative of sperm depletion), and failed requeening as indicative of low queen quality.

Another important characteristic that determines a queen’s quality is the degree to which she is inseminated, since queens with greater sperm stores can live longer and fertilize more eggs. Queens take mating flights early in life when they are approximately one week old, mating with multiple males on one or several flights away from their hive. Sperm is temporarily deposited in the median and lateral oviducts, then a small proportion migrate and are stored in the spermatheca. Resent research also found 19% of commercially produced queens had fewer than 3 million sperm (“poorly inseminated”), but they also found that 80% of the queens had fewer than 5 million sperm (“inadequately inseminated”), with an average of 4 million sperm. These numbers are consistent with commercially tested queens in CA in the mid-1980’s.

Insemination is one measure of a queen’s mating success, but emerging evidence suggests that mating diversity is also important for queen and colony productivity. The genetic diversity within a colony is a direct reflection of the number of drones that sire worker offspring, and several empirical studies have demonstrated that genetically diverse colonies increase the behavioral function of the worker force, reduce the likelihood for detrimental levels of inviable brood due to the primary sex-determiner (csd locus) and lower the prevalence of various parasites and pathogens.

Lineage Diversity and Economics

The European honeybee (Apis mellifera) is a key pollinator and has in the last decades suffered significant population decline. Our data confirm that a loss of genetic diversity has occurred during the last century, potentially increasing honeybees’ vulnerability to contemporary ecological and anthropogenic stressors. The Agricultural Research Service (ARS) researchers recently studied the U.S. honeybee’s genetic diversity to ensure that this crucial pollinator insect has sufficient diversity to overcome the growing number of stressors such as parasites, diseases, malnutrition, and climate change. The U.S. honeybee population has low genetic diversity, and this could have a negative impact on future crop pollination and beekeeping sustainability in the country. Researchers studied approximately 1,063 bees from hobbyist, and commercial beekeepers in 45 U.S. states, the District of Columbia (D.C.), and two US territories (Guam and Puerto Rico). The data showed that the nation’s managed honeybee populations rely intensively on a single honeybee evolutionary lineage. In fact, 94 percent of U.S. honeybees belonged to the North Mediterranean C lineage. Data reflected that the remainder of genetic diversity belongs to the West Mediterranean M lineage (3%) and the African A lineage (3%). In a more recent study conducted on unmanaged colonies from 12 states, 83% of the samples belonged to the C lineage, 7% to the M lineage, while 9% were attributed to the Oriental lineage O. Lineage C is by far the most dominant lineage found in the honeybee populations of the USA, with only traces of M and A lineages.

(Eastern Europe) North Mediterranean lineage C – North Mediterranean region of Southeast Europe. It includes several well-defined subspecies, such as A. m. ligustica (Italian Bee), A. m. carnica (Carniolan Bee), A. m. macedonica (Macedonian Bee), A. m. cecropia (Greek Bee), and A. m. cypria (island of Cyprus Bee).
(Western Europe) West Mediterranean lineage M – West Europe A. m. mellifera (European Dark Bee) and A. m. iberiensis (Spanish Bee)
(North Africa) African lineage A – African Bee A. m. lamarckii (Egyptian Bee), A. m. intermissa (African Bee), A. m. syriaca (Middle Eastern Bee), A. m. scutellate East African lowland Bee), A. m. capen (Cape of South Africa Bee), A. m. siciliana (Sicilian Bee) and A. m. unicolor (Madagascar Bee)

The challenge of U.S. honeybees’ weakened immunity has become an economic burden to bee producers and beekeepers. In the past, U.S. beekeepers suffered less honeybee colony losses and treated against varroa mite once per year. In 2023, colony losses and winter mortality are at a high peak and varroa mite requires multiple treatments per year to keep it under control. Researchers worry that 77 percent of our honeybee populations are represented by only two haplotypes, or maternal DNA, while over hundreds of haplotypes exist in the native range of this species in the Old World, or the honeybees’ native land of evolution. Many of these haplotypes have evolved over millions of years in their native lands and have developed astonishing adaptation traits that we should consider incorporating in our US honeybee stocks before it is too late.

Drones

Honeybee queens’ mate with many males, creating numerous patrilines within colonies that are genetically distinct. It’s also been found that swarms from genetically diverse colonies (15 patrilines per colony) founded new colonies faster than swarms from genetically uniform colonies (1 patriline per colony). Interestingly, selecting for non-swarming colonies/queens might be favoring queens that are poorly mated and a cause for commercial requeening annually. Accumulated differences in foraging rates, food storage, and population growth led to impressive boosts in the fitness (i.e., drone production and winter survival) of genetically diverse colonies. These results further the understanding of the origins of polyandry (mating with more than one drone) in honeybees and its benefits for colony performance

Phenotypes and Specialized Workers

Genetic diversity influences a wide range of phenotypes (physical or biochemical characteristics) in honeybee colonies, from expression of antimicrobial compounds, resistance to pathogens, thermoregulation, foraging behavior and colony defense, all essential to colony survival, and response to environmental stress, with lower genetic diversity reducing the variation of these phenotypes as well. Tasks within a colony, such as defense and hygienic behavior, are performed by a small subset of workers descendent from only some patrilineal lines. Differences in propensity for certain tasks are believed to be influenced by genetics. For example, hygienic and non-hygienic colonies have a difference in gene expression in the Cytochrome P450 gene and a limited number of other genes. This means that when genetic diversity is decreased the number of workers in a colony performing some tasks may decrease or less specialized workers will perform such tasks, decreasing the efficiency of the colony. This may originate from high selection pressure selecting for traits based only on queen performance but ignoring the genetic contribution of drones or failing to maintain sufficient levels of genetic diversity within a colony.

Local Adaptation

The genotype of the honeybee is the chemical composition of its DNA, which gives rise to the phenotype, or observable traits. The outward appearance, or phenotype, is the result of interactions of proteins being created by the DNA. In a Europe-wide experiment it was found that colonies of local origin survived significantly longer than colonies of non-local origin, clearly indicating the presence of genotype–environment interactions. Results of the experiment also strongly indicated that environment had a highly significant and much stronger effect on Varroa infestation rates than the genotype of the bees. Location matters. Colony life histories, driven by environmental conditions, have a significant influence on Varroa infestation rates. So simply bringing resistant honeybees into a new environment may not result in continued resistance.

Feral Honeybees and Swarming

Feral honeybee colonies carrying many parasites and/or high parasite abundances would be a nuisance to apicultural disease management and would pose a risk to the health of non-Apis wild bees. However, it’s been found that feral colonies have on average fewer parasites and lower parasite loads than managed colonies. This is partly explained by the effect of natural swarm reproduction and dispersal. Given that feral colonies have a relatively low parasite burden and that they make up a small fraction of the overall honeybee population (in Germany feral colonies make up about 5% of the whole honeybee population in summer), it’s unlikely that they significantly contribute to the spread of bee parasites. On the contrary, new disease agents are probably primarily propagated by managed colonies, as indicated by the higher prevalence of two emerging viruses, chronic bee paralysis virus (CBPV) and deformed wing virus (DWV-B), in managed hives. The management implication is that the prevention of epidemics is no suitable argument for the often-practiced removal or destruction of feral honeybee nests. In fact, there is no conflict between the promotion of wild-living honeybee populations and the management of bee diseases in apiculture. What remains unclear is how the various environmental differences between wild nests and hives at apiaries contribute to the reduced parasite burden in feral colonies. Some known natural parasite-reducing factors, for example the spatial separation of colonies and the periodic interruption of brood production, can readily be adopted by beekeepers to increase the health of managed honeybees and to reduce the risk of disease spread by apiculture.

Domestication

Even though honeybees have been used to produce honey and for pollination purposes for over 7,000 years, since at least Ancient Egypt civilizations, it was only when beekeeping techniques were perfected in the seventeenth and eighteenth centuries that it became possible to maintain large bee colonies giving rise to modern apiculture. More recent practices, such as the commercial mass rearing of queens, artificial selection of behaviors favoring honey production, and the presence of thousands of honeybees in limited spaces, may have altered the natural processes and affected the genetic diversity of domestic and wild (or feral) hives, increasing their susceptibility and the transmission rate of diseases between honeybees. There is an ongoing debate about whether European honeybees are domesticated (in the sense that selective breeding over generations has led to artificial selection) or not. Traits favorable to beekeepers, such as docility, lack of propensity to swarming, honey yield, and others may be selected for, but as it is difficult to have controlled mating, this is usually done through the import of stock from other areas, where these traits are more frequent. This has consequences for wild and local managed populations, as due to the wide freedom honeybees have even when in artificial hives, factors influencing one of them will have a similar effect on the other. Nevertheless, given that commercial beekeepers generally aim for high economic performance and desirable behavioral traits, the direction of evolution under domestication will almost certainly differ from that might have occurred in response to natural selection. Moreover, since human management generally strives for greater homogeneity, this will predictably lead to a reduction in population genetic diversity.

What can be done?

Since 1886, queen bees have been delivered by mail to beekeepers and breeders. Today it is estimated that about one million queen bees are annually sent by mail, mainly in the USA, Canada, Europe, and Australia. As a result, there is increasing evidence that the global honeybee trade has detrimental effects, including the spread of new diseases and pests. However, scientists from a half dozen European countries have found that honeybees that are adapted to the local environment fare much better than honeybees that have been purchased and imported from a completely different home area. Damage from importations may arise from accompanying pests and pathogens, but it is also inevitable that introduced honeybees represent a burden to the genetic integrity of local populations. The spread of imported genes into the local population is likely, and the resulting increase in genetic diversity is not universally beneficial. Since maladapted genes will be selected against, this process may well in the short term contribute to colony losses, and is in the long term, unsustainable.

Domestication and professional breeding aim at selecting individuals with specific traits, consciously or unconsciously narrowing genetic variation. Management practices have also changed to include regular application of acaricides, attempting to secure the survival of colonies. In consequence, though, mite-susceptible colonies are given the chance to propagate and transmit their susceptibility traits to the next generation. In the long term, this practice leads to an increasing dependency on medication and prevents the establishment of mechanisms of mite-tolerance.

Commercial operations, because of their large colony counts, management practices, and annual need for bees, will have many more pathogens and pests than any backyard beekeeper. In addition, the impact of migratory beekeeping, limited diversity breeding, and constant preference for specific traits increases the pressure on the honeybee’s health and increases the need for human management to survive like any other animal under the pressure of domestication practices. This is an obvious cycle and one that is both economic and supply based.

On the other hand, local honeybees fare better than imported ones. Research suggests that the way forward is to strengthen the breeding programs with local honeybees instead of imported queens. This would help maintain the honeybee population’s natural diversity. It would also contribute to preventing the collapse of honeybee colonies, optimize sustainable productivity, and make it possible to maintain continual adaptation to environmental changes.

So, what can the backyard beekeeper do? Here are a few ideas to consider.

Join or even organize a local breeding program focused on sustaining local honeybee colonies.
Prefer local swarm collection over importing queens and packages into your area.
Develop teaching apiaries where new beekeeper could acquire the first colonies.
Promote the health benefits of allowing colonies to swarm.
Increase hive to hive distances reducing drift of pathogens and pests.
Reduce medication and treatment of colonies allowing a natural increase in immune response.
Keep backyard apiaries and colony counts small.
Leave more resources for the bees to discourage robbing and winter losses.

If you are losing colonies every year, why not try something new – something more local.

We have never known what we were doing because we have never known what we were undoing. We cannot know what we are doing until we know what nature would be doing if we were doing nothing. ~Wendell Berry, “Preserving Wilderness” 1987

Referenced Materials

Robbing Stress

Harbo Assay, Varroa Sensitive Hygiene Testing

Mold, should I be concerned?

STOP PRETENDING.

START BEE-TENDING.

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