Social regulation of ageing by young workers in the honey bee, Apis mellifera

Organisms' lifespans are modulated by both genetic and environmental factors. The lifespan of eusocial insects is determined by features of the division of labor, which itself is influenced by social regulatory mechanisms. In the honey bee, Apis mellifera, the presence of brood and of old workers carrying out foraging tasks are important social drivers of ageing, but the influence of young adult workers is unknown, as it has not been experimentally teased apart from that of brood. In this study, we test the role of young workers in the ageing of their nestmates. We measured the impact of different social contexts characterized by the absence of brood and/or young adults on the lifespan of worker nestmates in field colonies. To acquire insight into the physiological processes occurring under these contexts, we analyzed the expression of genes known to affect honey bee ageing. The data showed that young workers significantly reduced the lifespan of nestmate workers, similar to the effect of brood on its own. Differential expression of vitellogenin, major royal jelly protein-1, and methylase transferase, but not methyl farneosate epoxidase genes suggests that young workers and brood influence ageing of adult nestmate workers via different physiological pathways. We identify young workers as an essential part of the social regulation of ageing in honey bee colonies.

Overwintering Is Associated with Reduced Expression of Immune Genes and Higher Susceptibility to Virus Infection in Honey Bees

The eusocial honey bee, Apis mellifera, has evolved remarkable abilities to survive extreme seasonal differences in temperature and availability of resources by dividing the worker caste into two groups that differ in physiology and lifespan: summer and winter bees. Most of the recent major losses of managed honey bee colonies occur during the winter, suggesting that winter bees may have compromised immune function and higher susceptibility to diseases. We tested this hypothesis by comparing the expression of eight immune genes and naturally occurring infection levels of deformed wing virus (DWV), one of the most widespread viruses in A. mellifera populations, between summer and winter bees. Possible interactions between immune response and physiological activity were tested by measuring the expression of vitellogenin and methyl farnesoate epoxidase, a gene coding for the last enzyme involved in juvenile hormone biosynthesis. Our data show that high DWV loads in winter bees correlate with reduced expression of genes involved in the cellular immune response and physiological activity and high expression of humoral immune genes involved in antibacterial defense compared with summer bees. This expression pattern could reflect evolutionary adaptations to resist bacterial pathogens and economize energy during the winter under a pathogen landscape with reduced risk of pathogenic viral infections. The outbreak of Varroa destructor infestation could have overcome these adaptations by promoting the transmission of viruses. Our results suggest that reduced cellular immune function during the winter may have increased honey bee’s susceptibility to DWV. These results contribute to our understanding of honey bee colony losses in temperate regions.

Predictive markers of honey bee colony collapse

Across the Northern hemisphere, managed honey bee colonies, Apis mellifera, are currently affected by abrupt depopulation during winter and many factors are suspected to be involved, either alone or in combination. Parasites and pathogens are considered as principal actors, in particular the ectoparasitic mite Varroa destructor, associated viruses and the microsporidian Nosema ceranae. Here we used long term monitoring of colonies and screening for eleven disease agents and genes involved in bee immunity and physiology to identify predictive markers of honeybee colony losses during winter. The data show that DWV, Nosema ceranae, Varroa destructor and Vitellogenin can be predictive markers for winter colony losses, but their predictive power strongly depends on the season. In particular, the data support that V. destructor is a key player for losses, arguably in line with its specific impact on the health of individual bees and colonies.

Dead or alive: deformed wing virus and Varroa destructor reduce the life span of winter honeybees

Elevated winter losses of managed honeybee colonies are a major concern, but the underlying mechanisms remain controversial. Among the suspects are the parasitic mite Varroa destructor, the microsporidian Nosema ceranae, and associated viruses. Here we hypothesize that pathogens reduce the life expectancy of winter bees, thereby constituting a proximate mechanism for colony losses. A monitoring of colonies was performed over 6 months in Switzerland from summer 2007 to winter 2007/2008. Individual dead workers were collected daily and quantitatively analyzed for deformed wing virus (DWV), acute bee paralysis virus (ABPV), N. ceranae, and expression levels of the vitellogenin gene as a biomarker for honeybee longevity. Workers from colonies that failed to survive winter had a reduced life span beginning in late fall, were more likely to be infected with DWV, and had higher DWV loads. Colony levels of infection with the parasitic mite Varroa destructor and individual infections with DWV were also associated with reduced honeybee life expectancy. In sharp contrast, the level of N. ceranae infection was not correlated with longevity. In addition, vitellogenin gene expression was significantly positively correlated with ABPV and N. ceranae loads. The findings strongly suggest that V. destructor and DWV (but neither N. ceranae nor ABPV) reduce the life span of winter bees, thereby constituting a parsimonious possible mechanism for honeybee colony losses.

Seasonal changes in juvenile hormone titers and rates of biosynthesis in honey bees

Honey bee colonies can respond to changing environmental conditions by showing plasticity in age related division of labor, and these responses are associated with changes in juvenile hormone. The shift from nest tasks to foraging has been especially well characterized; foraging is associated with high juvenile hormone titers and high rates of juvenile hormone biosynthesis, and can be induced prematurely in young bees by juvenile hormone treatment or by a shortage of foragers. However, very few studies have been conducted that study plasticity in division of labor under naturally occurring changes in the environment. To gain further insight into how the environment and juvenile hormone influence foraging behavior, we measured juvenile hormone titers and rates of biosynthesis in workers during times of the year when colony activity in temperature climates is reduced: late fall, winter, and early spring. Juvenile hormone titers and rates of biosynthesis decreased in foragers in the fall as foraging diminished and bees became less active. This demonstration of a natural drop in juvenile hormone confirms and extends previous findings when bees were experimentally induced to revert from foraging to within-hive tasks. In addition, endocrine changes in foragers in the fall are part of a larger seasonally related phenomenon in which juvenile hormone levels in younger, pre-foraging bees also decline in the fall and then increase the following spring as colony activity increases. The seasonal decline in juvenile hormone in foragers was mimicked in summer by placing a honey bee colony in a cold room for 8 days. This suggests that seasonal changes in juvenile hormone are not related to photoperiod changes, but rather to changes in temperature and/or colony social structure that in turn influence endocrine and behavioral development. We also found that active foragers in the late winter and early spring had lower juvenile hormone levels than active foragers in late spring. In light of recent findings of a possible link between juvenile hormone and neuroanatomical plasticity in the bee brain, these results suggest that bees can forage with low juvenile hormone, after previous exposure to some threshold level of juvenile hormone leads to changes in brain structure.