honey bee society) do no work and offer no long‐term value to the colony once the missile has launched (Hölldobler and Wilson 2009). Drones die during mating, while any survivors (those that have failed in their only life mission to pass on their genes) are cast aside as resources dwindle each autumn.
A short discourse on honey bee genetics is in order to make sense of how honey bees evolved a social system. Honey bees have a haplodiploid method of sex determination in which the queen bee dictates the sex of her own offspring by adding the drone’s sperm, or withholding it, as each egg is laid. The worker bees also influence colony demographics when they make a cell: standard size comb cells and round queen cups receive a fertilized egg that become future female bees (workers or queens, respectively), while larger comb cells are fashioned for unfertilized eggs that become male bees (drones). The latter process is known as parthenogenesis – passing on just a single set of chromosomes (those of the mother) to the drone bee. Put simply, haploid gives half the number of chromosomes while diploid gives double the number of chromosomes. It was long thought that only female bees (workers and queens) were the outcome of a fertilized egg with the resulting bee receiving two sets of chromosomes, one from each parent. But it is not that simple in honey bee society.
Along came diploid drones from inbreeding studies. With their appearance, it was discovered that the number of chromosomes itself did not dictate the sex of honey bees, but rather a single sex determination locus (SDL) determines the sexual fate of honey bee offspring, a process known as complementary sex determination (Whiting 1933; Hasselmann and Beye 2004). Fertilized eggs are heterozygous at the SDL making females, unfertilized eggs are hemizygous and become fertile drones. And those peculiar diploid drones? They are homozygous at the SDL and never survive beyond their first days as a larva; eaten by workers who recognize such anomalous drones would never contribute to colony reproduction. In the curious world of honey bee gene flow, a drone has no father but does have a grandfather and is a parent to daughters, granddaughters and grandsons, but never to sons.
The superorganism exhibits both altruism and inequality, inconsistencies that Darwin himself struggled with in his unifying theory of evolution (Wilson 1971; Ratnieks and Helantera 2009). Darwin's ideas surrounding natural selection focused on how small heritable traits in the individual offer a survival advantage that is passed on to future generations (Figure 2.1). How then, could individuals that do not produce offspring (worker bees) evolve body shapes and functions far different from their fertile parents (the queen and drones)? Part of the answer can be found in the matter of kinship or a high level of relatedness among worker honey bees. More than a century after Darwin's Origin of Species (1859), Hamilton (1964) wrote that natural selection may favor altruism, but only among related individuals: worker bees are half‐sisters having a single mother, and from a nonreproductive worker bee's perspective, success for her own sisters, and that of the queen, equates with success for herself and the colony. Yet, the kinship theory was cast aside with the subsequent discovery of other eusocial organisms having diplodiploidy (e.g. termites) as well as many haplodiploid species living in groups that failed to evolve a eusocial system (Hölldobler and Wilson 2009). Therefore, the extreme inequality and altruism observed in honey bee societies could not have emerged by close kinship alone. It must have been imposed on the sisterhood by the queen and other workers through coercion or “enforced altruism” – social pressures that essentially prevent workers from egg laying (Ratnieks and Helantera 2009). These pressures arise at both the larval stage by workers that control the level of feeding (phenotypically larger queens require more food) and at the adult stage through “policing,” whereby the queen and other worker bees destroy worker‐laid eggs.
Figure 2.1 Charles Darwin marveled at the superorganism. Recognizing the remarkable structure of honeycomb and the precision of its hexagonal shape, he nevertheless struggled to understand how it may have evolved. Darwin theorized that honeybees once had nests similar to bumblebees, with rough conglomerations of spherical cells. Honey bees, Darwin pondered, must have built circular cells closer and closer together over the generations until finally the cells became organized into the hexagon we see today.
It is fascinating to follow the evolution of the superorganism from solitary insect to primitive eusocial group living to the highly eusocial organism. But there is a difference between the evolution of, and maintenance of, eusociality. The multiple mating of queen honey bees and the resulting diversity of worker bees evolved after the formation of separate castes, a step in the journey to eusociality that Hölldobler and Wilson call “the point of no return.” And it is this diversity in the honey bee that led to improved resistance to disease and the protective nature of colony living – in fact, on the path to eusociality the potency of protective defenses against disease in bee populations rises steeply with multiple matings (Stow et al. 2007). Diversity also brought about improvements in productivity and the regulation of hive temperatures, the latter made possible by a worker bee force having innately different thresholds of response to temperature cues that modulate hive ventilating behavior (Jones et al. 2004).
Whether ants or bees, the superorganism must have offered the society key advantages over life as an individual. Ultimately the concept can be viewed from the level of the gene. Seeley (1989) concluded that the emergence of the superorganism must have arisen through suppression of conflict over reproduction (and thereby gene‐flow) among its constituent parts. “It seems correct to classify a group of organisms as a superorganism when the organisms form a cooperative unit to propagate their genes, just as we classify a group of cells as an organism when the cells form a cooperative unit to propagate their genes” writes Tom Seeley (1989). Now let's turn our attention to the marvelous ways in which honey bees work together as a cooperative unit to maintain a healthy organism.
Part 2: Social Immunity: Bees as Their Own Doctors!
Group living in insects with its consequent division of labor, cooperative care of brood, and the overlap of more than a single generation in time and space are the hallmarks of the superorganism. Insects living within a coordinated framework, where tasks are divided among different bee castes and communication networks are compartmentalized in a confined space, are susceptible to the spread of disease from one individual to another. Likewise, their strict control of the nest cavity environment necessary to maintain the stable temperatures for brood care can be compared to a pathogen incubator. The group living of honey bees predisposes the individuals and the entire organism to epidemics. Fortunately, honey bees and other social insects have evolved highly adaptive behaviors that range from “constitutive” (aka prophylactic) to “inducible” (aka activated) responses that help prevent disease (Simone‐Finstrom 2017). Behaviors that reduce or eliminate pathogen exposure or pest infestation at the level of the superorganism are collectively known as social immunity.
One of the advantages of a social (or group) response to preventing or actively eliminating an infection by a parasite or pathogen in honey bee(s) is a coordinated response from the colony. By doing so, the individual bee is able to conserve resources that it would otherwise expend on maintaining and delivering an individual response. The immune function of individual honey bees is costly and expressed to a lesser degree than in asocial insects; indeed, the mapping of the Apis mellifera genome revealed a surprising lack of immune specific genes (Evans and Pettis 2005; Simone et al. 2009). This does not mean that individual honey bees lack discrete methods for disease protection entirely. Like other insects, honey bees have a hard chitinous exoskeleton that protects against pathogen entry, possess hemocytes that can phagocytize foreign invaders (though they lack memory cells and any ability to produce protective antibodies like vertebrates), remove themselves from the colony when sick or dying, recruit specialized members to perform dangerous biosecurity tasks as guards and undertakers, and even mummify pests too large to carry out of the hive.
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