Honey Bee Medicine for the Veterinary Practitioner. Группа авторов
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Conditions inside the colony can likewise favor pathogen spread as the bees maintain strict control of the hive environment (brood nest T = 34–36 °C; outer winter cluster T > 10°C; RH = 60–80%) (Avitabile 1978; Li et al. 2016). This combination of a stable temperature and humidity is essential to support key hive processes including brood development and rearing, yet these conditions also set up a perfect storm for infectious and parasitic disease to thrive. How then do bees work together to prevent infection and maintain colony health within the framework of this environment of potential harmful organisms? Before we delve into the wonderful colony‐level adaptations that support health in the hive environment, we will begin by reviewing the structure of the superorganism.
The idea of the “superorganism” is more than a century old, having first been contemplated by the entomologist William Morten Wheeler (Wheeler 1911). Wheeler believed an ant colony is a system that possesses fundamentally the same properties of an individual organism – namely the complex system (the ant colony in Wheeler's studies) obtains and assimilates substances from the environment, produces offspring, and protects both the system and offspring from disruptions of the environment. Future researchers, including famed ecologists Edward O. Wilson and Bert Hölldobler, refined the definition of a superorganism (also known as eusocial insect societies) by outlining a fundamental couplet: division of labor whereby a small segment of a society produce offspring while the vast majority forgo reproduction to work for the hive – together with the overlap of generations (Wilson 1971; Hölldobler and Wilson 2009). The offspring of the reproductive queen must remain in the nest to help raise the next generation; it was the reproductive division of labor combined with the sibling care for younger siblings in the same nest that marked the rise of eusociality.
Honey bees represent the pinnacle of social evolution by taking this division of labor to the extreme with a single queen monopolizing the egg‐laying role (a healthy queen can lay upward of 2000 eggs per day) while 50 000 or more worker bees toil in the hive. The workers still possess ovaries, but rarely lay eggs, and those that are laid are unfertilized and will only produce males. Worker bees cannot function as individual organisms – their only survival is linked inextricably to a promiscuous queen by way of a tightly choreographed system of communication among closely related sisters. The role of the male is simply as a conveyor of genes offering a mechanism to induce diversity (a maiden queen will mate with up to 20 drones) – such diversity is an essential ingredient for Darwin's recipe of natural selection. In their book, The Superorganism, the authors equate the male of these female‐dominated societies as simply “sperm‐guided missiles.” While essential to the reproduction of the honey bee colony, the males (or “drones” in the 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)