By Heiloi Yip
Honeybees are well-known for their social hierarchy being consisted of two main castes: the queens and the female workers (the male drones will be ignored in this article). Each colony will typically consist of a single egg-laying queen, along with tens of thousands of workers that perform various tasks to maintain the colony’s health. Aside from the ability to produce new queens and workers, queen bees differ from worker bees by their life expectancies. While the workers typically live up to a month or two, a queen can continue reproducing up to a few years before succumbing to age. Considering that the queens are not very different from workers in terms of size, internal anatomy or genetics, the difference in life expectancy is an astounding phenomenon. Many researchers have picked up on this oddity, studying the honeybees to uncover what exactly drives this queen-worker difference in how quickly they age.
Before getting into some of the reasons why queens live longer than workers, the evolutionary causes of this should first be considered. The short lifespan of most insects selects for those that can reproduce as soon as possible and produce large batches of offspring at a time. Typically, the insect only lives until they have reproduced. However, for a honeybee colony to remain strong and functional, there needs to be a constant supply of workers to replace those lost to ageing or other causes of death. Thus, the queens need to reproduce on a more consistent schedule over longer periods of time, selecting for queens with long lifespans. In contrast, the workers normally are unable to produce their own offspring, instead devoting their entire lives to maintaining the reproductive capacity of the queen. With no need or prospect of reproduction, the workers are left with much shorter lifespans than the queen. 1 While this theory may explain why queen longevity arose in honeybees in the first place, this still leaves the question of what processes occur in the queen’s body to result in such extended life.
The intrinsic cause behind the queen’s longevity is one that has multiple explanations. Queens and workers are genetically very similar due to their mother-daughter relationship, yet the two castes display dramatic differences in morphology, behaviour and lifespan. Instead, epigenetic changes appear to drive queen-worker differences by resulting in different genes being expressed for these two castes. As such, a wide variety of proteins may be responsible for differentiation between queens and workers. For example, a gene regulatory network called the TI-J-LiFe (TOR/IIS–JH–Lifespan and Fecundity) comprises several transcription factors, growth factors and hormones interacting with each other to control ageing in insects. In honeybees and other social insects, there exists queen-worker differences in expression levels of the genes involved in the pathway, suggesting a correlation between epigenetics and longevity. 2
Studies comparing the morphology between queens and workers reveals a higher abundance of two types of cells in the queens: trophocytes and fat cells. It is suggested that presence of these cells plays a role in the prolonged age of queens due to a few reasons. For one, these cells rarely divide, as evident by similar telomere lengths between cells of new and old queens. Low activity of telomerase – an enzyme that alleviates telomere degradation – is also found in the two cell types.3 In addition, both cell types possess higher activity of antioxidant enzymes, which could be involved in preventing damages caused by age-onset oxidative stress.4
Interestingly, the mere ability of a bee to reproduce may play a role in the bee’s lifespan. In colonies where the queen is suddenly lost, a few workers become egg-laying in response to this drastic change in the hierarchy. While the workers can only lay unfertilized eggs, they also appear to attain a higher life expectancy than the average non-laying worker. It is suggested that perhaps egg-laying releases hormones that in turn promote longevity, but this is yet to be investigated. The takeaway point from this phenomenon is the multi-factorial nature of honeybee ageing, beyond just the honeybee’s internal factors.5
When researchers conduct studies on ageing, they typically use familiar model organisms such as Drosophila flies or Caenohabditis roundworms. However, honeybees are an example of how longevity can be manipulated without human interference, and perhaps something can be learnt from studying how this flexibility in ageing evolved in the first place. It goes without saying that this research could be expanded to other social insects: ants, wasps and termites exhibit the same queen-worker discrepancies in longevity as honeybees do, despite all groups having independently evolved their social hierarchies. By learning how other animals age, these findings will overall contribute to our understanding of ageing in organisms of interest such as humans, potentially revealing how it can be manipulated.
References:
1. Tautz J. The Buzz about Bees. Heidelberg: Springer-Verlag Berlin Heidelberg; 2008
2. Korb J, Meusemann K, Aumer D, Bernadou A, Elsner D, Feldmeyer B, et al. Comparative transcriptomic analysis of the mechanisms underpinning ageing and fecundity in social insects. Philosophical Transactions of the Royal Society B: Biological Sciences. 2021 Apr 26;376(1823):20190728.
3. Hsieh Y-S, Hsu C-Y. The changes of age-related molecules in the trophocytes and fat cells of queen honeybees (Apis mellifera). Apidologie. 2011 Nov 1;42(6):728–39.
4. Hsu C-Y, Hu T-H. Energy-regulated molecules maintain young status in the trophocytes and fat cells of old queen honeybees. Biogerontology. 2014 Aug 1;15(4):389–400.
5. Dixon L, Kuster R, Rueppell O. Reproduction, social behavior, and aging trajectories in honeybee workers. Age (Dordr). 2014 Feb;36(1):89–101.
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