Honey Bee Caste Systems: Part 2 – How Genetics and the Environment Shape Honey Bee Workers and Queens

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By Garett Slater, Former Midwest Tech Transfer Team June 5, 2019



In part 1 of my blog series, I wrote about how genetics can shape reproductive males (drones) and both reproductive (queens) and non-reproductive (workers) females within a colony. However, genetics only explains part of the story. I will describe why that is in the second installment of my 3-part series:

  • Part 1: The Genetic Book of Life-The basics to honey bee genetics (for Part 1 click here)

  • Part 2: How Genetics and the Environment Shape Honey Bee Workers and Queens

  • Part 3: The Differences Between Queens and Workers

Figure 1: Queen and Worker honey bees. As you can see, there are clear anatomical differences between these castes despite genetic similarities.

Figure 1: Queen and Worker honey bees. As you can see, there are clear anatomical differences between these castes despite genetic similarities.

Queen determination has always fascinated researchers and beekeepers. This is unsurprising considering queens and workers are genetically similar yet have distinct anatomical and physiological features (Figure 1). In fact, queens are highly fertile and lay more than 2000 eggs per day whereas workers have anatomical structures specialized for foraging, nursing and other colony tasks. So how does a colony produce either a fertile queen or a sterile, highly specialized worker, even if they have the same genetics?

Most beekeepers and queen producer know nutrition determines whether a fertilized larva develops into a queen or worker. Just place a 1-3-day old larvae into a queenless colony and watch as they develop a queen from a previously worker-destined larvae (Figure 2). However, how this nutrition determines queens has historically perplexed researchers. Since the 1890’s, diet quality has been thought to determine queen-worker castes in honey bees through a “biological active substance” found only in royal jelly. This quality hypothesis arose from early observations of queen and worker larvae receiving different proportions of water-clear and milky-white secretions from nurse bee glands. The milk-white secretion fed to queen-destined larvae was termed royal jelly whereas the water-clear secretion was termed worker jelly (Figure 3). Since then, royal jelly was thought to contain the major dietary components necessary for queen development.

Figure 2: Queen producers make queens by transferring worker-destined larvae into queen cells of breeder colonies.

Figure 2: Queen producers make queens by transferring worker-destined larvae into queen cells of breeder colonies.

Figure 3: Royal jelly (left) is fed to queen-destined larvae and worker jelly (right) is fed to worker-destined larvae.

Figure 3: Royal jelly (left) is fed to queen-destined larvae and worker jelly (right) is fed to worker-destined larvae.

The first person to empirically test differences between royal and worker jelly was Dr. Adolf van Planta in 1888 (Table 1). He concluded the food composition fed to workers changed drastically after the age of 4 days, which is when worker larvae cannot develop into a queen naturally in a colony. While the food fed to workers and queens seem striking, royal jelly content was only quantified for 1 day. However, once this study was published, researchers began to search for the substance in royal jelly that determines caste. In fact, the only other study after Dr. von Planta’s publication to compare differences between royal and worker jelly was Wang et al. 2016.

Royal jelly was then deemed special and necessary for queen development. In fact, royal jelly has been thought to contain a “pure determining substance” not found in worker jelly ever since. This has pushed scientists to find this active substance so we can truly understand how queens develop.  Researchers have tested nearly every major component in royal jelly on caste determination. In fact, most studies found positive results. They found lipids, proteins, carbohydrates, water, pantothenic acid (vitamins) and even p-coumaric acid (chemical in pollen) all contribute to queen development in honey bees under some experimental conditions. Despite positive results, how does every single macronutrient and micronutrient in royal jelly determine caste? This question perplexed me.

Table 1: Comparison between Royal and Worker jelly.

Table 1: Comparison between Royal and Worker jelly.

As I began researching royal jelly and queen development, I realized none of these studies controlled for diet quantity. This is surprising because queens are obviously fed more food than workers during development. In fact, queen-destined larvae are fed an excess of 300mg of diet and are fed 1400 times more frequently by nurse bees than worker-destined larvae. As I did a literature search, I couldn’t believe the impact of quantity on honey bee caste determination has not been formally tested. 

Figure 4: 48 well plate I use for rearing honey bee larvae on an artificial diet


Figure 4: 48 well plate I use for rearing honey bee larvae on an artificial diet

I decided to pursue this question during my masters. I used in vitrorearing techniques to test whether diet quantity causes queen development (Figure 4). In vitro rearing is a useful tool because I can become a “nurse” bee and control the food larvae receive. In fact, I can change the type of food and the amount of food larvae receive and see how that impacts development. So, I tested the relative contributions of diet quantity and quality by rearing honey bee larvae on diets that altered both quality and quantity.

Figure 5: The range of sizes I produced by changing diet quantity. I produced small workers, workers, intercastes, and queens.

Figure 5: The range of sizes I produced by changing diet quantity. I produced small workers, workers, intercastes, and queens.

A wide range of individuals were raised from my artificial diets. Not only were queens and workers reared, but also intercaste bees (part worker-part queen), or what I call them, princess bees (Figure 5). I have never seen intercaste bees in a natural hive setting nor have I seen miniature workers half the size of normal workers. Thus, it seems bees have the ability to develop into a wide range of body sizes. These results indicate 2 conclusions: 1) nutrition during development is extremely important for both workers and queens, and 2) colonies impressively control queen versus worker development. More importantly, it seems diet quantity plays a larger role in caste determination than expected.

Caste determination is fascinating and I truly enjoyed studying it, but why should the average beekeeper care? The main reason is queens are an integral component of a colony however we have little understanding about how a queen develops and which factors make a high-quality queen. This is important to improve queen quality and manifest important queen traits through breeding, selection, and alternative management practices.

Honey bee queen determination is an intersection between genetics and the environment. I hope you enjoyed reading this post and keep an eye out for the next installment on the key differences between queens and workers.

Citations

von Planta, A. (1888). Ueber den Futtersaft der Bienen. Zeitschrift für physiologische Chemie12(4), 327-354.

Wang, Y., Ma, L., Zhang, W., Cui, X., Wang, H., & Xu, B. (2016). Comparison of the nutrient composition of royal jelly and worker jelly of honey bees (Apis mellifera). Apidologie47(1), 48-56.

https://beeinformed.org/2019/06/05/honey-bee-caste-systems-part-2-how-genetics-and-the-environment-shape-honey-bee-workers-and-queens/

Honey Bee Caste Systems: Part 1 - Honey Bee Genetics

Bee Informed Partnership By Garrett Slater March 19, 2019

I have always been fascinated with queens and workers. In fact, I spent my master’s degree studying the mechanisms that produce queens and workers. I won’t bore you with my master’s thesis, but I did want to write about the fascinating differences between queens and workers. This topic includes a lot of information, so I decided to split this topic into 3 blog installment: 

  • The Genetic Book of Life-The basics to honey bee genetics

  • How genetics and the environment shape honey bee workers and queens

  • The differences between queens and workers 

Honey bees are unique living organisms. Some fascinating traits honey bees possess include: 1) distinct reproductive caste system, i.e. fertile queens that lay the colony’s eggs and sterile workers who forego their own reproduction but help raise their brothers and sisters instead, 2) they have a behavioral division of labor within the worker caste, and 3) they have distinct sexual dimorphism. As most beekeepers know, honey bees include many more interesting characteristics, but I included the three that I am most interested in! While honey bees are quite unique compared to any other animal or living form, the underlying material by which these traits are passed on to future generations is shared with all organic living organisms: Deoxyribonucleic Acid or DNA. DNA carries the genetic material necessary to produce the distinct and fundamental characteristics of honey bees. While all living organisms have DNA, honey bee genetics is unique.

Honey bees have a system of sex determination (male drones versus female queens or workers) known as haplodiploidy. This differs from human sex determination in several ways. With humans, both males and females carry two copies of every chromosome (they are both diploid), one inherited from the father, and one from the mother. Human males result because they have a specific sex chromosome (Y chromosome) that females lack. With honey bees, queen bees carry sperm inside a specialized compartment within her body that she obtained from earlier mating events, and she determines whether or not to fertilize each egg as it is being laid. Males develop from unfertilized eggs, and therefore only carry a single set of chromosomes (Haploid) and females develop from fertilized eggs and possess two copies of each chromosome (Diploid), Females receive DNA from both parents, while males receive DNA from just the mother. Therefore, this is referred to as a Haplodiploid genetic system.

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Figure 1: Depicted above is the genetics of honey bee workers and queens. Female workers and queens result from fertilization, which is the act of fusing female queen eggs with male drone sperm. This combination results in a diploid egg and contains chromosomes from both the male drone and the female queen. Unique to honey bees, diploid females can develop into either a queen or worker. This depends upon the nutrition they receive during development.

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Figure 2: The picture above is the genetics of a laying worker. A laying worker has underdeveloped reproductive traits, so they cannot mate with drones. Because of this, they cannot fertilize eggs and produce female workers or queens. The laying workers can, however, produce unfertilized haploid males. This is a last-ditch effort for the colony to pass along its genetic material to future generations because the colony will not survive.

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Figure 3: The picture above shows a queen laying drone eggs. Queens can either lay fertilized or unfertilized eggs. This typically depends upon cell size as queens lay unfertilized drone eggs into drone cells. In some situations, queens run out of viable sperm for many different reasons. Queens can only produce unfertilized drone eggs, which can spell doom for a once prosperous colony.  

Figures 1-3 summarize the genetic differences between diploid females and haploid males. In order for females to develop, they need a different genetic recipe from both the mother and father. Diploid males are a great example of how important these different genetic recipes are in sex determination. In certain cases, diploid males can result if they receive identical chromosomes from both the father and mother. This can result from very inbred populations, and results in infertile males.

Queens are the only individuals in the colony that can produce both diploid female workers or queens and also produce haploid males. I will touch on why workers cannot produce diploid females in a later blog, but I describe in some detail in Figures 2-4. Though, workers can lay drones because workers are able to lay unfertilized eggs. Essentially, workers cannot mate or store sperm, so they produce just haploid males.

Honey Bee genetics is fascinating. If you enjoyed reading this blog as much as I enjoyed writing it, keep an eye out for the next installment on how genetics and the environment shape honey bee workers and queen. 

Cheers!
Garett Slater
Midwest Tech-Transfer Team
University of Minnesota
Bee Informed Partnership

https://beeinformed.org/2019/03/19/honey-bee-caste-systems-part-1-honey-bee-genetics/