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.
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.
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.
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.
Midwest Tech-Transfer Team
University of Minnesota
Bee Informed Partnership