5.01 Honey bee evolution
- List environmental threats to honey bee populations.
- Define the three honey bee castes and their roles in the colony (both in terms of work and reproduction).
- Define eucosociality and haplodiploidy.
- Calculate relatedness of pairs of bees in a honey bee colony.*
- Explain how altruism and polyandry contribute to honey bee fitness.*
- Predict the unit of selection of humans informed by understanding of honey bee evolution.*
*To be done in class
Threats to honey bees
Humanity shares a special fascination with the honey bee. Honey bees collect and distill sweet flower nectar into one of the most concentrated forms of simple sugar, honey. Honey bees can also be easily observed buzzing from flower to flower, carrying pollen with them and allowing our favorite foods such as apples, tomatoes, beans and almonds to be fertilized and set fruit.
Recently, however, beekeepers around the globe have suffered colony losses at staggering rates. Last year saw the highest losses yet with over 40% of beehives dying during the year.(1) The causes are still unclear, but likely are a combination of factors including decreased bee habitat, pesticide exposure, disease and stresses involved with commercial beekeeping practices.
Here’s the fabulous Hank Green explaining the scary situation:
Honey bee castes and haplodiploidy
Honey bees are among a select group of insects (namely ants, bees, termites and wasps) that exhibit eusocial behavior. There are several features that define social insects: their colonies are multigenerational, there is a small proportion of reproductive individuals, and there is a distribution of labor. The vast majority of bees in a honey bee colony are female, and females are divided into two castes: queens and workers. Queens and workers develop from fertilized eggs and their differences arise due to the diet they are fed as larvae. Worker bees are fed a glandular secretion, royal jelly, for 3 days and later are fed “bee bread” which is a mixture of honey and pollen. Queen larvae are fed royal jelly throughout development. It was recently shown that p-coumeric acid, which is found in pollen, inhibited regulatory pathways required for queen development. (2) Each colony contains only one queen bee, the only reproductive female. Male honey bees, or drones, develop from unfertilized eggs and make up 0-5% of the colony’s population depending on the time of year. The remainder of the colony’s population is comprised of workers.
Because males develop from unfertilized eggs they are haploid. Females (queens and workers) are diploid. This mechanism of sex determination is called haplodiploidy.
Worker bees are sterile females and depending on their age do a variety of tasks essential for colony survival. Workers build comb, care for the queen and larvae, guard the hive and forage for food. The queen almost exclusively lays eggs and can lay up to 1000 eggs/day. Drones do not contribute to the hive, but rather consume honey and take daily mating flights until they are successful at mating with a queen. This division of labor is known as eusocial behavior. In normal conditions, worker bees do not reproduce and so the work they do to maintain the colony is considered altruistic behavior.
Honey bee genetics
Honey bees utilize a haplo-diploid mechanism of sex determination. Male honey bees, drones, develop from haploid eggs. Female honey bees develop from fertilized or diploid eggs. Honey bees are polyandrous and each queen can mate with up to 20 different drones during her mating flight. She stores the sperm from these drones in an organ called a spermatheca in her abdomen and uses them to fertilize eggs throughout her life (which can be up to 5 years). Honey bee mating occurs in the air in “Drone Congregation Areas”. Drones from many colonies assemble to mate with virgin queens from colonies other than their own. Since drones are haploid, all of their gametes are clonal and originate from mitosis. Gametes produced by queen bees (eggs) are produced by meiosis.
(2) Mao, Wenfu, Mary A. Schuler, and May R. Berenbaum. 2015. “A Dietary Phytochemical Alters Caste-Associated Gene Expression in Honey Bees.” Science Advances 1(7).