Honey Bee Development and Caste Determination Lecture

Honey Bee Development and Caste Determination

Learning Objectives

By the end of this lecture, students should be able to:

• Describe the four stages of honey bee development: egg, larva, pupa, and adult.
• Compare and contrast the different morphs/castes (workers, queens, drones) in terms of their development, morphology, and behavior.
• Explain the relative roles of genetics and environment/epigenetics in caste determination.
• Summarize how female larvae can develop into either workers or queens, highlighting the role of epigenetic modifications in this process.

Part 1: From Eggs to Adult

Stages of Development

All honey bee individuals, regardless of their caste, undergo the same four developmental stages:

1. Egg: The queen lays an egg in a wax cell.
2. Larva: The egg hatches. Workers feed the hatched larva.
3. Pupa: The larva reaches full growth, workers seal the cell, and the larva transforms into a pupa.
4. Adult: The adult bee emerges from the cell.

Hormonal Control of Growth and Molts

Growth and molting in insects, including honey bees, are regulated by two key hormones:

• Juvenile Hormone (JH): Produced by the corpora allata, located at the base of the larva's brain. JH controls development during the larval and pupal stages.
• Ecdysone: Produced by the larval prothoracic gland. Ecdysone controls molting in both larvae and pupae.

The relative amounts of Ecdysone and JH dictate the developmental progression:

• When JH levels are high and Ecdysone is present, larvae go through successive molts, growing larger with each stage.
• When JH levels are low, the larva molts into a pupa.
• When there is no JH, the pupa develops into an adult.

Total Development Time (Egg to Adult)

• Queen: Averages $16$ days.
• Workers: Averages $21$ days.
• Drones: Averages $24$ days.

Part 2: Description of the Three Castes

Genome and Sex Determination

Honey bees do not have sex chromosomes. Sex is determined by the complementary sex determiner (csd) gene, where two different alleles lead to females.

• Drones (Male): Haploid (n). They have $16$ chromosomes.
• Workers (Female): Diploid (2n). They have $32$ chromosomes.
• Queens (Female): Diploid (2n). They also have $32$ chromosomes.

Morphology

Each caste exhibits distinct morphological features, adapted to their specific roles.

Drones (Males)

• Numbers: Typically $0$ to about $2000$ per colony.
• Labor: Perform no labor for the colony.
• Sexual Maturity: Become sexually mature by day $12$. They undertake orientation flights (lasting $1$ to $6$ minutes in the afternoon) to learn the hive's location using landmarks.
• Mating: Mate with virgin queens outside the colony, in Drone Congregation Areas (DCAs). They die immediately after mating.
• Feeding: Are fed honey and bee bread by workers while in the hive, or feed themselves.
• Fate in Temperate Environments (e.g., Ithaca, NY): Drones that do not mate are culled from the hive as winter approaches, as they are a resource drain.
• Fate in Tropical Environments: Drones in tropical regions (between the Tropic of Cancer and Tropic of Capricorn) may not be subjected to the same kind of culling due to continuous foraging conditions.

Queens (Female)

• Numbers: Only $1$ per hive, typically.
• Primary Job: Lay eggs, typically $800$ to $2000$ per day.
• Labor: Perform no physical labor.
• Care: Groomed and fed honey and bee bread by worker bees.
• Colony Influence: The single most important individual for influencing colony organization through her pheromones.

Workers (Female)

• Numbers: Vary greatly, from $20,000$ to $80,000$ per colony.
• Roles: Perform virtually all tasks within the colony, other than mating and laying eggs. These activities were described in a pre-lecture video. Tasks include:
  • Handling food.
  • Making honey.
  • Making food for the brood (bee bread).
  • Feeding brood (e.g., mouth-to-mouth feeding).
  • Capping brood cells.
  • Tending and grooming the queen.
  • Making combs with wax.
  • Controlling hive temperature and humidity (e.g., fanning wings for evaporative cooling).
  • Guarding the colony.
  • Gathering nectar, pollen, water, and resin (foraging).
  • Cleaning cells.
  • Removing dead bees and debris.
• Pollen Use: Workers collect pollen and use it to make bee bread, which is stored in the colony's