Male bees, known as drones, possess only one set of chromosomes, a condition called haploidy. Female members of the colony—the queen and the worker bees—possess two sets, a condition called diploidy. This unique genetic arrangement, known as haplodiploidy, governs the entire reproductive system and social structure of the colony in honey bees, as well as many other insects in the order Hymenoptera. The difference in chromosome count is the foundational genetic mechanism that determines a bee’s sex and subsequent role within the complex social hierarchy of the hive.
Understanding Haploidy and Diploidy
The genetic terms haploid and diploid refer specifically to the number of chromosome sets contained within an organism’s cells. Organisms that are diploid, such as all female honey bees, inherit one set of chromosomes from each parent, meaning they possess two complete sets in total. Female honey bees are diploid, having 32 chromosomes in their somatic cells.
In contrast, a haploid organism possesses only a single set of chromosomes, having inherited genetic material from only one parent. Male honey bees are haploid, meaning they have only 16 chromosomes in total. This distinction means that a drone has a mother but no father, as its entire genetic makeup is derived solely from the queen. The number of chromosome sets is the sole determinant of sex in the honey bee.
The Unique Bee Sex Determination System
The process that results in haploid male bees is a form of asexual reproduction called arrhenotoky, a type of parthenogenesis where unfertilized eggs develop into males. The queen bee is responsible for laying all the eggs and possesses a specialized internal organ called the spermatheca, where she stores sperm from the multiple drones she mates with. As the queen lays an egg, she controls whether that egg is fertilized or not.
If the queen releases sperm from her spermatheca to fertilize the egg, the resulting embryo will have two sets of chromosomes—one from the queen and one from the drone—and will develop into a diploid female. If the queen chooses not to fertilize the egg, the single set of chromosomes already present in the egg will develop directly into a haploid male. A key consequence of the drone’s haploid state is that it produces sperm through mitosis rather than meiosis, meaning all of its sperm cells are genetically identical.
Genetic Implications for the Bee Colony
The haploidy of the drone has profound effects on the genetic relatedness within the colony, which is theorized to be a factor in the evolution of complex social behavior, or eusociality. Because a drone’s sperm is genetically identical, any two female offspring (full sisters) that share the same father inherit an identical set of genes from him. This means that full sister worker bees share 100% of their father’s genes and, on average, 50% of their mother’s genes, resulting in a unique relatedness of 75%.
This high degree of genetic similarity among sisters, often called “super-sisterhood,” is higher than the 50% relatedness a worker would have to her own daughters or sons. The theory suggests that a worker bee can pass on more of her own genes by helping to raise her highly related sisters, thereby favoring altruistic behavior and cooperation within the hive. The drone’s primary role is to mate with a new queen, ensuring a direct and unmixed genetic contribution to the next generation of female workers.