In bee school you learned that the sex of a bee is determined by the fertilization status of the egg. A fertilized egg becomes a female; an unfertilized one becomes a male (or drone). You may have also learned that this phenomenon is called haplodiploidy, and that all the hymenoptera (bees, wasps, and ants) are haplodiploid organisms.
What they didn’t tell you was that some of the fertilized eggs become diploid drones. This happens because the thing that actually determines sex is not the presence or absence of fertilization but the presence or absence of heterozygous alleles at the sex locus. Don’t go away yet; this isn’t difficult.
You see, instead of having an entire chromosome that determines sex (like the X and Y chromosomes in humans) bees have one gene on one chromosome that determines sex. Specific places on chromosomes are called loci (the singular is locus), so the “sex locus” is just the place (think location) on the chromosome where the sex gene is found.
The European honey bee species has about 18-20 different alleles of the sex gene. An allele is just a variation of a gene. All the sex alleles do basically the same thing, but the genetic coding is a little different in each one. Compare this to having 18 different recipes for chocolate cake—the end products are similar but the instructions for getting there vary.
- Haplodiploidy: Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid.
- Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
- Locus: The specific location of a gene or DNA sequence on a chromosome.
So different bees are running around with different alleles (or instructions) for the sex gene. If an egg is not fertilized, there is only one set of instructions and the bee becomes a drone. If an egg is fertilized and has two different sets of instructions, the bee becomes a female. But—and here’s the rub—if the egg is fertilized but receives two identical sets of instructions (two identical sex alleles) the bee becomes not a female but a diploid drone. Think of it like this: one set of instructions twice is not the same as two different sets of instructions.
These diploid drones do not survive. In colonies of social insects such as honey bees, the worker bees eat the diploid drones soon after the eggs hatch. Many diploid drones in a colony result in “shot brood” or “scattered brood”—brood combs that have lots of empties or brood of many different ages. In solitary bees, the diploid male may die in the cell, or may emerge and mature but be sterile.
The chart below shows what would happen when a honey bee queen (with two alleles) mates with five different drones, each with one allele. In this case, two of the drones have the B allele and the rest have different alleles.
|Drones||Queen Allele A||Queen Allele B|
|Drone Allele A||AA||BA|
|Drone Allele B||AB||BB|
|Drone Allele C||AC||BC|
|Drone Allele B||AB||BB|
|Drone Allele D||AD||BD|
Wherever you have homozygous alleles for the sex gene (two of the same alleles), you get a diploid drone. This chart shows an extreme example because it has a small number of alleles and a small number of matings, but it illustrates how homozygous alleles happen.
In real life, honey bees have about 18 alleles for the sex gene and a queen may mate twelve or more times, both of which lessen the likelihood of diploid males. But inbreeding decreases the number of alleles in a population and thereby increases the occurrence of diploid drones. Large numbers of diploid drones weaken a colony because the nurse bees waste resources raising these bees only to kill them later, and because the presence of so many drones reduces the number of worker bees that the colony can raise.
Large numbers of diploid males hasten a species toward extinction. Studies have shown that when population sizes of haplodiploid organisms become small, they go extinct more quickly than other species (Zayed and Packer 2004, 2005). With feral honey bee populations declining and inbreeding becoming even more common, the presence of diploid drones is a major concern to honey bee breeders.