Haplodiploidy

Which sex will eclose from the pupa, an adult male or female? In the order Hymenoptera, the female wasps, ants, and bees are produced from fertilized eggs, and the males are produced from unfertilized eggs. When an egg is fertilized by a sperm, two sets of chromosomes combine (one set from the mother, the other from the father) to produce a diploid individual. If the egg is not fertilized, the haploid egg containing a single set of chromosomes will develop into a male. This means the males are haploid, and the females are diploid—a condition called haplodiploidy (see sidebar).

The males look entirely normal, but they only inherit one set of genetic instructions solely from the mother. The main function of the males is to mate, so in one sense they are merely flying female gametes. They have a grandfather but no father. Haploid males seem like a perplexing trick of nature from the human perspective.

Diploid Males and Inbreeding

Male production in haplodiploid species can be complex—in some situations, diploid males can be formed. Diploid males occur when there is inbreeding in a population,with the result that a chromosome from the mother and one from the father have the exact genetic information in a critical place. The sameness cancels out the diploid state at that particular genetic location, or sex-determining locus, and the genetic instructions are only translated as if they came from a single, haploid chromosome.

Diploid males are generally useless; depending on the species, they are either killed by the females or do not propagate themselves. Inbreeding and resulting diploid male production might occur when there is severe disturbance in the environment such that only small, local populations of bees remain that must mate among relatives. The worst case scenario is that the local inbred population may become extinct (Zayed and Packer, 2005). It is extremely important to preserve enough natural habitats for bees so they can flourish and avoid mating with relatives. Our job is to be good stewards of our environment, which will allow bees to avoid the threat of inbreeding and production of diploid males on their own.

Adult bees have short or long life spans, depending on the species. In most bee species, the first thing the new adults do is fly off to forage and mate. Males will usually die soon after mating. The haploid set of genetic instructions in each male provides just enough guidance to enable him to nourish himself on flowers, locate a female of the same species, pass on his genes, and die; short and sweet. Females on the other hand have a number of options. For some species, after the female mates, she and her supply of stored sperm may enter hibernation, or diapause, for a variable amount of time, often until the next year. The following spring or summer, she will emerge from diapause, find a nest site, collect pollen, and deposit fertilized and unfertilized eggs in the nest for the next generation. After provisioning the nest, the female may leave it and die, as do the leafcutter and mason bees described in this book. In bumble bees, a queen emerges in late summer, mates, and enters diapause for the winter. In spring, she will emerge, find, and provision a new nest. She will remain with her offspring in a social setting, laying eggs until late in the summer when she will die after producing a new batch of daughter queens. In honey bees, mated queens return to the natal nest without ever entering diapause and spend their entire lives in a populous society, laying thousands of eggs almost slavishly.

How and where bees mate is a great topic for cocktail party conversation. After males eclose as adults, they may spend their days cruising around flowers where females of the same species will also forage for food. The males then pounce on a female right there in an aromatic floral cafeteria. Alternatively, males may cruise nest entrances, particularly when the nests are aggregated in the same area, waiting for a female to depart on her first flight. This may seem like an overly direct way of passing on one’s genes, but it is very efficient. Finally, males and females may meet on non-floral vegetation or in the sky, finding each other by cueing into common features of the landscape or other celestial cues, or by unique odors produced by the bees.

The honey bee mating story is worth even more detail. A 5 to 7 day-old virgin queen leaves her nest and is smart enough to circle around it several times, taking mental snapshots of where her nest is located, so she can find it again upon her return from her eventful day. Without prior knowledge or instruction as to where she should fly, she locates a drone congregation area where males from many neighboring colonies are flying in circles 20 to 30 feet (~6 to 9 meters) in the air. To this day, the cues the females and males use to find these areas remain a mystery. Every afternoon in the summer months, when temperatures are warm and skies are clear, the males approach the entrance of the nest, check out the weather, groom themselves, and then fly out to meet other males in these congregation areas. They return to the nest a time or two to tank up on honey to fuel another 20 to 30 minutes of cruising before calling it a day. If they are lucky, they will encounter a virgin female and pass on their genes before dying. If they do not mate, at least they pass their afternoons trying. The female honey bee does not stop mating after encountering one male. She mates with 10 to 20 males, sometimes more, within one hour before returning to the nest. She may take another mating flight on a subsequent day if she did not encounter sufficient drones the first day. After mating, her behaviors change, and she will not fly out to mate again. She stores a portion of each male’s sperm in a receptacle, called a spermatheca, in her abdomen for the remainder of her life. As eggs mature in her ovaries and pass down one by one through the oviduct, she can release sperm (or not) from her spermatheca to fertilize eggs (or not). This is an excellent example of planned parenthood, with no political ramifications.

The females in most bumble bee species mate once. Females and males find each other in the air, fall to the ground, and remain in copula for hours. Leafcutter and mason bees quickly mate with one or more males at the nest site and get on with things.