Sex Determination in Honeybees — Haplodiploidy

Honeybees (Apis mellifera) display an entirely different, and genetically quite unusual, mechanism of sex determination called haplodiploidy, which relies not on any distinct, dedicated PAIR of sex chromosomes at all — unlike both the XX-XY system of humans and the ZW system of birds discussed in the previous section — but instead on the total NUMBER of complete chromosome sets (the ploidy level) that a particular individual bee happens to develop from. A fertilised egg in a honeybee colony arises, exactly as in most sexually reproducing organisms, from the union of a haploid egg cell (produced by meiosis in the queen) and a haploid sperm cell (produced by a drone); such a fertilised egg is therefore diploid, carrying the full (2n) chromosome complement, and every diploid individual in a honeybee colony develops as a FEMALE — becoming either a fully reproductive queen or a sterile worker bee, with this particular distinction between queen and worker being decided not by any further genetic difference between them at all, but purely by the different diet (specifically, whether or not the larva is fed royal jelly throughout its larval development) that the two types of female larvae happen to receive.

An UNfertilised egg, by sharp contrast, develops by a wholly different route: it develops through parthenogenesis, meaning it begins embryonic development without ever having been fertilised by any sperm cell at all, and consequently it remains haploid, carrying only a single (n) chromosome set, throughout its entire development from egg to adult. Every haploid individual arising this way in a honeybee colony develops as a MALE — the drone. Because a drone possesses only ONE single complete set of chromosomes rather than the usual paired, diploid set, it has no homologous chromosome PARTNERS available at all with which normal meiotic pairing and segregation could occur; consequently, drones produce their sperm cells by ordinary MITOSIS rather than by meiosis, since there is simply no second homologous chromosome set present for meiosis to act upon. A direct and genetically significant consequence of this mitotic sperm production is that every single sperm cell produced by any one given drone carries EXACTLY the same genetic complement as every other sperm cell that same drone produces, and indeed exactly the same genetic complement as the drone itself — the sperm of a single drone are, in effect, all genetically identical clones of one another, quite unlike the genetically varied sperm produced by meiosis in a diploid male such as a human. …