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A drone bee is male and is the product of only a queen bee. How is it that the drone bee can be male. I know that a drone is produced by an unfertilized egg, but this does not explain where the two X chromosomes come from. To me, this means that the one X chromosome from the queen must some how be duplicated.

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    $\begingroup$ The drone bee is haploid, they only have 1 copy of each chromosome, so the chromosomes aren't duplicated. See this. $\endgroup$ – user137 Feb 10 '15 at 19:44
  • $\begingroup$ In the general case sex chromosomes in animals are different from the human XX/XY. Birds, some reptiles, some insects use ZW, where the female is ZW and the male is ZZ. Others use XO where XX is female and X is male. There are others. Fungi have hundreds of 'sexes' because they have a particularly weird system. $\endgroup$ – Resonating Feb 10 '15 at 21:24
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The short answer, which has already been pointed out by user137, is that females are diploid while males are haploid. Bees do not have sex chromosomes per se; rather sex is determined by the csd gene on chromosome III (csd stands for complementary sex determination). Diploid bees that are heterozygous at the csd locus (i.e. contain two different alleles (versions of a gene) for csd) are female (workers).

Bees that are haploid at the csd are males. Interestingly, diploid bees that are homozygous at the csd (i.e. contain two copies of the same csd allele) are also males. But in the honey bee, these bees do not survive as they are killed by workers before they mature. In some other haplodiploid species, however, both haploid and diploid males are viable (the bumble bee Bombus terrestris for example). Also note that diploid honey bee drones can be viable if they (somehow) make it to maturity without being killed by workers (e.g. see here).

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  • $\begingroup$ Do you have a source for the diploid males? $\endgroup$ – user137 Feb 11 '15 at 21:40
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    $\begingroup$ Here is one source link.springer.com/article/10.1007%2FBF01240420 $\endgroup$ – yidryi Feb 11 '15 at 21:42
  • $\begingroup$ That's interesting, looks like 50% of all larva in those colonies are male, that would seriously handicap those colonies. Since the odds of diploid males goes up when inbreeding happens, do bees have some sort of defense against inbreeding? How many versions of the CSD gene are there? Because more versions would lead to reduced chances of diploid males. Also makes me want to know how the bee detects heterozygosity vs homozygosity. $\endgroup$ – user137 Feb 11 '15 at 22:02
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    $\begingroup$ Yes the honey bee csd is highly polymorphic, which reduces the odds of inbreeding (e.g. ncbi.nlm.nih.gov/pmc/articles/PMC1626638). It's an interesting point within the context of bottlenecked populations (e.g. imagine an invasive bee swarm transferred by boat that has no conspecifics around) because, as you point out, if the population has low csd variability then it increases the probability that a diploid bee will be an inviable male rather than a viable worker. But as queens mate with multiple drones, a single invading queen can have multiple csd alleles in her spermatheca. $\endgroup$ – yidryi Feb 11 '15 at 22:27
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    $\begingroup$ Also note that diploid honey bee drones can be viable if they (somehow) make it to maturity without being killed by workers (e.g. ncbi.nlm.nih.gov/pubmed/16475107). It looks like workers tell diploid and haploid males apart by pheromones. $\endgroup$ – yidryi Feb 11 '15 at 22:32

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