why are honeybees haplodiploidy?

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Basically, this means that males develop from unfertilised eggs and are haploid (a cell has one set of chromosomes), however females develop from fertilised eggs and are diploid (a cell has two sets of chromosomes)

In: Biology

Haplodiploidy is seen not just in honey bees, but in all hymenopterans (bees, ants, wasps, and sawflies), and appears to have only evolved once in the ancestor of this group. I don’t think it’s really possible to get a definitive answer for *why* this system evolved, but there is a fair bit of theoretical work on *how* it might have appeared. For example, from a theoretical standpoint, it’s reasonably easy to imagine that the frequency of haploid males in a variable population (assuming that they are viable) would be able to grow much more quickly than the number of diploid males, because females can produce haploid males without having to mate. See [Brown 1964](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1210615/pdf/797.pdf) for more detail on this idea. Other theories have suggested that haplodiploidy might be expected to arise within species that have high levels of inbreeding, e.g. [this paper](https://www.nature.com/articles/6886430).

Either way, the ability to produce haploid males was probably just a rare fluke. In most cases, a haploid individual of a normally diploid species would not be viable, because any recessive mutations with serious negative consequences it inherited would not be recessive anymore! However, at some point in a group ancestral to hymenopterans, presumably viable haploid males were produced anyway, and then spread through some combination of the methods described in the above and other studies.

I also came across [this study](https://onlinelibrary.wiley.com/doi/full/10.1111/evo.12792), which looked at mites (in which haplodiploidy has evolved multiple times, rather than just once as in hymenopterans). They showed that there do seem to be some features that make the transition to haplodiploidy more likely. One of these is male heterogamy (basically, having males with two different sex chromosomes, e.g. X and Y, rather than females having two different sex chromosomes as in birds). Another feature they mention is that haplodiploid species tend to have fewer chromosomes than diploid species, which seems to make sense for complicated reasons I won’t go into. I can’t say for sure if either of these features were present in the ancestor of hymenopterans, but it might be part of the explanation.