How to manipulate the sex of a wasp

Species: Nasonia vitripennis or the jewel wasp

Sex determination: PSR (in some individuals)

Male (small) and female (big) jewel wasp. If this male possesses an accessory chromosome called PSR, he will not be able to transmit any of his genes and all the offsprings will become males. Photo Source:

A lot of us have probably learnt at school that bees determine sex differently from humans and in a very peculiar way. Embryos with only one set of chromosomes (no egg fertilisation) become males, while females have the two sets of chromosomes from fertilisation (sex determination system called haplodiploidy — it sounds smarter).

This system is actually widely distributed in other hymenopteran groups (including ants and wasps). For one species in particular, it sometimes becomes much more complicated. Scientists have discovered that Nasonia vitripennis, the parasitoid jewel wasp, is the victim of different types of sex manipulation. Today, I will only talk about one: PSR for Paternal Sex Ratio.

Haplodiploidy by its nature makes the wasp vulnerable to external genetic elements that come live in the cell in order to replicate and be transmitted. PSR in particular is the name given to a selfish element that appears in a form of an additional chromosome (scientists also call them B chromosomes). In this case, a male jewel wasp has normally five chromosomes and the individuals that contain B chromosomes have six.

PSR is only transmitted by males. In fertilised eggs — destined to become females — PSR manipulates the eggs’ fate by eliminating the complete chromosome set coming from the father. And as you already know, an embryo with only one set of chromosomes becomes a male. As a result, a normal haplodiploid system produces 90% of females and 10% of males, while if PSR is present in the father, he will produce 100% of males.

Sex determination of Nasonia in the presence of PSR. The green dot is the PSR chromosome. (source: Aldrich et al., 2017)

This selfish element wins the trophy of selfishness as to maximise its transmission chances, it completely prevents half of the host’s genome to be transmitted by taking advantage of the haplodiploid system.

To summarise, PSR is a selfish additional chromosome found in some populations of jewel wasps and modifies the fate of female embryos by pushing out the dad’s chromosomes so it guarantees its own transmission!


Aldrich, John C., et al. “A ‘selfish’B chromosome induces genome elimination by disrupting the histone code in the jewel wasp Nasonia vitripennis.” Scientific Reports 7 (2017).

Let us know what you think of this sex determination system by commenting

  1. Crazy!

2. Cool

3. Boring

4. Sad …

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