Saturday genetics lesson: A gynandromorph stag beetle

Matthew sent this tw**t by entomologist Gil Wizen:

Which highlighted the picture below (I can’t read the Japanese; if you can, please translate).

Look carefully:

“Gynandromorphs” are animals that are part male and part female, with the parts strongly demarcated; they’re not “interesexes”. Both Matthew and I have written about these anomalies before (see here and here), and, as we and this longer piece explain, such mosaic animals usually result from loss of a sex chromosome during embryonic development, which results in some parts bit being male and others female. How it works depends on the system of sex determination.

In the fruit fly Drosophila, and many insects, secondary sex characteristics (those traits that make you appear either “male” or “female”) depend on the ratio of X chromosomes to autosomes (non-sex chromosomes). If you’re XX and have the normal set of paired autosomes, then the ratio of Xs to autosomes is 1, and you look female. If you’re XY, that ratio is 0.5 and you look male. But if an XX female embryo loses one X at some stage of development, the descendant cells will be XO (“O” means “no Y’) and your ratio will become 0.5—male. Thus, the cells losing the X, and their descendants, will produce male body parts, while the original XX tissue will produce female body parts. (XO males, however, are sterile, as they lack the Y chromosome genes necessary for male fertility.)

If the X is lost in one of the first two cells, then exactly half the body will be male and half female: split right down the middle. this produces the “bilateral gynandromorphs” that I used to see about once a year while pushing flies. This is what they look like:

Males are smaller than females, so that parts of the right side of the body above, which has lost an X, are smaller than those on the left (note the shorter wings). Also, the fly has male-like pigmentation on the right (dark pigment on the posterior abdomen) and typical female striping on the left. The “sex comb” (a tuft of stiff bristles on the foreleg, probably used to grasp females during mating) is present on the shorter right foreleg but not on the left.

Here’s a photo of a real fly gynandromorph (source here):

The male half has white eyes because the original female was heterozygous for a recessive allele on the X chromosome that eliminates eye pigmentation: it was Ww, where “w” is the recessive “white” allele. The X chromosome containing the W allele was lost on the right side of the body, producing an XO male half that expresses the w allele. Note the shorter wings and darker pigmentation on the right side. If you turned the fly over, you’d see that half of its genitalia was female and the other half male.

You can use this technique to investigate where in the body male pheromones and behaviors are “produced”. I won’t get into the details of that technique (developed by Seymour Benzer), but an example of such work, done by myself and my undergraduate student Ryan Oyama, can be seen here. (I have to say, it was a clever experiment but the idea of studying sex-specific chemicals and behaviors came from Benzer).

Now as for that stag beetle: it’s was probably produced the same way the flies above were: loss of an X chromosome in a female embryo. Most beetles have the same kind of X/autosome ratio sex determination as do Drosophila, so that loss of an X chromosome in part of a beetle produces male secondary sexual traits. In the stag beetle above, an X was probably lost on the left side at the two-cell embryonic stage, and so the left side became male while the right remained female.

The clearest difference in their morphology is seen in the mandibles: the right side has the typical large and fearsome mandibles of a male, while the left bears the smaller mandibles of a female. There are probably other differences between the sides that you can’t see, as in the shape of the genitalia. Such “split” animals are a very clear demonstration of sex differences, and, in this case, the mandible difference probably resulted from sexual selection.

Dorcus is a genus of stag beetle (family family Lucanidae), and the name “stag beetle” comes from the antler-like mandibles of the males. They use these to fight over females or good mating sites (a form of sexual selection called “male/male competition”), and sometimes over food, though the sexual dimorphism was undoubtedly produced by competition for females. Here’s what those battles look like (I don’t think the beetles actually damage each other):

There will be a quiz later.


  1. Joseph Stans
    Posted September 9, 2017 at 9:05 am | Permalink

    Oh Gynandromorphs are so difficultto shop for.

  2. Posted September 9, 2017 at 9:12 am | Permalink

    What about fiddler crabs’ claws? Did one claw win for fighting and the other for waving?

    • Posted September 9, 2017 at 10:17 am | Permalink

      I think this one is a case of genetically based assymetry, where it is almost always either the right or left claw that is large (not sure which right now). Anyway, the big claw is waved to attract females and to defend their burrow against rival males. The small claw is used for feeding.
      For lobsters, the assymmetric claws are determined by differences in excercise so either the left or right claw can be the big ‘crusher’ claw.

  3. frednotfaith2
    Posted September 9, 2017 at 9:42 am | Permalink

    This is the first time I’ve even heard of such creatures! A bit surprised no one has produced a comedy about a half-male/half-female human. Or maybe someone already did and it sank without notice.

    • Torbjörn Larsson
      Posted September 9, 2017 at 11:09 am | Permalink

      I do not know, but there was a sad story of half-and-half colored beings in Star Trek who ended with “racial” mass extinction. (The ones with a side one color could not abide the ones with same side the other color, and vice versa. Further complication was that each color scheme were inherited, and that the two variants had geographically separated.) [ ]

    • Posted September 9, 2017 at 2:08 pm | Permalink

      As a matter of fact both Carl Reiner and Dick Van Dyke used to do a comedy sketch about this.

  4. ThyroidPlanet
    Posted September 9, 2017 at 10:04 am | Permalink


  5. Posted September 9, 2017 at 10:05 am | Permalink

    Very interesting!

  6. sgo
    Posted September 9, 2017 at 10:25 am | Permalink

    The Japanese means something like : I can feel the female side (actually, 維持 means preservation, maintenance, but I know too little of the context to translate properly):

    • Darren Garrison
      Posted September 9, 2017 at 12:35 pm | Permalink

      Yeah, Google Translate gives “Female maintenance is felt.” (Babelfish gives “Can feel the knife maintenance.”)

  7. Liz
    Posted September 9, 2017 at 11:40 am | Permalink

    “In the fruit fly Drosophila, and many insects, secondary sex characteristics (those traits that make you appear either “male” or “female”) depend on the ratio of X chromosomes to autosomes (non-sex chromosomes).” – Thank you for this post.

  8. Liz
    Posted September 9, 2017 at 11:48 am | Permalink

    I don’t know if I’m “allowed” to ask this but does this relate, in any way, to why people are trans-gender and if so, in what way? I am genuinely curious. Any answers appreciated.

    • jaxkayaker
      Posted September 9, 2017 at 5:16 pm | Permalink

      These specific examples probably don’t relate specifically to why people are transgender. These examples are about biological sex and how that is affected by chromosomes.

      In humans (and mammals generally), having two X chromosomes (XX) usually results in a female, while having 1 X chromosome and 1 Y chromosome (XY) usually results in a male. Sex is usually determined in mammals by the presence of a Y chromosome. If an embryo lost one chromosome and only an X chromosome remained, the individual that results (XO) is a female with Turner syndrome, resulting in a shortened lifespan and infertility. If only a Y chromosome is present, the embryo isn’t viable.

    • nicky
      Posted September 10, 2017 at 5:16 am | Permalink

      If I understand it correctly these are ‘mosaics’, where one part of the embryo is different from another part. Trans-sexuals have a different genetic make-up than the classical XX/XY (as mentioned by jaxkayak), or more commonly(?) have problems with sex-hormone receptors.

      • Liz
        Posted September 10, 2017 at 8:52 am | Permalink

        Thank you.

      • jaxkayaker
        Posted September 11, 2017 at 7:39 am | Permalink

        I don’t think the biological bases of transsexuality in humans is well understood, though some specific cases are, such as insensitivity to sex hormones. I believe there is little knowledge of the chromosomal/genetic causes of human transsexuality, and nothing I said above should be so construed.

        • nicky
          Posted September 11, 2017 at 9:10 pm | Permalink

          Sorry for the misunderstanding, I was talking about “intersexuals” and/or “hermaphrodites” and ” pseudo-Hermaphrodites”, not really transsexuals.
          I guess that hormonal influences during brain development may play a role there, in trans-sexuality. In some other it apearrs just a fashion statement, wanting to be seen as ‘different’ or ‘special’. It is very difficult for the layman to tell the difference in the latter cases.

  9. Posted September 9, 2017 at 5:10 pm | Permalink

    Do they fly in circles?

  10. gravelinspector-Aidan
    Posted September 9, 2017 at 9:36 pm | Permalink

    I used to see about once a year while pushing flies.

    A Sisyphean task if ever I head of one.
    I’m now waiting to hear of a restaurant with a “Tantalising menu”.
    And then a “Tantric menu” when someone really detaches from Classical mythology.

  11. gravelinspector-Aidan
    Posted September 9, 2017 at 9:43 pm | Permalink

    I feel as if I’m making progrees in my German!


    KÄFER = beetle or bug ;
    KAMPF = war or contest ;
    and by elimination, HIRSCH ~= STAG ?

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