It’s a Male. It’s a Female. No…it’s a Gynandromorph!

Don’t eat me cuz I’m beautiful…

You could make a case for “looking weird” being an effective survival strategy. Just ask Calvin the lobster whose calico-patterned shell spared it from dining table destiny. Rather than being dunked into a pot of boiling water, Calvin is now on display at the New England Aquarium. Calico-patterned lobsters are extremely rare, occuring about 1 in 30 million. For comparison, the rarest are albino lobsters, which occur at a rate of about 1 in 100 million whereas blue lobsters are more commonly found (~1 in 2 million).

But even stranger looking lobsters are lurking out there in our oceans. Take for instance this two-toned lobster that looks like only one half of it was cooked :

As odd as this lobster looked initially, the more I looked at this picture the more familiar it seemed to me. Where have I seen something like this before? I paced around the lab a bit yesterday and it dawned on me. I’ve seen something like this happen in Drosophila. Every once in a while I’ll find a fruit fly where one half of its body is yellow and other half is brown–split right down the middle. Or more strikingly one eye is white and the other is red. And what’s even more peculiar, one half of the fly will be male with male specific structures like the sex comb and the other half is female. The genitalia of these individuals can vary from having “two complete sets of genitalia, one male, one female. Most of the time, you get weird mish-mashes of tissue that don’t look like male or female genitalia (h/t @DaveMellert).” This bilateral sexual asymmetry is a form of gynandromophy, where an organism abnormally displays both male and female characteristics and is not to be confused with hermaphrodites which are organisms that have both male and female sexual reproductive organs.

Drosophila gynandromorph

In Drosophila, sex is determined using the XX/XY sex-determination system (also used in humans) in which females have 2 X chromosomes and males have 1 X chromosome and 1 Y chromosome. Bilateral gynandromorphy in Drosophila occurs when there is a spontaneous, anomalous loss of an X chromosome during the first mitotic division in a female zygote. This results in one daughter nucleus* containing 2 X chromosomes (denoted as XX) and the other daughter nucleus containing only 1 X chromosome (denoted as XO). In this situation, cells derived from the XX nucleus will give rise to the female body plan in half of the fly while cells derived from the XO nucleus will give rise to the male half despite the lack of a Y chromosome. This is because in Drosophila the most important factor in sex determination is the number of X chromosomes. This phenomena also indicates that by the first mitotic division the left and right side of the Drosophila has been determined since one half will become male and the other female. All of the descendants of one cell will makeup the entire left side of the animal while all the descendants of the other cell will makeup the right side!

XX/XY sex-determination system in Drosophila

Although this explains how bilateral sexual asymmetry occurs, you might be wondering why in the Drosophila gynandromorph above one half of its body is yellow (left) and the other is brown (right) or why one eye is white and the other is red. This is because the genes that determine eye and body color are found on the X chromosome. So in the case of eye color, the original zygote was heterozygous for the gene controlling eye color–it has the recessive allele or gene variant for white eyes (w) on one X chromosome and the wildtype allele for red eyes (w+) on the other X chromosome. During mitosis, one of the w+ -bearing X chromosomes is lost and so cells derived from this XO nucleus will carry only the white eye gene, therefore giving rise to 1 white eye. The other eye is heterozygous for the eye color gene, but since w is recessive the eye will be red by virtue of also carrying the wildtype w+ allele. Of course, loss of the X chromosome can also occur after the first mitotic division, in which case the animal will be a mosaic gynandromorph having patches or regions that are male or female instead of the stark left-right division of male and female body plans.

Gynandromorphy occurs in other animals also, although the specific details depend on the species in question. In butterflies, gynandromorphy is a result of sex chromosome aneuploidy as well but their situation is reversed: XX cells are male and XY or XO cells are female.

In birds, however, gynandromorphy occurs by a different process. Sex determination in birds is different from insects and humans: females have a Z and W chromosome whereas males have 2 Z chromosomes. This chicken is a mosaic of both normal male (ZZ) and female (ZW) cells, with male cells concentrated on 1 side and female cells concentrated on the other.

The exact details of this phenomena in birds is unknown. One hypothesis is that it is due to an error that “occurs in the formation of an egg, which normally carries one chromosome to unite with the single chromosome carried by the sperm. But if an egg accidentally ends up with two chromosomes — a Z and a W — and if this aberrant egg is fertilized by two Z-carrying sperm, the bird that results will have some ZZ cells and some ZW cells, he explained.” Another hypothesis is that the egg is abnormally fertilized by two sperm. 

Coming back to lobsters, bilateral gynandromorphs can occur in crustaceans as well. As for humans, however, despite Conrad Lycosthenes’s claims that a bilateral human gynandromorph existed, Natalie Reed explains why this phenomena in all likelihood would not happen, “Back to this not happening in humans: yes, intersexual chimerism can happen in humans. You can even end up with human beings who have one ovary and one testicle. But given that almost all sexual differentiation is a result of hormones, which are more or less evenly distributed throughout the body, you would never see any kind of stark split down the middle of a human with, say, a breast on one side and a flat chest on the other.”

*I say nucleus rather than cells because Drosophila do this strange thing during early embryonic development where the chromosomes duplicate and segregate into separate nuclei without the normal cell division (cytokinesis) that occurs after mitosis. This results in a cell called the syncytial blastoderm that has multiple nuclei without cell membranes separating the nuclei from each other. After 13 rounds of nuclear divisions, cell membranes are finally erected (cellularization) to partition the nuclei into individual cells.

Here is a video of mitosis in the early Drosophila embryo. Note the lack of cell membranes.

Here is a video of cellularization.