Sexual forms are not necessarily either male or female.

This Kentish glory moth (Endromis versicolora) appears female on the left and male on the right.

Gianpiero Ferrari/FLPA/Alamy Stock Photo

Gynandromorphs (“gyne” from Greek meaning female, “andro” for male, and “morph” meaning variety) are individual animals that have both genetically male and female tissues and often have observable male and female characteristics. They may be bilateral, appearing to divide down the middle into male and female sides, or they may be mosaic, with patches characteristic of one sex appearing in a body part characteristic of the other sex. Gynandromorphs occur in insects, spiders, crustaceans, and other arthropods as well as in birds, but they are extremely rare, and discovering one in the field or in the laboratory is a major event. Estimating how frequently they occur is difficult because they usually go unnoticed in species where sexual dimorphism is less pronounced. Gynandromorphs have been reported in mosquitoes, fruit flies, and in other insects, but they are most dramatic in those butterfly species in which the male and female wing colors and patterns are dramatically different.

Gynandromorph Catonephele numilia

Alex Bik
All sexually reproducing organisms begin as a single cell, a zygote, a fused sperm and egg, which then divides repeatedly, eventually differentiating into all the types of cells in the body; muscles, nerves, intestinal lining, brain, and so forth. In higher order animals, the signals from the sex chromosomes inside the cell that cause cells to sexually differentiate may be modulated from outside the cell by hormonal signals coming from the ovaries or testes. Hormonal actions during the early stages of development can have a permanent “organizational” effect on sexual differentiation, and later in life sex differences are caused by the “activational” effects of hormones as occurs in puberty, pregnancy, and menopause.

Female Catonephele numilia

Kim Garwood

Male Catonephele numilia

Kim Garwood

In some insect species, the destiny of each cell is determined when the zygote divides for the first few times. The first split determines the left and right sides of the body, the second creates the front and back, and the third division determines top and bottom, so the eight cells are the origin of the front upper left of the body, front lower left, and so on. If damage occurs to one of these early cells but development proceeds, the corresponding portion of the insect’s body will be missing or deformed. 

The dorsal side of a nearly bilateral gynandromorph Diana fritillary (Speyeria diana) with female coloration on the upper left (male) hindwing.

Andrei Sourakov, Florida Museum of Natural History
 Before a cell divides, its chromosomes contain two complete copies of its genetic information (DNA). These two copies separate during normal cell division, but occasionally the two copies of a chromosome do not detach (nondisjunction), with the result that one of the new cells will be missing a chromosome; this is usually lethal to that cell. The other cell now has an extra chromosome and it may be unaffected, or abnormalities may occur.

Male sex in insects is determined by the presence of two (or more) Z chromosomes, and females have a pair of two different chromosomes (ZW) or just one Z and no W. If a non-disjunction occurs in a male (ZZ), then one of the cells after the division (and all its descendants) will end up with one Z (female), while the other cell could have three Zs and develop as male. If this occurs during the first division of the zygote, it could result in an individual that appears half male and half female. In such a bilateral gynandromorph, the sex-related characteristics appear on half of the insect and are clearly visible on the wings. A mosaic individual may occur if a non-disjunction occurs one or more times during subsequent cell divisions.

Male and Female Diana Fritillary

Andrei Sourakov, Florida Museum of Natural History
There are other ways in which a gynandromorph can occur in an insect. A female insect’s egg occasionally may have two nuclei, and if one is a Z and the other is a W, both are fertilized, and the cell starts dividing, the resulting embryo might develop into a gynandromorph. In another scenario, when an egg is formed, the cell is meant to discard half its chromosomes in a bag of DNA called the polar body. However, in rare cases the egg may retain the polar body as well as its own nucleus. A gynandromorph may occur when sperm cells fertilize both the egg and the polar body and the two zygotes may interact and swap cell combinations. 

Hybridization, bacterial and viral infections, temperature variation, and mutations have also been implicated in the development of gynandromorphs. Evidence gathered after the nuclear disasters at Chernobyl and Fukushima suggest that fundamental environmental changes, because of nuclear fallout, can be associated with mutations in butterflies whose larval stages fed on radioactive plants.

A unique method of sex determination occurs in the Hymenoptera, the third largest order of insects that includes wasps, bees, and ants. In this order, males normally result from unfertilized eggs, and females come from eggs that have been fertilized. Temperature during development has been linked to the development of gynandromorphs in wasps, as has a symbiotic bacterial infection that results in feminization of male embryos.

Phasmids, like the gynandromorph Malayan Jungle Nymph (Heteropteryx dilatata) above, are insects known as Walking Sticks or Leaf Insects. Female Malayan Jungle Nymphs are much larger than males, are colored bright lime green and are flightless, whereas males are mottled brown and they can fly.

The Malayan Jungle Nymph (Heteropteryx dilatata) is an example of a leaf insect, commonly referred to as a “walking stick.” The bright lime green females are much larger than the males and are flightless, whereas males are mottled brown and are able to fly. The striking gynandromorph illustrated clearly shows the wing on the male side and the bright green, wingless female side.

Within hours after birds mate and fertilization occurs, the new embryo starts to develop by simple cell division, but the process is more complex than in insects. In the chicken, for example, after four hours in the uterus the embryo has already grown to 256 cells. Initially the dividing cells form one layer over the yolk, but as cell division continues three layers are formed out of which the organs and tissues of the bird will develop.

Gynandromorph Chicken (Gallus gallus domesticus)

Courtesy of Roslin Institute at the University of Edinburgh

Some of the physical characteristics (phenotype) that indicate a bird’s sex are determined in the first few days of the embryo’s life by key genes in the cell. In normal chicken embryos, for example, molecular changes indicate maleness or femaleness as early as 18 hours after fertilization, long before gonads begin to form and sex hormones begin to exert an influence. In the chicken, the gonads that produce the sex hormones begin to differentiate into ovaries or testes starting at day 6 of the 21-day embryonic period.

The predominant theory of how gynandromorphy occurs in birds is that an error occurs in the formation of an egg. The egg normally carries one sex chromosome to unite with the single sex 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 and it may have both male and female physical characteristics.

Recent analysis of a rare gynandromorph Zebra Finch (Taeniopygia guttata) demonstrated that both Z chromosome– and W chromosome containing cells were present. Although this finch’s body appeared to be half female and half male, it was a mixture of genetically male and female cells. The right half of its brain was genetically male, and the left half genetically female. In this species, only the males sing, and the brain regions and muscles controlling song are much larger in males than in females. This bird sang a masculine song and courted and copulated with a female, although the eggs they produced were infertile.

Gynandromorph House Finch (Haemorhous mexicanus)

Peter Post

Gynandromorph Zebra Finch (Taeniopygia guttata)

Arthur Arnold

Gynandromorphy in crustaceans is caused by the loss or damage of sex chromosomes during early cell division of the zygote, but sex hormones have an influence on the development of sex-related characteristics. Bacterial and viral infections, temperature variation, and photoperiod cues are known to affect sex determination in crustaceans and may play a part in the occurrence of gynandromorphs. Female lobsters extrude egg masses whether or not they have mated, and the occasional gynandromorph lobster may extrude half a batch of eggs that attach to only one side of its abdomen. Normally the eggs die, turning bright orange and falling away, but in one instance Maine scientists were able to keep the eggs of a gynandromorph lobster alive until two eggs hatched. One offspring was male and one was female, suggesting that perhaps the gynandromorph had mated rather than simply producing a clone. In examples of gynandromorphs found in different crustacean families, the gonads may be divided into ovaries and testes and they each may contain mature gametes (cells that are capable of fusing with another cell during fertilization).

Bilateral gynandromorph lobster (Homarus gammarus) may be the result of a minor chromosomal anomaly that occurred early in development but is unrelated to sex.

Courtesy of Richard Palmer, University of Alberta

Because of the complex nature of crustacean development, the apparently bilateral gynandromorph lobster pictured may simply reflect a minor chromosomal anomaly that occurred early in development but is unrelated to sex.

Gynandromorphs are found in arthropods and birds but they do not occur in humans and other higher organisms in which sex determination is more complex. When the human sperm and egg fuse, their combined chromosomes mark the zygote as either female (XX) or male (XY), but this first cell and the early cells into which it divides are still flexible developmentally (they can become bone cells, brain cells, muscle cells, etc.). This means that if one of these cells is damaged or destroyed, it is likely to have no impact on development because the destiny of the other cells is still not determined. Developing embryos are initially ‘sexless’; nipples in male mammals are an example of a feature that develops in all embryos before sexual differentiation occurs.

It has been established that therian mammals (those that give birth to live young without using a shelled egg) have a master testis-determining gene that is carried by the malespecific Y chromosome. At some point, this gene is activated and it signals the embryo to develop testes; if there is no Y chromosome the embryo develops into a female.

In the developing human embryo, the testes and ovaries begin to develop and secrete hormones about seven weeks after conception, and the sex hormones play a key role in determining sex in humans and higher order vertebrates. The dominant hormones can overwrite an abnormality in the sex chromosomes, and because of the influence of hormones that diffuse throughout the body, an individual cannot end up with distinct male and female halves.

Several other anomalies in sexual development can and do occur among humans, such as when external and internal sex organs are not consistent with one or the other sex. This can result from an atypical set of sex chromosomes so that an individual has both testicular and ovarian tissue. In rare instances, an infant at birth may have ambiguous-looking external genitals such that it is unclear whether to assign male or female gender on the baby’s birth certificate.

The term “gender” is only used in reference to humans; scientific literature about animals only describes sexual characteristics or traits that indicate sex. Gender refers to social and cultural factors rather than purely biological differences that determine sexual identity, and gender introduces the issue of emotion into sexual identity. Gender dysphoria involves a conflict between a person’s physical traits or assigned sex and the gender with which that person identifies, and emotional distress is not uncommon in people as they try to reconcile the way they feel and think of themselves. Options some people choose are cross-dressing, socially transitioning into being addressed by the chosen gender’s pronouns and living accordingly, and medically transitioning with sex-change surgery and/or hormone treatment.

Just as conventional notions of gender are being challenged, our ideas about sex and reproduction are expanded by studying species whose individuals are neither simply male nor female. Gynandromorphs are just one variety of unusual sexual forms. There are fish and snails that are sequential hermaphrodites; they are born with male or female traits, but they can change sex in response to environmental conditions in the local population when one sex is needed for reproductive purposes. Some earthworms and other animals are simultaneous hermaphrodites, having both male and female sexual organs. Although mating with another individual is always genetically more desirable, there are a few organisms that reproduce alone (parthenogenetically) if no suitable mate is available. Research into gynandromorphy and other unusual sexual forms has important implications for understanding and treating diseases and developmental disorders. We are only at the beginning of learning where it will lead.--CAB