Half-siders: A tale of two birdies
“Halfsiders” are actually two genetically distinct individuals — twins — fused into one living being
I recently stumbled across a video that has been attracting quite a bit of discussion. This video shows a captive-bred pet budgerigar, Melopsittacus undulatus, that is half green and half blue, and the colours are split down the middle. This bird is composed of two distinct individuals fused into one living, breathing being. Yes, you read that correctly. Although this phenomenon is rare, it does pop up often enough amongst captive birds that aviculturists have given these strange birds their own name: “half-sider”.
This video is interesting but the narrator’s script is quite melodramatic and contains a number of factual errors, which was my impetus to share it with you:
After watching that video, you may first ask whether this bird is a hoax: perhaps a fancy dye job, for example?
Actually, no. Twinzy is comprised of non-identical (fraternal) twins. Scientifically, he’s a tetragametic chimæra — a term that describes a living being originating from two (or more) fertilised eggs or early-stage embryos that merged during development. In fact, Twinzy’s “half and half” bilateral condition resulted when fertilised twin embryos fused very early during development — between the 2-cell and the 64-cell stage — to form one embryo. Because this event happened at such an early developmental stage, it gave rise to the near-perfect midline division between Twinzy’s two distinct cell populations. (Read a detailed discussion of how this bilateral condition happened). When this co-mingling of cells occurs later in development, we instead see a patchier distribution of the different cell populations.
When cells from twins fuse into one chimæric individual, both populations of cells retain their individual characteristics. One such characteristic is plumage colour. Twinzy’s two distinct cell populations are visible because they possess two distinct plumage colour genes: one encodes the normal “wild type” green colour whilst the other encodes the blue colour morph. (The blue colour morph is a structural colour, known as a schemochrome, created when light is scattered by the feathers’ microstructure. The gene that creates the blue colour morph stops production of yellow plumage pigments but leaves the feather structure intact, thus a normally green coloured bird appears to be blue.) If Twinzy resulted from twins that had just one set of colour genes, he’d still be a chimæra, but we obviously couldn’t see it.
This video of another bilateral chimæra budgerigar, Houdini, nicely captures how symmetrical a half-sider’s colour patterns can be when marked with two plumage colour morphs:
Scientists have identified several causes for chimærism. First, this phenomenon depends upon the presence of more than one embryo. Second, the tendency towards chimærism may be inherited or it can result from environmental insults — exposure to certain drugs or chemicals during particular stages of embryonic development.
Chimærism is not the same as mosaicism: mosaics are comprised of genetically distinct cell populations that originated from just one fertilised egg. Cancers, for example, are mosaics made up of genetically distinct cell lines, yet they all originated from just one cell population.
Chimærism is not a peculiar congenital event that is limited to just domestic budgies, nor to parrots. Further, it also is not limited to embryos of the same sex. In fact, sharp-eyed observers may have noticed that Houdini appears to be half male and half female (in budgerigars, the sexes are colour-coded; males have blue ceres whilst females have brown ceres). But bird species that are sexually dichromatic, where males and females have distinct plumage colours or patterns, can be particularly dramatic. One such example is this captive-bred Gouldian finch, Erythrura gouldiae, where males have brightly coloured plumage on their body and head, whereas females have subdued body plumage colours and darker head plumage:
Like Twinzy, this finch’s markings are bilaterally symmetrical, indicating embryonic fusion at a very early stage of development. Unlike Twinzy, who results from the fusion of two male twins, this finch is half male and half female — a bilateral gynandromorph. (Read a detailed discussion of how a bilateral gynandromorph occurs, or read the original literature; doi:10.1038/nature08852).
Chimæras and bilateral gynandromorphs are also seen in free-roaming wild birds. A few years ago, one particularly striking northern cardinal bilateral gynandromorph was photographed by a number of bird watchers in the eastern USA:
The male and female sides of this sexually dichromatic species are clearly discernible. Here’s another bilateral gynandromorph northern cardinal that showed up at a birdfeeder in Texas in 2011:
By now, you’re probably wondering if chimæras can breed?
In fact, some can, potentially. Both of Twinzy’s cell populations are male, so he is a male chimæra, and as such, he can probably reproduce. But the plumage colour of the offspring that he fathers will partially depend on which cell population gave rise to his gonads. Since Twinzy is so symmetrical, it is probable that one of his testes arose from his wild-type green twin, whereas the other testis originated from his blue-morph twin.
It is also important to keep in mind that the wild-type green plumage colour is dominant to the blue plumage colour morph in budgerigars, so Twinzy’s mate must have blue plumage for us to see which of Twinzy’s gonads produced the sperm that fertilised her eggs.
But could Houdini produce eggs? To answer that question, I have to tell you about an avian physiological peculiarity: in most bird species, only the left ovary is functional. Thus, if an avian bilateral gynandromorph is female on its left side, the bird may be capable of produce eggs, whereas a bilateral gynandromorph that is female on the right side would never be capable of producing eggs. Of course, things are more complicated than this in real life, so it is difficult to know how such a physiologically and behaviourally complex animal could successfully reproduce. For example, like all the other cells in a gynandromorph’s body, neural cells have their own genetic sex, even though they all are exposed to the same hormonal milieu. So this raises the question: can a chimæric brain adequately integrate and respond to hormonal signals so that a bilateral gynandromorph would show the appropriate behaviours necessary for successful reproduction? (doi:10.1073/pnas.0636925100 & doi:10.1210/en.2003–1491.)
Although different genetic and developmental mechanisms, especially for determining sex, are in play amongst other vertebrates and in invertebrates, chimæras and bilateral gynandromorphs have been noted in a variety of animals. For example, crabs (doi:10.2307/1540801) and lobsters:
Chimærism also occurs in mammals — in housecats, for example. Chimæras even occur in people: some published studies estimate the occurrence of chimærism in humans to be as high as 10 percent of the population (doi:10.1093/humrep/dei370). The fact is that chimeras are not visibly different from the rest of us. Unless their fused cell populations contain gene(s) that creates a visibly distinct phenotype, such as green versus blue plumage in a pet budgie, they are impossible to differentiate from those animals that have a single genotype. In fact, I argue that chimæras (and possibly even gynandromorphs) are more common than we may suspect, whether they live in birdcages in our front rooms or are flying free through our gardens — or even if they may be sitting next to us on the train.
Video courtesy of Rudy’s Pet Supply: Home of Twinsy the “one of a kind” parakeet.
Read more about how avian feathers show colours, especially violet, blue and white: Schemochromes: the physics of structural plumage colours.
A detailed discussion of the science underpinning a special sort of tetragametic chimærism, bilateral gynandromorphism, in birds: Gender-Bending Chickens: Mixed, Not Scrambled.
Bilateral gynandromorphism in butterflies: Cal Academy Butterfly Collection.
Zhao D., McBride D., Nandi S., McQueen H.A., McGrew M.J., Hocking P.M., Lewis P.D., Sang H.M. & Clinton M. (2010). Somatic sex identity is cell autonomous in the chicken, Nature, 464 (7286) 237–242 | doi:10.1038/nature08852
Agate R.J., Grisham W., Wade J., Mann S., Wingfield J., Schanen C., Palotie A. & Arnold A.P. (2003). Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch, Proceedings of the National Academy of Sciences, 100 (8) 4873–4878 | doi:10.1073/pnas.0636925100 [OA PDF]
Arnold A.P. (2003). Minireview: Sex Chromosomes and Brain Sexual Differentiation, Endocrinology, 145 (3) 1057–1062 | doi:10.1210/en.2003–1491
Macklin M.T. (1923). A description of material from a gynandromorph fowl, Journal of Experimental Zoology, 38 (3) 355–375 | doi:10.1002/jez.1400380302
Johnson P.T. & Otto S.V. (1981). Histology of a Bilateral Gynandromorph of the Blue Crab, Callinectes sapidus Rathbun (Decapoda: Portunidae), Biological Bulletin, 161 (2) 236 | doi:10.2307/1540801
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Originally published at The Guardian on 31 January 2014.