The Half-Sider

From time to time, a Budgerigar known as a “Half-sider” arises; that is, a Budgerigar that is green on one side and blue on the other in the vertical plane. The line that divides the two sides is the Midline, which specifies the left and right sides of the body. Half-siders may be Grey-Green on one side and Grey on the other, and they can be Normals, Cinnamons, Opalines Spangles, and so on. For many years, scientists have discussed the genetics of the Half-sider, and different explanations have been proposed.

Prof. T G Taylor’s Genetics for Budgerigar Breeders provides an explanation for the genetic makeup of the Half-sider. To recap Prof. Taylor’s reasoning, the initial fertilized egg cell begins as a heterozygous Green/Blue split, and during the first division of this cell into two “daughter” cells, one of the two daughter cells loses the chromosome containing the green gene. All descendants of this cell will have just the blue gene and no opposing chromosome. Furthermore, it is proposed that after the initial cell division, the daughter cells of the fertilized egg instantly differentiate into the origins of the left and right sides of the body, respectively. This hypothesis, however, has three glaring problems.

  1. Many additional genes would be lost if the chromosome encoding the green gene was removed. It is unusual that a creature could grow and survive without so much genetic information, yet it is well known that a single gene may have a significant impact on the body.
  2. After the sperm fertilizes the egg, the cell splits into two new cells, which divide into two new cells each, and so on to produce the Blastula, a ball of cells. No differentiation into future body parts occurs in the Blastula during the first few cell divisions, therefore the right or left side of the body does not arise from a single cell.
  3. If Prof Taylor’s idea is right, one would expect to observe Half-siders of various types, such as half-Opaline and half-Normal, or half-Spangle and half-Normal, and so on. As far as I know, such birds have never been observed.

In contemplating the enigma that the Half-sider provides, it seems that the riddle has always been thought to have a genetic answer. Is this correct? Are there any other possibilities?

Half-sider is Congenital

The disease is congenital since the Half-sider is born this way, but not all congenital disorders are of main genetic (and hence heritable) origin. Most are developmental disorders of varied severity that happened halfway between egg fertilization and delivery, but were not severe enough to prohibit future development.

In this post, I would want to argue that the Half-sider does, in fact, exhibit such a developmental failure that was induced at an early stage of development inside the egg, and that the disease is not genetic.

I remember knowing a lady with a white spot in her hair when I was a kid. This occurred in the back of her skull and was the consequence of being struck by a tennis ball on this place when she was 8 years old. Following that, the hair always turned white. The white patch, obviously, does not have a hereditary relationship since the hair was previously brown and the permanent change of color was caused by harm to the hair generating cells. In this example, the hair-producing cells were irreversibly destroyed and could no longer create melanin pigment, although otherwise functioning normally. As a result, small damage may have a long-term impact on melanin pigment synthesis.

A Type Of Pied

The Half-sider is a Pied Budgerigar in the broadest sense, but not of the Dominant Pied or Recessive Pied kinds, which are caused by a failure to synthesize melanin pigment in the medulla of the feathers in some regions of the body and have a recognized and proven genetic basis. The recognized Pied types breed according to genetic theory, producing offspring with the predicted color production ratios. However, the markings in the Dominant and Recessive Pied variants are bilateral and do not distinguish at the Midline.
The nervous system is the most evident delineation of the midline in any higher form of life. The classic illustration of the absolute difference between the neural systems on the left and right sides of the body is when a person gets a “stroke” and paralysis of one side of the body only occurs at the Midline.

The whole body is supplied with a complex nerve system, which may be divided into two primary groups: Cranial nerves and Spinal nerves.

The Nervous System is further subdivided into two primary groups: the Sympathetic and Parasympathetic nervous systems. The Sympathetic nervous system normally tries to “speed up” the body and prepare it for “fight or flight,” while the Parasympathetic nervous system seeks to slow the body down (digestion and temperature control).

The sympathetic nervous system collaborates with the endocrine system. Adrenaline, the hormone that we all know prepares the body for “fight or flight,” comes to mind. The point I’m trying to make here is the link between the hormone Adrenaline and the nerve supply to one half of the body, which is defined by the Midline. This is the same location where the Yellow pigment is not evenly distributed in the Half-sider.

Melanin is generated in the body by Melanocyles, which originate in the Neural Crest during vertebrate embryonic development. Melanocytes migrate from the Neural Crest to cover the different pigmented areas of the body, with the exception of retinal melanocytes (eye pigmentation cells). Melanin, the pigment responsible for most mammalian coat coloration, is also present in many other living things such as birds, insects, and molds.

Enzymes inside melanocytes are capable of producing Melanin via a sequence of chemical reactions from the amino acid Tyrosine. The enzyme Tyrosinase slowly oxidizes Tyrosine to Dihydroxyphenylalanine, often known as D.O.RA in the first of these chemical processes. It just so happens that D.O.RA. may be converted into Melanin and Adrenaline. As a result, there is a relationship between pigmentation in the body, nerve distribution, and a hormone. Two chemical processes are required to convert D.O.RA. to Adrenaline, while many more are required to convert D.O.RA. to Melanin. Each chemical reaction necessitates the use of a separate enzyme, and each of these enzymes is the result of a different gene.

Melanin Is A Polymer

The substance Melanin is a polymer, which is a chemical molecule made up of many separate subunits that all have the same chemical structure and are connected together. Indole Quinone is the subunit in the case of Melanin. Melanin polymerizes and mixes with other proteins to generate Melanoprotein.
Various distinct melanoproteins exist, which accounts for the variation in body color found in many animal life forms. Geneticists have done extensive research on mice, who have many different coat colors, and discovered that there are multiple distinct genes that have the impact of modifying the diverse chemical processes at various phases between D.O.RA. and Melanoprotein synthesis. Each gene results in a particular chemical structure of Melanin, resulting in a huge range of coat colors and patterns. Melanin exists in two fundamental forms in mammals: a brown-black form known as Eumelanin and a yellow-reddish form known as Phaeomelanin. The two Melanins, Eumelanin and Phaeomelanin, are, of course, groupings of Melanoproteins with vastly differing chemical compositions, resulting in a wide range of coat colors ranging from black to white.

In the absence of contradictory data, I would propose that the Melanin granules discovered in the medulla of Budgerigar feathers belong to the Eumelanin group and the Yellow pigment detected in the cortex of Budgerigar feathers belongs to the Phaeomelanin group. Budgerigars’ yellow colouring has never been recognized. It is plausible to believe that the yellow pigment is of the Phaeomelanin group, since the metabolic route for the production of both Eumelanin and Phaeomelanin is the same, except for the last few chemical steps. The generation of yellow pigment inside the Budgerigar’s feathers is controlled by a single gene, as shown by the fact that the dominant wild Green gene altered to generate the recessive blue gene, which prevents the creation of all yellow feather pigment.

Eyes Can Be Different Colours

Referring back to the retinal melanocytes’ beginnings, the eyes are one of the few typically pigmented nerve tissues in the body. As a result, the connection between nerve tissue and pigmentation is clear in this tissue. We’re all aware that most people’s eyes are either blue or brown, and the two eyes are generally extremely similar in color. However, there are a few individuals that have various colored eyes. David Bowie, the music star, is perhaps the most renowned example. As a result, this condition may be compared to the Half-sider. Brown eyes are formed by Melanin pigment in the iris of the eye, while blue eyes are caused by a lack of pigment in the iris. The blue coloration is caused by the visibility of the fluid inside the eyeball. The brown or blue color of the eye is caused by a single gene system. The dominant gene results in brown eyes, whereas the homozygous recessive gene results in blue eyes. There are various hues of eye color, such as grey, green, and hazel, which are caused by lower levels of melanin pigment in the iris than in brown eyes. Such hues are created by altering genes that work in tandem with the Brown/Blue gene and, if present, influence the creation of the healthy eye.
The eyes are very sensitive to harm at all phases of development and life. Disease conditions or injury to the surface of one’s eye may induce color changes in the afflicted eye, either by increasing Melanin production and making the injured eye look darker, or by decreasing Melanin production and making the Iris appear lighter (similar to the woman hit by the tennis ball). This will result in different colored eyes. Obviously, harm to Melanin-producing cells may permanently alter their function. If a person is born with dissimilar eyes, it is quite likely that the two eyes were exposed to distinct pressures inside the womb, since the eyes only have the eyelids for protection. Babies are typically born with pressure marks of many different sorts, from being so tightly-folded inside the womb, and they are generally referred to as “birth marks”. If the melanocytes in the Iris were slightly injured at a vital time of development, the eye, like any other fragile foetal tissue, may reveal such birth scars in the form of different eye color.

It is commonly known that after bird eggs have started to incubate, they must be rotated on a regular basis or the embryo would die in the shell. Embryo mortality may occur at any point before hatching, and it is dubious that any surviving chick has developed normally throughout its 17-day stay in the shell.

Living Creatures Are Not Perfect

Minor structural flaws must exist in all living beings, which is why we are all slightly different from one another; even identical twins.
In my experience, around half of all Budgerigar eggs that I have been able to certify as viable fail to hatch. Because the egg cell is fertilized before the shell is formed over it, and a viable egg can only be identified by handling on the third or fourth day at the earliest, the number of embryonic Budgerigars who die must be larger than the number that hatch. However, I have only seen one living chick with a birth structural abnormality, particularly an underdeveloped wing. Mother Nature seems to have a very high quality control standard, rejecting sub-par biological beings. Perhaps there is a lesson here for Budgerigar enthusiasts who want to make a “new” version of a proven and successful product.

It is my hypothesis that the Halfsider is produced by a very little lesion to the extremely fragile embryonic tissues of the Neural crest on one side exclusively, at a key stage of melanocyte development. When a growing melanocyte cell is irreparably injured, all future direct descendants of that cell will exhibit the same malfunction; this is why scars permanently reveal places of earlier harm. This damage might be produced by the most little local freezing of embryonic tissue inside the egg or by extremely minor pressure variations within the egg. This kind of damage might be caused by the hen not sitting correctly, being away from the eggs for an extended amount of time, or a knock on the egg.

While we’ve been discussing the link between pigmentation and the nervous system, I’d like to suggest that this direct association may account for many of the distinctive patterns of markings observed in the animal world, particularly when the perimeter of certain marks is so precisely defined. The Budgerigar is an amazing example of several diverse, carefully defined markings. To highlight the probable relationship between nerve supply to the body and local pigmentation, I shall try to explain the Budgerigar’s head and facial markings.

I’m not sure about the nerve supply to the Budgerigar’s head. I’ll suppose it’s comparable to most other animal forms, which have a high degree of resemblance since they originated from the same ancestral roots. If we consider the sensory nerve supply to the human face and head, then a map of the nerve supply can be drawn.

The nerve in question is the 5th Cranial Nerve, the Trigeminal Nerve. Birds do have the same cranial nerves as do other creatures. This nerve leaves the brain, and later divides into 3 separate nerves, each having specific, clearly defined areas of sensitivity as shown in the diagram. There are in fact, two Trigeminal nerves: left and right. There is very little overlap where the two nerve supplies meet along the Midline. If we now assume that the nerve supply to the Budgerigar face and head is the same Trigeminal nerve, and superimpose a similar nerve distribution map over the face of a Normal Budgerigar, the clearly defined nerve supply shows exceptionally close correlation to the different types of feather and markings that appear on the head and face.

Likewise, the markings on the back of the head, neck, wings and back in the Normal Budgerigar could be associated with the nerve supply to these areas; i.e., the cervical and thoracic spinal nerves. The primary flight feather markings to the nerve supply equivalent to the human hand, and so on.

Many species of the Parrot family, display similar elaborate, clearly defined patterned markings, similar to those which the Budgerigar displays, perhaps associated with different nerve distributions in each species.

I have considered all of the many different colour patterns that can be seen in the exhibition Budgerigar, and it is possible to account for every single variety of colour pattern, in direct relationship to the pigmentation patterns seen, and the maps of nerve distribution served by the Cranial nerves and the Spinal nerves; however, this will be the subject of a future article.

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