Gynandromorphs, Mosaics & "Halfsiders"

gynandromorphs are invariably known as "half-siders:' The enor Gynandromorphs, Mosaics mous numbers ofbirds reared in con finement ensures that there will be several examples produced every & "HalfSiders" year. However, only some get to be reported. Recently I saw a "half by George A. Smith sider" Zebra Finch. The reason for Broadway Veterinary Clinic the owner bringing the decomposing Peterborough, Cambridge, England bird to my attention was not to demonstrate his curious, perhaps unique, possession; but because he The name gynandromorph (mean History at Tring. This skin is of the wanted to know what had killed it. I ing in shape both male and female) is wide-ranging, highly sexually dimor have seen two "half-sider" cinna given to those curious creatures phic, colony nesting grackle Quis mon/normal canaries and have been whose body halves seem to be of dif calus lugubris. In this species, the told ofa third. ferent sexes. Although, in at least one brown females are about three In Cockatiels (Nymphicus hollan text book on genetics, gynandro quarters the size of the black males. dicus), there are two sequential 1986 morphs have been called "half This pied gynandromorph is, there Parrot Society Magazine reports siders;' we shall see later that there fore, ridiculously lop-sized. The (Smith: p. 60 - a color photograph is are Budgerigars that, although they imaginative might get the impression included - and Whitby: p. 266). The have sides of different colors, are not that it had spent its life going in cir sex-linked genes involved were sexually divided. Therefore, we shall cles; for on its left it has a big, black "pearl" and" cinnamon." The restrict the term gynandromorph to wing, a long leg and a black body. On accounts are sufficiently detailed to those cases where the sexual division the right side they are partnered by a prove their genetic structure. goes straight down the middle of the smaller brown wing, a shorter leg By far, the most common "half body. Such a separation is so precise and a brown body. The tail, too, is sider" parrot is the exhibition Bud that it looks just as if someone had asymmetric: the left half black and gerigar. Although I can recall several severed the members of a sexual pair long, and the right brown and short. involving sex-linked genes (lutino, into two longitudinal halves, then Another striking feature of this cinnamon, opaline and laceWing), skillfully joined their separated grackle is how the bill is curved the list also contains two where the halves together to make two mirror sideways, to the right, because the bicoloration of the bird was blue and images. bigger male half-portion of the beak green. These were not gynandro Although gynandromorphs are bends over to accommodate the morphs. The gene for blue is inher best known in butterflies, moths and smaller female half. ited as a recessive autosomal. One other insects, they have been Another particularly impressive Budgerigar was wholly male (to have recorded in several vertebrates. A gynandromorph was a caged Austra a completely blue cere) and the other lovely demonstration in birds is the lian Eclectus. It would have been female (with a brown cere). European Bullfinch Pyrrhula pyrr more unusual had not the mutation An obvious question for anyone to hula on display in the bird section occurred slightly later than the first ask about "half-siders;' mosaics and near the school entrance to the cell division. Perhaps it took place at gynandromorphs is: are they fertile? American Museum of Natural His the four-celled stage? Although the If they are, will they prove to be as tory, New York. As the Bullfinch is body was absolutely split-colored true hermaphrodites as snails and highly sexually dichromatic, this (green on one side and blue and red earthworms to function as both male monster has the right hand side of on the other), the eclectus' head was and female? Or are they, despite the belly, chest and cheeks pink (the almost wholly green as in a normal them seeming to have a female half, male color), while the left is the male. Such fractional gynandro wholly male? Or, perhaps, they may female brownish-grey. morphs are scientifically known as have ovaries, not testes, and are The cause of this extraordinary mosaics. female? fifty-fifty gynandromorphy is that the Mosaics form at any stage in cell In answer, all the dozen orso ' 'half first two cells to form in the earliest division. As the number of embryo siders" studied by myself have been stages of embryo development differ nic cells build up, the greater, obvi fertile males, prodUcing the usual in their sexual genetics. When, by ously, will become the chance ofone 50/50 ratio of male to female chicks. cell diVision and growth, each pro developing a "sex change" mutation. What they have never done is to yield duces its half of the body, one por This is why mosaics are th~ most further gynandromorphs. tion is male and the other looks common form of gynandromorphy. The color of the chicks show that female. Most of us will have seen at least one both testicles function in a "half As this astounding early defect hap example as in a few patches of sider:' One half of the father's body pens extremely infrequently (and female feathers on an otherwise (with its contained testicle) is hetero because most of the few that occur male-colored bird. zygous (' 'split'') for a recessive color wild will not be noticed by anyone), Obviously, a gynandromorph gene. The other half of the body the avian examples tend to be of would not be discernable in a sex (with the second testis) is homozy domesticated birds: mostly ducks, ually monomorphic bird. However, gous (" double factor'') for the same pheasants and poultry. Amongst the they do show up in domesticated gene. most bizarre must be a preserved birds that are bred to reveal mutant Before explaining "half-siders;' we bird in the British Museum, Natural colors. Such mutant, captive-bred must realize that each testis has 72 October/November 1992 directly descended (by division and general, is contradictory. The fact is growth) from one of the first two that certain genes seem exception cells of the fertilized egg. At concep ally susceptible to allelomorphic tion, the "half-sider" yielding egg is change. Getting back to our exam homozygous (' 'double factored'') for ple, the" palette" of color mutations a recessive, color-determining gene. in parrots proves always to be rather However, at the first cell division, limited in range. It is not as broad as one of these genes then mutates it would be were all mutations pos (reverts back) to the "normal;' domi sible. nant state. The consequence of this limitation Although rare, there still are so is that color mutations do not form many examples of "half-siders" it an infinite series. The same ones suggests that "reversed mutations" recur in all species ofparrot. Perhaps for color are not exceptionally infre not more than ten exist. Moreover, quent. This idea is contrary to the there is a "bias" even with these so teaching that genetic mutations are that certain colors are far more likely invariably quite haphazard and to appear than others. Likewise, we always of an extremely rare occur know that identical "sports" occur rence. The evidence, and not from in other avian Orders and that, again, just "half-siders" but mutations in these mutations match the parrots in their relative frequency. The"ino" (albino and lutino) mutation has Johnny with male oftrue coloring on been found in parrots as different as right. the Kakapo and Indian Ringneck; also it is one of the commonest 2. In birds, the primary difference recorded in penguins, Ostrich, Goul between the sexes is that males have dian and Zebra Finch. two sex chromosomes ("XX") and It may be that some alleles revert females have but one (and a "Y". back to the" normal state" more Hens, therefore, are' 'XY' '). As "Y" readily than the "normal" alleles is generally smaller than"X", it is mutate. This might explain why considered to be almost genetically ,'half-siders" occur more frequently inert. So inconsequential is it that its than color mutations. The blue-green absence does little or no harm to the ,'half-sider" Budgerigars depend on cell. If we rel)resent the absence of a an autosomal (not a sex linked) gene sex chromosome by the zero symbol reverting back to normal. "0", then' 'XY" containing cells are, This simplistic explanation, of biologically, closely approximate to ,'half-siders" being the visible conse ones with "XO". Botl1 "XY" and quence of a mutation of a recessive "XO" are"fenlale" cells. gene, may not necessarily explain Very rarely the fertilized "XX" wild-type gynandromorphs such as ovum is irregular in the way in which the bullfinch and grackle. To know it distributes its sex chromosomes at wllat happens with these we would the first cell division. In this uncom need a chromosomal analysis of cells mon event, one of the first two taken from different sides of their embryonic cells gets its correct ' 'XX" body. Until this is done, I offer three and the otller but one' 'X" and is (from many) possible explanations. "XO". Whether "XO" would cause 1. A single gene could be responsi an ovary to be present is doubtful; ble for causing sexual dimorphisnl in but it would give female coloration some birds. Certainly, and this is to that side ofthe body. despite the obvious difficulty in dis 3. The complete loss (gain, frac cerning such examples, there are a tioning and joining together) of a considerable number of cases where, chromosome is a well-known phe in a sexually-dimorphic speCies, odd nomenon in genetics.

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