Revised 22nd of April 2007

Sex-chromosomes in Birds and Mammals; Differences and Similarities

By: Inte Onsman, Research coordinator
Research & Advice Group

Article about a remarkable difference between the sex-chromosomes of birds and mammals.

In the fifties new technologies improving chromosome research and two important developments opened the way to new discoveries. The first discovery was that colchicine (formerly a drug against rheumatism) blocks cell mitosis in the stadium of metaphase. This implicates that a great number of cells treated with colchicine and prepared for microscopical examination are "frozen" in metaphase. Putting them in a special salt solution they can take up water, swell and adventually the chromosomes in the nucleus spread out and become visible.
Many of these techniques were performed by dr. Susumo Ohno, a japanese cellscientist who went to the U.S.A. in 1953 to do research on sex-chromosomes and his work would support a number of remarkable discoveries and hypotheses.

Already in 1949 Murray Barr and E.G.Bertram found in the nervecells of a female cat a small dark body as well as in most of the body cells. They identified this later on as sex-chromatin (a sex-chromosome in shrinked condition). This sex-chromatin is also called the Barr-body after its discoverer. Later on it was also found in the cells of other mammals but only in females.
Eventually it was Ohno who proved in 1959 that strictly speaking the Barr-body is one of the two sex-chromosomes in a female.
The observations of Ohno soon were confirmed for other female mammals including humans. Although one expects to find two sex-chromosomes in all body cells of a female mammal, only one is to be found the other is the so called Barr-body.

During evolution in placental mammals the Y-chromosome has lost all genes which were allelic to the genes on the X-chromosome. The result is that most and maybe all X-linked genes in hemizygous state (XY) occur in males. Each X-linked gene must have been adapted to the hemizygous state by doubling the amount of "product output".
When this redoubling was achieved in an efficient way (during evolution), the genetic difference between the male having only one X and the female having two X-chromosomes became to large.
A certain need to compensate the dosage effect for X-linked genes between the two sexes arose.
In mammals this is achieved by inactivation of one of the two X-chromosomes in individual somatic cells in the female. In consequence this means that the expression of X-linked genes in individual somatic cells of both sexes is hemizygous making the female mammal a genetical mosaic dependent on the sex-linked genes.
A good example are cats with orange and black marks on their fur. The orange-black fur is caused by two alleles of one gene on the X-chromosome, one for orange and one for black. Dependent on which X-chromosome is deactivated, cellclones appear with either the active gene for orange or the active gene for black. The mechanism of inactivation is rather involuntary.

In birds the Z-linked genes (Z=X) occur in hemizygous condition in hens. Yet there is no indication at all for dosage compensation of these genes. On the contrary, the Z-chromosome of birds even shows a certain dosage effect. The full expression of an Z-linked mutant phenotype requires even the presence of two dosages of a mutant gene in the heterozygous cock. The phenotype of the hemizygous hen with a single dosage of a mutant gene simulates the effect of the heterozygous cock.
Ohno even considers the possibility that the X-chromosome of mammals and birds posses the same series of gene loci which are involved in a certain phase of pigmentsyntheses [5]. However, the answer from comparative gene mapping proved otherwise [4]. Of the 17 genes now mapped to the chicken Z, most lie on human chromosome 9 and none lie on the mammalian X. Conversely, of the six genes on the human X that have been mapped in chicken, three lie on chicken 1 and three on chicken 4; none lie on the chicken Z. Thus, the bird Z and the basic mammalian X do not share any genes at all.
Many sex-linked factors are not related to the sex of an individual at all. The recessive cinnamon factor in Budgerigars [6] is expressed as having an effect similar to, but quantitatively less than, that of the wild-type allele. Black eumelanin is replaced by brown in cinnamon birds; two doses in the male give a distinctly darker shade of brown (a closer approach to the wild-type black) than one in females [2]. The Pilgrim breed of geese, unlike most other domestic breed of geese, is sexually dimorphic in plumage colour: females are pale grey, males white [3].

Consulted and cited literature:

[1] Baverstock P.R., Adams M., Polkinghorne R.W. (1982)
    A Sex-Linked Enzyme in Birds - Z-chromosome Conservation but no
    Dosage Compensation
    Nature Vol.269 : p.p.763-766

[2] Cock A.G. (1964)
    Dosage compensation and sex-chromatin in non mammals.
    Genet.Res.,Camb.Vol.5: p.p.354-365

[3] Jerome F.N. (1959)
    Colour inheritance in geese.
    Canad.Journ.Genet.Cytol.Vol.1.: p.p.135-141

[4] Marshall Graves J.A., Shetty S.(2001)
    Sex From W to Z: Evolution of Vertebrate Sex Chromosomes
    and Sex Determining Genes.
    Journal of Exp.Zool. 290: p.p.449-462

[5] Ohno S. (1967)
    Sex Chromosomes and Sex-linked Genes.
    Springer Verlag Berlin. Heidelberg. New York

[6] Taylor T.G., Warner C. (1961/1986)
    Genetics for Budgerigar Breeders
    Publ.The Budgerigar Society, England

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