Sex-chromosomes in Birds and Mammals; Differences and Similarities
By: Inte Onsman, Research Coordinator
MUTAVI
Research & Advice Group, The Netherlands
Article about a remarkable difference between the sex-chromosomes of birds and mammals
[Taken from www.euronet.nl/users/hnl/barr.htm. The original page has been edited and links added.]
In the fifties new technologies improving chromosome research and two important developments opened the way to new discoveries. The first discovery was that of colchicine (formerly a drug against rheumatism) blocks cell mitosis in the stadium of metaphase. This implies that a great number of cells treated with colchicine and prepared for microscopic examination are "frozen" in metaphase. Putting them in a special salt solution they can take up water, swell and eventually the chromosomes in the nucleus spread out and become visible.
Many of these techniques were performed by Dr. Susomo Ohno, a Japanese cell scientist who went to the USA 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 nerve cells, as well as most of the body cells, of a female cat, a small dark body. They identified this later on as a sex-chromatin (a sex-chromosome in a shrunken 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 were confirmed for other female mammals besides 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 its 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 occur in males. [See original page for further information on mammals.]
In birds the Z-linked genes (Z=X) occur in hemizygous condition in hens. Yet there is no indication at all for dosage compensation [inactivation of one of the doubled chromosomes] of these genes. On the contrary, the Z-chromosome of birds even shows a certain dosage effect [1]. The full expression of a 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 possess the same series of gene loci which are involved in a certain phase of pigment synthesis [5]. However, the answer from comparative gene mapping proved otherwise [4]. Of the 17 genes now mapped in the chicken Z, most lie on the human chromosome 9 and none lie on the mammalian X. Conversely, of the 6 genes on the human X that have been mapped on the chicken, three lie on chicken 1 and three on chicken 4; none on the chicken Z. Thus, the bird Z and the mammalian X do not share any genes at all.
Many sex-linked factors are not related the sex of an individual at all. The recessive cinnamon factor in Budgerigars [6] is expressed as having an effect similar to, but quantitavely 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 other domestic breeds of geese, is sexually dimorphic in plumage colour; females are pale grey, males white [1].
References
Also from the same site: Crossing over in the Sex-chromosome of the Male Budgerigar
Of interest from elsewhere:
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