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Structure and evolution of the avian W- and Z-sex chromosomes, especially as this relates to molecular, DNA-based sex identification of birds.

Most, but not all, birds have differentiated sex chromosomes. Among these (neognathous) birds males are characterized by the occurrence of two Z chromosomes whereas in females one of the Z chromosomes is replaced by a female-specific W chromosome. Whereas the Z chromosome carries many genes, the W chromosome carries only a few. Current evidence suggests strongly that the W chromosome has been derived from an ancestral Z chromosome as a result of loss of a large percentage (>98%) of the Z-chromosomal genes. It is thought that this loss of genes has been the result of the cessation of recombination between the two chromosomes early in avian evolution, approximately 100 - 150 million years ago, which caused the accumulation of mutations in the W chromosome and the inactivation and the loss of the affected genes. Since male birds contain two Z chromosomes this latter chromosome can still recombine and consequently avoid the accumulation of genetic damage. The loss of identifiable genes on the W chromosome has however been accompanied by the acquisition of DNA components of unknown function. Often this DNA component consists of long stretches of repeated short sequence elements. Such sequence elements are characteristic for the heterochomatic, dense segments of the chromosomes. This makes it possible to apply staining procedures which render the W chromosome uniquely visible by microscopic observation, and consequently allow for the cytological sex identification of neognathous birds.

Although most of the Z-chromosomal genes therefore lack a counterpart on the W chromosome, this latter chromosome is not completely genetically inert. There are still a few genes which occur in an active W-chromosomal and Z-chromosomal form. These genes include e.g. the CHD gene, an ATP synthase gene, a protein kinase gene and the spindlin gene. Not all birds contain the W-chromosomal form of all these genes. Research at Avian Biotech has shown that in certain parrots, notably the African grey parrot (Psittacus erythacus) the W-chromosomal form of the ATP synthase gene has also been lost (1). But where found the W- and Z-chromosomal form of these genes have a considerable difference in nucleotide sequence as a result of their long (100 - 150 million years), separate evolution. This sequence difference is the basis of modern molecular, DNA based sex identification procedures.
In addition to the neognathous birds there exists another group, the palaeognaths which includes the ratites (ostriches, emus etc.) and the tinamous. Palaeognathous birds have W and Z chromosomes, which are largely identical and microscopically indistinguishable. Instead of having lost most of the genes found on the Z chromosome as in the neognaths, the palaeognath W carries most of these genes. Contrary to what is found in neognathous birds, recombination between the palaeognathous W and Z chromosome occurs over their entire length, which explains the retention of the genes on the W chromosome. Although not characterized, the paleognath W chromosome must however carry some genetic activity involved in femaleness, therefore original evidence for the existence of a palaeognathous W chromosome was initially genetic. Recent research has identified several DNA sequence elements which are unique for the palaeognath female, although none of these elements has any known, identifiable function in female sex differentiation. Most of these female specific elements are specific for the individual species. Research at Avian Biotech (2) identified e.g. a DNA sequence element which is female specific for the emu (Dromaius novaehollandiae), but not female specific for the ostrich (Struthio camellus), kiwis (Apteryx) or rheas (Rhea americana), although it is also female specific for the different cassowaries (Casuarius) which are closely related to the emu.

Tinamous are classified traditionally, based on certain anatomical evidence, as palaeognathous avian species. Microscopic evidence has shown that the tinamou W chromosome is structurally intermediate between the highly heterochromatic W chromosome of the neognathous avian species and the extended, euchromatic form of the ratites. Such studies have also revealed that the tinamous W and Z chromosome recombine only over a segment of their length. Further analysis at Avian Biotech (3, 4) has shown that most of the genes which occur on the neognathous W and Z chromosome in different forms are identical in tinamous, except for the spindlin gene, which occurs in a W- and Z-chromosomal form in these birds. This difference between the W and Z spindlin gene allows therefore the molecular, DNA based sex identification by procedures which can also be used for the neognathous species. Since tinamous are paleognaths, which are an entirely different group of birds than the neognaths, these results suggest strongly that the cessation of recombination, which is most likely the underlying reason for the degeneration of the avian W chromosome, occurred independently in these two avian groups, a finding in line with what is known about the evolution of sex chromosomes in general, where such degeneration has occurred numerous times in different animal families.


1) De Kloet, S. R. 2001. Loss of the gene for the alpha subunit of ATP synthase (ATP5W1) from the W chromosome of the African grey parrot (Psittacus erithacus). J. Mol. Evol. 53:135-143.

2) De Kloet, S. R. 2001. Development of a CAPS (cleaved amplified polymorphic sequence) assay for sex identification of the emu (Dromaius novaehollandiae). Mol. Ecol. Notes 1:273-274.

3) De Kloet, S. R. 2002. Molecular sex identification of tinamous with PCR using primers derived from the spindlin gene. Mol. Ecol. Notes 2:465-466.

4) De Kloet, R. S., and S. R. de Kloet. 2003. Evolution of the spindlin gene in birds: independent cessation of the recombination of sex chromosomes at the spindlin gene in neognathous birds and tinamous,
a palaeognathous avian family. Genetica 119:333-342.

5) De Kloet, R. S. and de Kloet, S. R. 2005. The evolution of the
spindlin gene in birds: Sequence analysis of an intron of the
spindlin W and Z gene reveals four major divisions of the
Psittaciformes. Mol. Phyl. and Evol. 36: 706-721.

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