Not dead yet: The decaying W-chromosome of songbirds can still acquire new genes

Researchers report the transfer of genes from the Z- to the W-chromosome.

The avian W-chromosome is slowly decaying. This female-specific chromosome (males have two Z-chromosomes) has lost almost 90 percent of its gene content and accumulated large stretches of nonsensical repeated sequences. The few genes that still reside on the W-chromosome play a role in cellular housekeeping or are involved in female-specific processes, such as the development of ovaries. It thus seems that this chromosome is destined to keep degenerating, losing more and more genes over time. However, a recent study in the journal Genes reported the transfer of several genes from the Z-chromosome to the W-chromosome. Could there still be hope for the W-chromosome?

Duplication and Deletion

While exploring the genomes of several birds-of-paradise, including the Raggiana Bird-of-paradise (Paradisaea raggiana) and the Red Bird-of-paradise (P.rubra) , Luohao Xu and his colleagues noticed something odd. A genomic region on the Z-chromosome showed unusual high sequencing coverage in females. Because females only have one Z-chromosome, you would expect less sequences in your data to map to this chromosome. Why did this specific region exhibit such high coverage? One explanation could be that the region has been duplicated, resulting in twice as many sequences mapping to it. Since this pattern was only apparent in females, the researchers suggested that this duplicated region ended up on the female-specific W-chromosome.

A closer look at the duplicated region revealed that it was about 700,000 DNA-letters long, whereas the original region on the Z-chromosome spanned 1.3 million DNA-letters. One section of the region on the W-chromosome was thus deleted, removing five complete and two partial genes. An additional two genes – ANXA1 and ALDH1A1 – survived the deletion event and were thus successfully transferred from the Z-chromosome to the W-chromosome. One of these genes (ANXA1) is active in the ovaries and might thus be preserved on the W-chromosome because of its female-specific function.

The high sequencing coverage (top graph) and elevated heterozygosity (middle graph) in females pointed to the transfer of genes from the Z-chromosome to the W-chromosome. A subsequent deletion removed several of the transferred genes (bottom figure). From: Xu et al. (2020) Genes.

Transposable Elements?

Next, the researchers repeated their analyses for a handful of other songbird species. They found similar patterns in the Medium Ground Finch (Geospiza fortis) and the Great Tit (Parus major), where other genes moved from the Z-chromosome to the W-chromosome. The relatively large sizes of the transferred sections suggest the involvement of transposable elements which are known to move large regions across the genome. More detailed sequence analyses will be needed to unravel the details of these transposition events. Whatever the mechanism, this study nicely shows that the degenerating W-chromosome can still acquire new genes.


Xu, L., Irestedt, M., & Zhou, Q. (2020). Sequence transpositions restore genes on the highly degenerated W chromosomes of songbirds. Genes11(11), 1267.

Featured image: Raggiana bird-of-paradise (Paradisaea raggiana) © David J. Stang | Wikimedia Commons

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