Birds-of-paradise, belonging to the songbird species, are well known for their diversified sexual traits such as extraordinary plumage forms and colors, sophisticated songs and mating rituals. In a collaborative study, involving the University of Vienna, the University of Zhejiang and the Swedish Museum of Natural History, genomes of 11 songbirds, including five birds-of-paradise, were analyzed to reconstruct the evolutionary history of their sex chromosomes. The study, published in Nature Ecology & Evolution, was enabled by the SciLifeLab National Genomics Infrastructure (NGI).
Female birds have two different sex chromosomes, called Z and W, while males have two copies of the Z chromosome. Just like the mammalian Y chromosome, the W chromosome is much smaller and contains fewer genes. By sequencing the female songbird genomes, the researchers were able to obtain information about how the W and Z chromosomes evolved over time.
During sperm and egg production sex chromosomes are partially blocked from genetic recombination, a process where genetic material is exchanged between chromosomes to increase diversity. This blockage prevents male sex characteristics from appearing in females and is thought to have been caused by four separate “suppression of recombination”-events that occurred before colonization and diversification of the species. When investigating these events, the researchers found evidence that “junk DNA” might have played an important role in the evolution of sex chromosomes.
Once chromosomes are blocked from recombination they lose protection against long-term genetic erosion leading to a permanent loss of many genes. The study shows that the remaining genes tend to be more broadly or highly expressed than in other species where both sets of genes still exists, indicating that they have more important functions than other genes.
The results provide insight into the dynamic evolutionary history of songbird sex chromosomes and new clues about the mechanisms of recombination suppression.
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