New research reveals evolutionary insights into the early development of songbirds

Zebra finch 500

An international team including researchers from the University of Sheffield has unravelled the evolutionary history of the germline-restricted chromosome (GRC), bringing new evolutionary insights into the early development of songbirds. 

Contrary to popular belief, not all cells in an organism contain the same genome. In some species, entire sections of DNA are eliminated when cells become either germline cells or somatic cells. In certain species, entire chromosomes are specific to the germline, such as the GRC found in zebra finches - the largest chromosome constituting more than 10% of the genome. For the first time, the team, led by scientists at Uppsala University in Sweden have performed a comprehensive genomic, transcriptomic, and proteomic analysis of the zebra finch GRC, uncovering its evolutionary history and function.

The ability to reproduce is a fundamental trait of all life. How reproduction has evolved and how it functions on a genetic level is therefore of great interest to biologists. During the early development of an animal embryo, cells are divided into two major types, germline and somatic cells. Germline cells are present in the reproductive organs and hold genetic information which is passed on to the next generation, whereas somatic cells are the cells which make up the rest of the organism. Biologists have discovered that in some organisms, specific genes and repetitive DNA-sequences are eliminated when cells become either somatic or germline, which means that some cells in the same organism contain a substantially different genome than others.

By identifying specific genes and comparing them with genomic data from other species, the scientists could unravel the evolutionary history of the GRC. The results showed that the GRC is tens of millions of years old and likely present across all songbird species, which represent half of all species of bird. The scientists also believe that the GRC became an important factor in bird development because many genes associated with early embryonic development are found on this chromosome. Because the GRC is not present in somatic cells, expression of its genes will only affect germline cells thereby possibly resolving a genetic conflict between the soma and the germline.

Alexander Charles from the Department of Animal and Plant Sciences at the University of Sheffield said: “DNA encodes different proteins that result in different physiological functions. I analysed the proteins present in zebra finch reproductive organs and was excited to find evidence for GRC-specific variants. This means that the GRC is functional and that the GRC is likely performing germline specific roles. ”

Dr Nicola Hemmings from the department added: "One of the most striking things about songbirds is their incredible diversity - they represent around half of all bird species and live in a wide range of habitats across the world. Our results show that the GRC likely emerged in the common ancestor of all songbirds and is still rapidly evolving, suggesting it has played a crucial role in this group's remarkable success."

The paper: Programmed DNA elimination of germline development genes in songbirds is published today in Nature Communications and brings together collaborators from: Uppsala University, Sweden, the University of Granada, Spain, Cornell University, USA, the University of Sheffield, UK, Cancer Science Institute of Singapore, National University of Singapore, Max Planck Institute for Ornithology, Germany, the University of Amsterdam, the Netherlands, the Université de Moncton, Canada and Bielefeld University, Germany.

Read the full paper in Nature Communications: Programmed DNA elimination of germline development genes in songbirds

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