Germline specific chromosome sheds light on songbird evolution

Published: 2019-12-02


In an international collaboration, led by SciLifeLab researcher Alexander Suh (Uppsala University), researchers have sequenced a germline-restricted chromosome (GRC) – a chromosome only found in germline cells. The GRC, found in zebra finches, turned out to be millions of years old and plays a key role in songbird biology.

To an evolutionary biologist the evolution of reproduction might be one of the most important aspects. During the early developmental stages of an animal embryo the cells are divided into two major types, germline and somatic cells. Germline cells exists in the reproductive organs and are responsible for passing on the genetic information to the next generation while somatic cells make up the rest of the organism.

During recent years, biologists have discovered that certain genes or repetitive DNA-sequences only exists in the germline cells or vice versa, meaning that not all the cells in an organism contains the same genome. In certain species, such as the zebra finch, entire chromosomes are specific for the germline cells. These chromosomes are known as germline-restricted chromosomes or GRC. 

In a recent study, published in Nature Communications and led by Alexander Suh (SciLifeLab/Uppsala University), researchers have, for the first time, performed a comprehensive genomic, transcriptomic, and proteomic analysis of the zebra finch GRC. The GRC constitutes more than 10 percent of the genome, making it the biggest chromosome in the zebra finch.

“The GRC is a very strange chromosome. We found that some of its genes are repeated tens or even hundreds of times, whereas the somatic cells have only one gene copy.” says Cormac Kinsella, one of the first authors of the study, in a press release from Uppsala University.

By comparing specific genes with genomic data from other species, the researchers were able to unravel the evolutionary history of the GRC. The results suggest that the germline chromosome is tens of millions of years old and likely present across all songbird species, which represent half of all bird species.

The GRC was also found to contain many genes associated with early embryonic development which means it became an important factor in bird development. Because the GRC is only present in germline cells, GRC-specific genes cannot be expressed in somatic cells rendering them protected from possible negative effects related to those genes.

“Because we found GRC expression on the RNA and protein level, we expect our evidence for selection acting on the GRC to become the starting point of further exciting discoveries.” says Francisco Ruiz-Ruano, the other first author of the study.

Photo: Wolfgang Forstmeier