An international team of researchers has discovered that all songbirds have an additional chromosome in their germ cells — the ‘germline restricted chromosome’ (GRC).

Somatic (normal) cells have two copies of each chromosome. Germ cells typically have the same set of chromosomes as a somatic cell, but only one copy of each chromosome when developed into sperm or eggs, depending on gender.

GRC, a chromosome found in the germ cells, is transmitted to offspring from mothers and is discarded from all somatic cells of the offspring in the early stages of development.

GRCs are also present in male germ cells, however GRC is thrown away before they turn into sperm and so do not pass to offspring via the father.

The first GRCs were reported for zebra and Bengalese finches — two related species of the family Estrildidae — but were considered a genetic oddity until this new study.

To answer the questions about the origin, architecture, and distribution of GRCs in bird lineages, the authors performed a comprehensive comparative cytogenetic study of GRCs from 24 bird species representing eight orders.

To further examine the degree of GRC conservation between distinct species, they made a sequence-based comparison of the GRC probes from four songbird species.

“Our study shows that GRCs are a key feature which helps to identify songbirds as a phylogenic group,” the researchers said.

“We did not find GRCs in any of non-songbirds, for example within chicken, terns, swifts, falcons and parrots.”

The team concluded that the GRC has formed in the common ancestor of all songbirds about 35 million years ago as a small additional chromosome.

Over evolutionary time, this chromosome underwent changes in size and genetic content and transformed into an important component of the songbird germ cell genome.

“It is tempting to hypothesize that GRC enabled songbirds to become the most species-rich order of birds,” said co-author Dr. Denis Larkin, from the Royal Veterinary College at the University of London.

“It allowed them to create endless, wonderful forms, and invade various ecological niches on all continents because their germ genomes have more genes than other birds meaning more opportunity for change.”

“These findings lead to other questions such as with birds being the only survived dinosaurs, did extinct dinosaurs also have GRCs which made them so speciose?”

The study appears in the Proceedings of the National Academy of Sciences.

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Anna A. Torgasheva et al. Germline-restricted chromosome (GRC) is widespread among songbirds. PNAS, published online April 29, 2019; doi: 10.1073/pnas.1817373116