Scientists have found an extra chromosome in songbirds

The research is published in the scientific journal ‘Proceedings of the National Academy of Sciences’.

The scientists studied the DNA of germline cells of 16 songbird species of nine singing passerine suborders, including martin, finch, tit, and siskin. They also studied eight species of other orders which included chicken, terns, swifts, parrots and falcons. It was found that the more ancient species which lived about 34 million years ago have an equal number of chromosomes in somatic and germline cells. Songbirds appeared on the Earth later, and the species studied have an additional chromosome in germline cells.

‘Unlike other birds and most other animals, all songbirds have a different number of chromosomes in somatic and germline cells. Each songbird species has an additional germline-restricted chromosome (GRC). It is transmitted solely by oocytes and is ‘eradicated’ from somatic cells. Males have this chromosome only in sperm precursor cells, but it is absent in mature spermatozoids. There are no other endothermic organisms except songbirds demonstrating such a phenomenon,’ said Svetlana Galkina, Candidate of Biology, Associate Professor of the Department of Genetics and Biotechnology of St Petersburg University.

Biologists first found an extra chromosome studying the zebra finch in 1998 and thought it was a genetic oddity. Only in 2014, when it was discovered that Japanese finch also has a forty first GRC pair, the researchers started a more thorough investigation.

The additional GRC is thought to have evolved about 35-30 million years ago. Later, it underwent significant changes with each species developing in different ways. This extra chromosome has different sequencing between songbird species, each species having a unique sequence. It is quite possible that this chromosome enabled songbirds to spread on Earth: five out of ten thousand existing bird species are passerines.

‘Birds fly and that is their evolutionary benefit. In terms of genomics, songbirds have a very small and compact genome: it is three times smaller than the genome of a mammal. The human genome in each cell weighs 3.5 pg (1pg = 10^-12 gr), while the weight of an avian genome is around 1.2 pg.  There is a hypothesis that explains why the weight of an avian genome is so different from that of mammals. It is that avian organisms ‘eradicate’ in the process of evolution everything that might complicate their flying abilities. It is just a hypothesis, but we should give it some consideration,’ said Svetlana Galkina.

Now scientists have to discover the real function of the chromosome. It is very likely that it affects gender determination as GRC is only transmitted from mothers and is found in the oocyte. It is part of what makes a female bird a female. ‘It is unlikely that it determines the ability of a bird to sing, as only male birds sing,’ noted Svetlana Galkina.

Researchers plan to determine the stage when the chromosome disappears from somatic cells. ‘It is necessary to establish the stage of embryogenesis when the additional GRC is discarded from somatic cells. Current research suggests that by the moment a fertilised egg is laid this chromosome remains only in germline cells and is discarded from somatic cells. However, the process of embryogenesis starts during the passage of the ovum down the oviduct which makes it difficult to study the ongoing processes,’ said the biologist.

The scientists also plan to study how GRC functions in the ovum. ‘There is a scientific school at St Petersburg University headed by Professor Elena R. Gaginskaia that studies chromosomes in developing oocytes. It is the only school worldwide which has developed specific methods to study chromosomes in developing oocytes of birds. We have to understand how this chromosome functions in the ovum and how it is transferred without a homologue in spermatozoids,’ said Svetlana Galkina.