Multicellular organisms usually have the same genetic information in all cells of an individual. There is, however, a growing list of exceptions, where parts of the genome are removed from some cells. This programmed DNA elimination has evolved multiple times across animals and plants, but we still know very little about its function, proximate mechanisms and evolutionary significance. Here we propose to study programmed DNA elimination in songbirds, where a whole chromosome is removed from the somatic cells during embryogenesis. This germline restricted chromosome (GRC) shows extraordinarily dynamic evolution and unstable meiotic and mitotic inheritance. Yet, it has not been lost from the genome for over 30 million years of songbird evolution, suggesting that it has an important function. Using a combination of novel cytogenetic and genomic approaches including single-cell multiome profiling, we aim to (i) reveal the function of the GRC, (ii) describe the mechanisms of its elimination, (iii) clarify its modes of inheritance, and (iv) assess its potential role in songbird radiation.
