Present in all songbird species tested to date, Germline-Restricted Chromosomes (GRCs) are characterized by their elimination from somatic tissues during development. Their intriguing biology and evolutionary history remain unsolved. Recent work revealed that genetic diversity in the zebra finch GRC is dramatically lower than the rest of the genome (A-chromosomes), suggesting a recent spread of a single GRC haplotype. Another peculiarity of the zebra finch GRC is its inheritance pattern. Thought to be mainly maternal inherited, the recent description of a paternally inherited haplotype complexified the picture. This discovery was further supported by genetic data with the GRC phylogeny being strongly incongruent with the mitochondrial tree. To reconcile those results, it was proposed that a single GRC haplotype might have swept through the zebra finch population through an increased rate of paternal inheritance (paternal spillover hypothesis). Nonetheless, previous studies had important limitations: the GRC containing paralogs to the A-chromosomes in various copies, only strongly divergent, single-copy sequences could be assembled, representing <1% of its estimated size. Those same regions being the most conserved between species, the possibility that the lower genetic diversity observed is driven by strong purifying selection could not be excluded. Moreover, due to the low number of polymorphic regions, the GRC phylogeny contained multiple nodes with poor bootstrap support. In this project, I propose to address those limitations by taking advantage of a new PacBio HiFi assembly containing ~54 % of the expected GRC length. Using Illumina re-sequencing data from the same 9 matrilines as the previous study, I will measure GRC diversity across a much larger portion of the GRC and construct a GRC phylogeny with much stronger support. I will address expected mapping issues by 1) including both the GRC and the A-chromosomes in the reference assembly and testing multiple combinations of reference version and mapper 2) taking advantage of a control somatic sample (i.e. without GRC) for each individual to identify GRC regions with higher rates of somatic mismapping. This approach should allow us to measure nucleotide diversity across all assembled GRC regions present in single-copy and non-identical to their A-chromosomal paralog. As the first large-scale population genetic study of a GRC, this project will provide more definitive answers to the paternal spillover hypothesis as well as useful methodological resources for population genetics analyses of GRCs in other species.