Towards a complete diploid chicken genome

The repetitive and heterochromatic parts of microchromosomes and dot chromosomes have impeded the creation of a complete, haplotype-resolved chicken genome. To overcome these challenges, we employed a trio-sequencing approach, utilizing long reads from a female chick, supplemented by parental long-read data for accurate haplotype phasing. This strategy produced nearly complete, haplotype-resolved paternal and maternal assemblies, with 14 residual gaps. Telomeric sequences remain missing in two paternal and four maternal chromosomes, suggesting a still incomplete assembly. GGswu2 resolved more than 50 Mb of previously unresolved regions, predominantly within dot chromosomes. The dot chromosomes exhibit low nucleotide diversity but a high density of structural variants between haplotypes. We found that the centromere sizes and relative positions of centromere cores sometimes differ between the two haplotype genomes. These findings suggest that comparative genomics must incorporate the high-resolution of these dynamic complex regions to fully understand avian genome evolution and genetic complexity.