Living representatives of the spiralian ancestral karyotype revealed by two Lima genomes

Conserved synteny enables the reconstruction of ancestral chromosome organization across deeply diverged lineages, as demonstrated across metazoans. In Spiralia, comparative genomic analyses have suggested an ancestral karyotype comprising 20 chromosomes. We report high-quality genome assemblies for two Lima species (Lima caribaea and Lima lima) with 20 chromosomes and show that their chromosome organization matches this ancestral state, including four previously described fusion-with-mixing events. Interestingly, other Limidae genus did not retain the ancestral karyotype: they have undergone subsequent lineage-specific fusions. The conserved organization of Lima chromosomes provides a rare genomic anchor for reconstructing ancestral chromosome organization and lineage-specific chromosome evolution across Spiralia and, more broadly, Metazoa.