Human-specific remodeling of an endogenous retrovirus shapes structural diversity at acrocentric nucleolar organizer regions.

Human acrocentric short arms harbor K111, an endogenous retrovirus that became an integral component of nucleolar organizer region (NOR)-associated architecture across all five acrocentric chromosome types. Here we combine complete human and primate genomes, haplotype-resolved population assemblies and parent-offspring trios to reconstruct its evolution and transmission. Predominantly full-length ancestral elements underwent human-specific expansion and remodeling, generating recurrent mosaics and individual-specific multicopy configurations. Population structural variation is concentrated in surrounding satellite landscapes rather than within K111-derived sequences themselves. Trio-resolved genomes reveal direct parental transmission, single-parent remodeling and, crucially, maternal-paternal remodeling in distinct configurations: large reciprocal domains with contrasting parental affinities and de novo sequence exchange within an offspring locus. Raw long reads support both maternal-paternal configurations. K111-derived loci associate with nucleolin-positive compartments. Together, these findings establish K111 as an evolutionarily dynamic component and molecular record of acrocentric NOR architecture, revealing unrecognized remodeling and sequence generation across human generations.