Insights into natural neocentromere evolution from a cattle T2T X chromosome

The cattle genome is crucial for understanding ruminant biology, but it remains incomplete. Here we present a telomere-to-telomere haplotype-resolved X chromosome and four autosomes of cattle in a near-complete assembly that is 431 Mb (16%) longer than the current reference genome. Using this assembly (UOA_Wagyu_1) we identify 738 new protein-coding genes and support the characterization of centromeric repeats, identification of transposable elements, and enabled the detection of 2397 more structural variants from 20 Wagyu animals than using ARS-UCD2.0. We find that the cattle X centromere is a natural neocentromere with highly identical inverted repeats, no bovine satellite repeats, low CENP-A signal, low methylation, and low CpG content, in contrast to the autosomal centromeres that are comprised of typical bovine satellite repeats and epigenetic features. Our results suggest it likely formed from transposable element expansion and CpG deamination, suggesting dynamic evolution. We find eighteen X-pseudoautosomal region genes have conserved testes expression between cattle and apes. We also find all cattle X neocentromere protein-coding genes are expressed in testes, which suggests they potentially play a role in reproduction.