T2T genome of the basal nematode Xiphinema index reveals major adaptive events associated to plant parasitism and epigenetic regulation

Nematodes constitute the most speciose animal phylum, occupying an exceptionally broad range of biomes and encompassing free-living microbe-feeders, predators, and notorious plant or animal parasites. However, the limited availability of genome-scale resources for early-branching nematode clades has constrained our understanding of the molecular determinants underlying these multiple evolutionary adaptations. Here, we present the genome of the plant-parasitic nematode Xiphinema index, constituting the first telomere-to-telomere assembly for the order Dorylaimida, an early-branching nematode lineage. Phylogenetic analyses reveal that horizontal gene transfers (HGT) have played a pivotal role in the emergence of plant parasitism in this phylum. Remarkably, we identify the first documented case of a plant-derived HGT in a nematode genome. This gene encodes a predicted secreted parasitic effector, underscoring the diversity of evolutionary events that contributed to the parasitic arsenal. Our results suggest that, beyond bacteria and fungi, plants themselves have provided crucial genes for parasitism that may now be used by the nematodes to manipulate them. Our comparative genomic analyses across more than 70 nematode species further uncover an unprecedented epigenetic toolkit in an invertebrate genome. Indeed, X. index harbors a complete set of canonical DNA methyltransferases, a full-length ATRX chromatin remodeler, and 3D-chromatin architectural CTCF protein coding homolog. Consistent with this repertoire, we detected methylation at CpG genome-wide, which inversely correlates with gene expression and chromatin accessibility. Together, these findings reveal a vertebrate-like epigenetic machinery in an early-branching nematode and call for a fundamental reassessment of the prevailing epigenetic paradigm in nematodes and, more broadly, in animals.