A telomere to telomere assembly of Oscheius tipulae and the evolution of rhabditid nematode chromosomes

Eukaryotic chromosomes have phylogenetic persistence. In many taxa, the number of chromosomes is related to the number of centromeres. However, in some groups, such as rhabditid nematodes, centromeric function is distributed across multiple sites on each chromosome. These holocentric chromosomes might, a priori, be expected to be permissive of large-scale chromosomal rearrangement, as chromosomal fragments could still partition correctly and fusions would not generate lethal conflict between multiple centromeres.

Here, we explore the phylogenetic stability of nematode chromosomes using a new telomere-to-telomere assembly of the rhabditine nematode Oscheius tipulae generated from nanopore long reads. The 60 Mb O. tipulae genome is resolved into six chromosomal molecules. We find evidence of specific chromatin diminution at all telomeres. Comparing this chromosomal O. tipulae assembly with chromosomal assemblies of diverse rhabditid nematodes we identify seven ancestral chromosomal elements (Nigon elements), and present a model for the evolution of nematode chromosomes through rearrangement and fusion of these elements. We identify frequent fusion events involving NigonX, the element associated with the rhabditid X chromosome, and thus sex-chromosome associated gene sets differ markedly between species.

Despite the karyotypic stability, gene order within chromosomes defined by Nigon elements is not conserved. Our model for nematode chromosome evolution provides a platform for investigation of the tensions between local genome rearrangement and karyotypic evolution in generating extant genome architectures.