Background: Mesorhabditis is known for its somatic genome being only a small portion of the germline genome due to programmed DNA elimination. This phenotype may be associated with the maintenance of telomeres at the ends of fragmented somatic chromosomes.

Objective: To comprehensively investigate the telomeric regions of Mesorhabditis nematodes at the sequence level, we endeavored to collect a Mesorhabditis nematode in the Republic of Korea and acquire its highly contiguous genome sequences.

Methods: We isolated a Mesorhabditis nematode and assembled its 108-Mb draft genome using both 6.3 Gb (53 ×) of short-read and 3.0 Gb (25 × , N50 = 5.7 kb) of nanopore-based long-read sequencing data. Our genome assembly exhibits comparable quality to the public genome of Mesorhabditis belari in terms of contiguity and evolutionary conserved genes.

Results: Unexpectedly, our Mesorhabditis genome has many more interstitial telomeric sequences (ITSs), specifically subtelomeric ones, compared to the genomes of Caenorhabditis elegans and M. belari. Moreover, several subtelomeric sequences containing ITSs had 4-26 homologous sequences, implying they are highly repetitive. Based on this highly repetitive nature, we hypothesize that subtelomeric ITSs might have accumulated through the action of transposable elements containing ITSs.

Conclusions: It still remains elusive whether these ITS-containing units are associated with programmed DNA elimination, but they may facilitate new telomere formation after DNA elimination. Our genomic resources for Mesorhabditis can aid in understanding how its distinct phenotypes have evolved.