Active and unusually expanded PIF/Harbinger transposable elements in the Caenorhabditis inopinata genome

BackgroundTransposable elements (TEs) serve as powerful drivers of genome innovation but also threaten genome integrity. The PIF/Harbinger superfamily is distinctive among DNA transposons because mobilisation typically requires proteins, a DDE transposase and a MADF DNA-binding protein. Caenorhabditis inopinata, the closest known relative of C. elegans, has a TE-rich genome and lacks multiple components of the ERGO-1-class endogenous small-RNA pathway, making it a useful system for examining TE dynamics in a distinct host context. We identified a spontaneous dumpy mutant of C. inopinata caused by insertion of a PIF/Harbinger-family element into the coding region of Cin-dpy-11. The inserted element, designated Harbinger-1M_cIno, belongs to the Turmoil2 lineage originally defined in C. elegans and retains a MADF domain but lacks a recognisable DDE transposase ORF. Genome-wide curation recovered 258 related copies, revealing a strongly asymmetric family structure. Short noncoding derivatives were predominant, MADF-bearing derivatives were expanded and only one DDE-bearing locus retained an apparently intact transposase gene, suggesting that DDE and MADF functions are partitioned across distinct elements and may be supplied in trans during mobilisation. We also identified a second PIF/Harbinger-derived family, Harbinger-2M_cIno, associated with the Turmoil1 lineage. This family comprises 1,376 copies and therefore records substantial past amplification, but it lacks a detectable DDE source, shows greater sequence divergence and more degraded terminal structures than Harbinger-1M_cIno. Together, these data indicate that the two PIF/Harbinger lineages in C. inopinata differ not in whether amplification occurred, but in when it occurred and whether present-day mobilisation competence has been retained.