Bergerac strains of Caenorhabditis elegans revisited: expansion of Tc1 elements imposes a significant genomic and fitness cost

The DNA transposon Tc1 was the first transposable element to be characterized in Caenorhabditis elegans and to date, remains the best-studied transposable element in Caenorhabditis worms. While Tc1 copy-number is regulated at approximately 30 copies in the laboratory Bristol N2 and the vast majority of C. elegans strains, the Bergerac strain and its derivatives have experienced a marked Tc1 proliferation. Given the historical importance of the Bergerac strain in the development of the C. elegans model, we implemented a modern genomic analysis of three Bergerac strains (CB4851, RW6999, and RW7000) in conjunction with multiple phenotypic assays to better elucidate the (1) genomic distribution of Tc1 and (2) phenotypic consequences of transposable element deregulation for the host organism. The median estimates of Tc1 copy-number in the Bergerac strains ranged from 451 to 748, which is both (1) greater than previously estimated and (2) likely to be an underestimate of the actual copy-numbers since coverage-based estimates and digital droplet polymerase chain reaction results both suggest higher Tc1 numbers. All three Bergerac strains had significantly reduced trait means compared with the N2 control for each of four fitness-related traits, with specific traits displaying significant differences between Bergerac strains. Tc1 proliferation was genome-wide, specific to Tc1, and particularly high on chromosomes V and X. There were fewer Tc1 insertions in highly expressed chromatin environments than expected by chance. Furthermore, Tc1 integration motifs were also less frequent in exon than noncoding sequences. The source of the proliferation of Tc1 in the Bergerac strains is specific to Tc1 and independent of other transposable elements. The Bergerac strains contain none of the alleles that have previously been found to derepress transposable element activity in C. elegans. However, the Bergerac strains had several Tc1 insertions near or within highly germline-transcribed genes which could account for the recent germline proliferation.