Evolutionary Dynamics of Transposable Element Activity and Regulation in the Apennine Yellow-Bellied Toad (<i>Bombina pachypus</i>)

Transposable elements (TEs) expansion and accumulation represent one of the main drivers of genomic gigantism. Different host genome silencing mechanisms have evolved to counteract TE amplification, leading to a genomic arms race between them. Nevertheless, the evolutionary relationship between TEs and host genome silencing pathways remains poorly understood. Here, we investigate the activity of TEs and TE-silencing mechanisms in somatic and germline tissues of Bombina pachypus, a 10 Gb anuran genome. Our findings reveal a higher activity of TEs in the gonads compared to the brain, with retrotransposons as the most active class in both gonads (∼15% increased expression compared to the brain) and DNA transposons showing a 2-fold higher activity in ovaries. However, analysis of differentially expressed TEs between male and female gonads revealed a greater number of overexpressed TEs in testes (231 vs. 169), with maximum fold changes up to 22 in testes versus 8 in ovaries. This suggests a more permissive environment for TE expression in male gonads. Accordingly, increased activity of TE-silencing pathways was observed in ovaries compared to testes, with the KRAB-ZFP complex showing not only the highest overall expression levels but also a distinct ovary-specific expression pattern. Summarizing, while the higher TE activity in the male gonad may result from the lower efficiency of the KRAB-ZFP complex, the elevated activity of KRAB-ZFPs in ovaries, along with growing evidence of the functional role of TEs in the germline, suggests the existence of a broad range of host-TE dynamics going beyond the arms race model.