Helitrons are enriched in lichenized fungi with long generation lengths and small distribution sizes.

Transposable elements have the potential to drive genome evolution by introducing mutations and causing structural instability and chromosomal rearrangements, particularly under conditions like environmental or genetic stress. In this study, we generated 18 new long-read-based metagenomically assembled reference genomes for lichenized fungi, which form obligate mutualistic symbioses with algae or cyanobacteria. We used the new genomes and 10 publicly available genomes to investigate the relationships between species traits (i.e. dominant reproductive mode, distribution size, and generation length) and the abundance and spatial distribution of transposable elements using a phylogenetic comparative framework. We found that species with smaller distribution sizes and longer generation lengths had a higher genomic DNA transposon load. Specifically, their genomes were enriched with Rolling Circle transposons, which contradict previous research that has identified high proportions of retrotransposons in rare species. Disproportionate distributions of transposable elements in rare and range-restricted species may disrupt genomic stability, decrease fitness, and be reflective of species experiencing a greater degree of stress. Conversely, greater transposable element activity may be an important source of novel genetic diversity in isolated populations with limited gene flow. Further research is needed to understand the potential mechanisms driving transposable element proliferation in rare species' genomes and if transposable element content is predictive of increased extinction risk.