Extreme haplotype variation in the desiccation-tolerant clubmoss Selaginella lepidophylla.

Plant genome size varies by four orders of magnitude, and most of this variation stems from dynamic changes in repetitive DNA content. Here we report the small 109 Mb genome of Selaginella lepidophylla, a clubmoss with extreme desiccation tolerance. Single-molecule sequencing enables accurate haplotype assembly of a single heterozygous S. lepidophylla plant, revealing extensive structural variation. We observe numerous haplotype-specific deletions consisting of largely repetitive and heavily methylated sequences, with enrichment in young Gypsy LTR retrotransposons. Such elements are active but rapidly deleted, suggesting "bloat and purge" to maintain a small genome size. Unlike all other land plant lineages, Selaginella has no evidence of a whole-genome duplication event in its evolutionary history, but instead shows unique tandem gene duplication patterns reflecting adaptation to extreme drying. Gene expression changes during desiccation in S. lepidophylla mirror patterns observed across angiosperm resurrection plants.