Stepwise emergence of recombination suppression precedes fissiparous asexuality in the planarian Schmidtea mediterranea

A central paradox in evolutionary biology is the rarity of asexual reproduction, which is often attributed to developmental constraints and long-term costs. Yet, fissiparous asexuality--where animals split and regenerate--is widespread among planarians, hinting at genomic features predisposing them to asexuality. We investigated the genomic underpinnings and evolutionary consequences of asexuality in the planarian Schmidtea mediterranea, which exists as both obligately fissiparous and sexual strains. We generated a haplotype-phased genome assembly of the asexual strain and collected population genomic data from laboratory and wild populations to uncover extensive heterozygous chromosomal rearrangements affecting all chromosomes. We show that these rearrangements arose in a sexually reproducing ancestor without directly disrupting reproductive genes but instead progressively suppressing recombination across the genome. The asexual genome exhibits minimal deleterious mutation accumulation, indicating a low cost of asexuality. Population-genomic data show this strain lacks any detectable sexual reproduction and originated recently (0.17-0.4 Ma), but the young age is insufficient to explain the low mutational burden. Instead, planarians may be able to exploit the lack of a single-cell bottleneck in fissiparous reproduction to mitigate the costs of asexuality. Altogether, our results support a model in which stepwise recombination suppression due to structural rearrangements eroded the benefits of sex and enabled the emergence of fissiparous asexuality in S. mediterranea.