Variation in phenotype, genotype, and somatic diversity among asexual planarians.

Most multicellular life reproduces sexually, utilizing a single-celled stage that acts as a genetic bottleneck. This bottleneck limits the evolution of selfish cell adaptations by ensuring all cells descend from a single progenitor. We investigated an obligately asexual strain of planarians that reproduces by self-bisection and lacks a single-cell bottleneck. Using 2.5 yearlong data on planarian reproductive behavior combined with genotyping, we revealed two previously undescribed genetically distinct substrains within the CIW4 strain. One substrain showed reduced fitness, which correlated with substantial losses of heterozygosity and increased somatic diversity. By genotyping consecutive head offspring over multiple generations, we found that only ∼9% of potential mutations were transmitted to the next generation via the tail, suggesting that fission acts as a genetic bottleneck. Our study uncovers significant diversity in a fissiparous animal and proposes how somatic diversity can be controlled in the absence of a single-cell bottleneck.