Cells in multicellular organisms are genetically heterogeneous owing to somatic mutations. The accumulation of somatic genetic variation in species undergoing asexual (or clonal) reproduction (termed modular species) may lead to phenotypic heterogeneity among modules. However, abundance and dynamics of somatic genetic variation under clonal growth, a widespread life history in nature, remain poorly understood. Here we show that branching events in a seagrass clone or genet leads to population bottlenecks at the cellular level and hence the evolution of genetically differentiated modules. Studying inter-module somatic genetic variation, we uncovered thousands of SNPs that segregated among modules. The strength of purifying selection on mosaic genetic variation was greater at the intra-module comparing with the inter-module level. Our study provides evidence for the operation of selection at multiple levels, of cell population and modules. Somatic genetic drift leads to the emergence of genetically unique modules; hence, modules in long-lived clonal species constitute an appropriate elementary level of selection and individuality.

Twenty-four seagrass (Zostera marina) ramets were collected from Finland. They were ramets belonging to the same genet. somaticMutation_nh.vcf: The DNA was extracted and sequenced at an average coverage of 81x on Illumina Hiseq4000 platform. For each ramet, a fixed genotype means the genotype is shared by all the cells within the ramet. The vcf file stores the loci with different fixed genotypes among the 24 ramets.04_s8x_rmGametic.recode.vcf: One ramet (R08) was sequenced at a coverage of 1,238x on Novaseq6000 platform. The vcf file stores the intra-ramet SNPs within the ramet.04_s10x_rmGametic.recode.vcf: One ramet (R10) was sequenced at a coverage of 1,295x on Novaseq6000 platform. The vcf file stores the intra-ramet SNPs within the ramet.04_s12x_rmGametic.recode.vcf: One ramet (R12) was sequenced at a coverage of 1,578x on Novaseq6000 platform. The vcf file stores the intra-ramet SNPs within the ramet.

Supplement to: Yu, Lei; Boström, Christoffer; Franzenburg, Sören; Bayer, Till; Dagan, Tal; Reusch, Thorsten B H (2020): Somatic genetic drift and multilevel selection in a clonal seagrass. Nature Ecology & Evolution, 4(7), 952-962