# The mutation landscape of Daphnia obtusa reveals evolutionary forces shaping genome stability

## TL;DR

This study explores how mutations shape the genome of Daphnia obtusa, revealing insights into mutation rates, selection pressures, and genome stability.

## Contribution

The study provides a comprehensive analysis of mutation dynamics and selection forces in Daphnia obtusa using mutation-accumulation lines and natural population data.

## Key findings

- Spontaneous single-nucleotide mutation rate is 2.45 × 10−9 per site per generation.
- Nonsynonymous mutations are underrepresented, indicating detectable purifying selection during mutation accumulation.
- Observed GC content is higher than mutation-predicted levels, suggesting natural selection stabilizes base composition.

## Abstract

Spontaneous mutations are the primary source of genetic variation and play a central role in shaping evolutionary processes. To investigate mutational dynamics in Daphnia obtusa, we generated a chromosome-level genome assembly spanning 129.4 Mb across 12 chromosomes, encompassing 15,321 predicted protein-coding genes. Leveraging whole-genome sequencing of eight mutation-accumulation (MA) lines propagated for an average of 482 generations (spanning over 20 years), we estimated a spontaneous single-nucleotide mutation (SNM) rate of 2.45 × 10−9 and an indel mutation rate of 3.34 × 10−10 per site per generation. The SNM spectrum was strongly biased toward C:G > T:A transitions. Despite the design of MA experiments to minimize selection, nonsynonymous mutations were strongly underrepresented, providing rare evidence that purifying selection can act detectably even during mutation accumulation. Comparative analyses with natural population data revealed that exonic mutations observed in the MA lines were significantly less likely to be present in standing variation than intronic or intergenic mutations, suggesting that purifying selection in natural populations acts to remove deleterious alleles. We also identified 48 loss-of-heterozygosity (LOH) events, comprising 8 heterozygous deletions and 40 gene-conversion events, yet found no evidence of GC-biased gene conversion. Instead, while mutation predicts a substantially lower equilibrium GC content, the observed GC level is maintained at higher values, implicating natural selection as the primary force stabilizing base composition. Together, these results provide one of the most comprehensive assessments of the interplay among mutation, selection, and genome stability in an ecologically important species.