Low Spontaneous Mutation Rate in Complex Multicellular Eukaryotes with a Haploid-Diploid Life Cycle

Area of Science:

• Evolutionary biology
• Genetics
• Marine biology

Background:

• The spontaneous mutation rate (µ) is vital for understanding evolution and biodiversity.
• Mutation rates vary significantly across species, influenced by selection, drift, and life history traits.
• Asexual reproduction and haploid selection are hypothesized to impact mutation rates, but empirical data are scarce.

Purpose of the Study:

• To determine the spontaneous mutation rate in brown algae, representing a complex multicellular eukaryotic lineage outside plants and animals.
• To evaluate the impact of life cycle characteristics, including alternation of haploid and diploid stages and modes of reproduction, on mutation rate evolution.
• To test hypotheses regarding the effects of asexual reproduction and haploid selection on mutation rate.

Main Methods:

• Whole-genome sequencing of a parent-offspring pedigree in Ectocarpus sp. (30 individuals).
• Whole-genome sequencing of an interspecific cross in Scytosiphon (137 individuals).
• Estimation of base substitution rates (µbs) per site per generation.

Main Results:

• Ectocarpus sp. exhibited a base substitution rate of µbs = 4.07 × 10⁻¹⁰ per site per generation.
• Scytosiphon showed a higher base substitution rate of µbs = 1.22 × 10⁻⁹ per site per generation.
• Both species displayed unusually low mutation rates for complex multicellular eukaryotes.

Conclusions:

• The haploid-diploid life cycle and extensive asexual reproduction in brown algae may be significant drivers of their low mutation rates.
• Effective population size alone did not fully explain the low mutation rate observed in Ectocarpus.
• Brown algae provide a valuable model system for studying the evolution of mutation rates in eukaryotes with complex life cycles.