Very small spontaneous structural mutation rate in green algae

Structural mutations led to major innovations, but our knowledge of the spontaneous structural mutation rate is very limited. We cannot have a complete view of the adaptive potential of species and new variants in a population without addressing this question. Here, we used Illumina genomic data from mutation accumulation experiments of five green algae species and measured very low structural mutation rate, between 5x10-12 to 2x10-11 mutations per site, at least ten times lower than the nucleotide mutation rate in these species. Such low rates of structural mutations may be a consequence of selection. Assuming that structural mutations have higher fitness effect than point mutations because they impact a larger portion of the genome, selection toward a small structural mutation rate may be stronger than for point mutations. A second explanation could be the rate of true positive detection of de novo structural mutations, that is globally poor and variable between methods. This highlights the necessity of long read sequencing in future mutation accumulation studies. SIGNIFICANCE STATEMENTMutations are the first source of all biodiversity. Biologists tried since a century to measure the spontaneous mutation rate, but our knowledge on the structural spontaneous mutation rate, st, is still poor. Here we measured st in five species of phytoplankton, and found a very small rate compared to nucleotide and short insertion-deletion mutations.