Department of Plant Sciences, University of Oxford, United Kingdom.
Department of Earth Sciences, University of Oxford, United Kingdom.
Genome Biol Evol. 2020 Jul 1;12(7):1051-1059. doi: 10.1093/gbe/evaa131.
Genetic diversity is expected to be proportional to population size, yet, there is a well-known, but unexplained lack of genetic diversity in large populations-the "Lewontin's paradox." Larger populations are expected to evolve lower mutation rates, which may help to explain this paradox. Here, we test this conjecture by measuring the spontaneous mutation rate in a ubiquitous unicellular marine phytoplankton species Emiliania huxleyi (Haptophyta) that has modest genetic diversity despite an astronomically large population size. Genome sequencing of E. huxleyi mutation accumulation lines revealed 455 mutations, with an unusual GC-biased mutation spectrum. This yielded an estimate of the per site mutation rate µ = 5.55×10-10 (CI 95%: 5.05×10-10 - 6.09×10-10), which corresponds to an effective population size Ne ∼ 2.7×106. Such a modest Ne is surprising for a ubiquitous and abundant species that accounts for up to 10% of global primary productivity in the oceans. Our results indicate that even exceptionally large populations do not evolve mutation rates lower than ∼10-10 per nucleotide per cell division. Consequently, the extreme disparity between modest genetic diversity and astronomically large population size in the plankton species cannot be explained by an unusually low mutation rate.
遗传多样性与种群大小成正比，然而，在大型种群中存在着一个众所周知但尚未解释的遗传多样性缺乏的现象，即“莱文廷悖论”。人们预计较大的种群进化出较低的突变率，这可能有助于解释这一悖论。在这里，我们通过测量一种普遍存在的单细胞海洋浮游植物——埃玛藻（甲藻门）的自发突变率来检验这一假说，尽管埃玛藻的种群规模巨大，但它的遗传多样性适中。对埃玛藻突变积累系的基因组测序揭示了 455 个突变，具有不寻常的 GC 偏向突变谱。这得出了每个位点的突变率µ=5.55×10-10（95%置信区间：5.05×10-10-6.09×10-10）的估计值，这对应于有效种群大小 Ne∼2.7×106。对于一种普遍存在且丰富的物种，其在海洋中的全球初级生产力中占比高达 10%，这样适中的 Ne 令人惊讶。我们的结果表明，即使是异常大的种群，其突变率也不会低于每个核苷酸每细胞分裂约 10-10。因此，浮游生物物种中适中的遗传多样性与巨大的种群规模之间的极端差异不能用异常低的突变率来解释。
