Plazzi Federico, Puccio Guglielmo, Passamonti Marco
Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy.
Genome Biol Evol. 2017 Apr 1;9(4):956-967. doi: 10.1093/gbe/evx051.
Bivalves and gastropods are the two largest classes of extant molluscs. Despite sharing a huge number of features, they do not share a key ecological one: gastropods are essentially epibenthic, although most bivalves are infaunal. However, this is not the ancestral bivalve condition; Cambrian forms were surface crawlers and only during the Ordovician a fundamental infaunalization process took place, leading to bivalves as we currently know them. This major ecological shift is linked to the exposure to a different redox environoments (hypoxic or anoxic) and with the Lower Devonian oxygenation event. We investigated selective signatures on bivalve and gastropod mitochondrial genomes with respect to a time calibrated mitochondrial phylogeny by means of dN/dS ratios. We were able to detect 1) a major signal of directional selection between the Ordovician and the Lower Devonian for bivalve mitochondrial Complex I, and 2) an overall higher directional selective pressure on bivalve Complex V with respect to gastropods. These and other minor dN/dS patterns and timings are discussed, showing that the Ordovician infaunalization event left heavy traces in bivalve mitochondrial genomes.
双壳类和腹足类是现存软体动物中最大的两个纲。尽管它们有大量共同特征，但它们并不共享一个关键的生态特征：腹足类基本上是表栖生物，而大多数双壳类是底内生物。然而，这并非双壳类的原始状态；寒武纪的双壳类是表面爬行生物，只是在奥陶纪才发生了一次根本性的底内生物化过程，从而形成了我们现在所知的双壳类。这一重大的生态转变与暴露于不同的氧化还原环境（缺氧或无氧）以及泥盆纪早期的氧合事件有关。我们通过dN/dS比率，针对一个经过时间校准的线粒体系统发育树，研究了双壳类和腹足类线粒体基因组上的选择特征。我们能够检测到：1）在奥陶纪和泥盆纪早期之间，双壳类线粒体复合体I存在一个主要的定向选择信号；2）相对于腹足类，双壳类复合体V总体上具有更高的定向选择压力。我们对这些以及其他较小的dN/dS模式和时间进行了讨论，结果表明奥陶纪底内生物化事件在双壳类线粒体基因组中留下了深刻的痕迹。
