Department of Biological Sciences, University of Arkansas, SCEN 601, Fayetteville, AR 72701,, USA.
Am J Bot. 2019 Apr;106(4):560-572. doi: 10.1002/ajb2.1267. Epub 2019 Apr 8.
Loss of photosynthesis is a common and often repeated trajectory in nearly all major groups of photosynthetic eukaryotes. One small subset of "apochloritic" diatoms in the genus Nitzschia have lost their ability to photosynthesize and require extracellular carbon for growth. Similar to other secondarily nonphotosynthetic taxa, apochloritic diatoms maintain colorless plastids with highly reduced plastid genomes. Although the narrow taxonomic breadth of apochloritic Nitzschia suggests a single loss of photosynthesis in their common ancestor, previous phylogenetic analyses suggested that photosynthesis was lost multiple times.
We analyzed genes from the nuclear, plastid, and mitochondrial genomes for a broad set of taxa to test whether photosynthesis was lost one or multiple times in Bacillariales. We also sequenced and characterized the plastid genome of a nonphotosynthetic Nitzschia species.
Phylogenetic analyses showed that genes from all three genetic compartments either supported or failed to reject monophyly of apochloritic Nitzschia species, consistent with a single loss of photosynthesis in this group. The plastid genomes of two apochloritic Nitzschia are highly similar in all respects, indicating streamlining of the plastid genome before the split of these two species.
A better understanding of the phylogeny and ecology of apochloritic Nitzschia, together with emerging genomic resources, will help identify the factors that have driven and maintained the loss of photosynthesis in this group of diatoms. Finally, some habitats host diverse communities of co-occurring nonphotosynthetic diatoms, reflecting resource abundance or resource partitioning in ecologically favorable habitats.
光合作用的丧失是几乎所有光合真核生物主要类群中常见且经常重复的轨迹。菱形藻属中的一小部分“无叶绿素”硅藻已经失去了光合作用的能力，需要细胞外碳来生长。与其他次生非光合分类群相似，无叶绿素的硅藻保持无色的质体，其质体基因组高度简化。虽然无叶绿素菱形藻的狭窄分类范围表明其共同祖先中发生了一次光合作用的丧失，但先前的系统发育分析表明光合作用已经多次丧失。
我们分析了广泛类群的核、质体和线粒体基因组中的基因，以测试 Bacillariales 中光合作用是否发生了一次或多次丧失。我们还测序并描述了一种非光合菱形藻的质体基因组。
系统发育分析表明，来自所有三个遗传区室的基因要么支持要么未能拒绝无叶绿素菱形藻物种的单系性，这与该组中光合作用的单次丧失一致。两种无叶绿素菱形藻的质体基因组在所有方面都高度相似，表明在这两个物种分化之前，质体基因组已经简化。
更好地了解无叶绿素菱形藻的系统发育和生态学，以及新兴的基因组资源，将有助于确定导致并维持该组硅藻光合作用丧失的因素。最后，一些栖息地存在着多种共存的非光合硅藻群落，反映了生态有利栖息地中资源丰富或资源的分区。
