A single loss of photosynthesis in diatoms
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 diatoms suggests a single loss of photosynthesis in the common ancestor of these species, previous phylogenetic analyses suggested that photosynthesis was lost multiple times. We sequenced additional phylogenetic markers from the nuclear and mitochondrial genomes for a larger set of taxa and found that the best trees for datasets representing all three genetic compartments provided low to moderate support for monophyly of apochloritic Nitzschia, consistent with a single loss of photosynthesis in diatoms. We sequenced the plastid genome of one apochloritic species and found that it was highly similar in all respects to the plastid genome of another apochloritic Nitzschia species, indicating that streamlining of the plastid genome had completed prior to the split of these two species. Finally, it is increasingly clear that some locales host relatively large numbers apochloritic Nitzschia species that span the phylogenetic diversity of the group, indicating that these species co-exist because of resource abundance or resource partitioning in ecologically favorable habitats. A better understanding of the phylogeny and ecology of this group, together with emerging genomic resources, will help identify the factors that have driven and maintained the loss of photosynthesis in this group, a rare event in diatoms.