Diatoms are the largest group of heterokont algae with more than 100,000 species. They are photosynthetic, unicellular eukaryotes that contribute ~ 45% of global primary production and inhabit marine, aquatic and terrestrial ecosystems. Despite their ubiquity and environmental significance very few diatom plastid genomes (plastomes) have been sequenced and studied. This study explored the pattern of diatom plastid nucleotide substitution rates across the entire suite of plastome protein-coding genes for 40 taxa representing the major clades. Substitution rate acceleration was lineage specific with the highest rates in the araphid 2 taxon Astrosyne radiata and radial 2 taxon Proboscia sp. Rate heterogeneity was also evident in different functional classes of genes. Similar to land plants, proteins genes involved in photosynthetic metabolism have substantially lower rates than those involved in transcription and translation. Significant positive correlations were identified between rates and measures of genomic rearrangement, but not plastome size. This work advances the current understanding of diatom plastomes and provides a foundation for future studies of their evolution.

Cartilaginous fishes have a very high phenotypical diversity, a phenomenon for which the mechanisms have been largely unexplored. Here, we report the genome of the white-spotted bamboo shark as the first chromosome-level genome assembly of cartilaginous fish. Using this genome, we illustrated a dynamic chromosome rearrangement process in the bamboo shark, which resulted in the formation of 13 chromosomes, all of which were sparsely distributed with conserved genes and fast-evolving. We found the fast-evolving chromosomes to be enriched in immune-related genes with two chromosomes harboring the major genes for developing the single-chain antibody. We also found chromosome rearrangements to have resulted in the loss of two genes ( p2rx3 and p2rx5 ) which we also showed were involved in cartilage development using a CRISPR/Cas9 approach in zebrafish. Our study highlighted the significance of chromosome rearrangements in the phenotypical evolution of cartilaginous fishes, providing clues to inform further studies on mechanisms for fish diversification.