Complete organellar genomes of Cystophora brownii: Insights into mitochondrial and plastid structure and evolution in brown algae.

Cystophora (order Fucales) is one of the most diverse genera of habitat-forming brown algae and is endemic to temperate reefs of Australia and New Zealand. Species in this genus form complex habitat structures that support diverse marine communities and contribute to coastal productivity. However, Cystophora spp. are highly sensitive to environmental stress, and recent declines across parts of their historical range highlight their vulnerability to ocean warming. Despite their ecological importance, genomic resources for Cystophora remain scarce, limiting insight into their evolutionary history, adaptive capacity, and resilience to environmental change. Here, we sequenced, assembled, and annotated the complete plastid and mitochondrial genomes of the widespread species Cystophora brownii using PacBio HiFi long-read sequencing. We characterized organellar genome architecture and gene content, conducted comparative analyses with other Fucalean taxa, and reconstructed phylogenetic relationships within the Fucales. The mitochondrial genome is 36,047 bp and contains 66 conserved genes (GenBank accession PZ196221). The plastid genome is 124,536 bp and contains 171 conserved genes, including two inverted repeat regions (GenBank accession PX975689). Comparative analyses indicate conserved gene content, order, and structure relative to other Fucalean organellar genomes. Maximum likelihood phylogenetic analyses robustly place C. brownii within the Fucales with strong support, resolving it as sister to the remaining Sargassaceae, suggesting an early divergence within the family that may date to the Miocene. These genomic resources provide a foundation for resolving the evolutionary placement of Cystophora and advancing studies of organellar genome evolution in brown algae.