Comparative Plastid Genomics of Glaucophytes

Diverse studies of plastid data suggest that the photosynthetic organelles of red algae, viridiplants, and glaucophytes, the three lineages comprising the Archaeplastida supergroup, share a common ancestor. Glaucophyte plastids are unique among archaeplastidians due to the presence of a vestigial peptidoglycan wall and the accumulation of RuBisCO in the stroma that resembles cyanobacterial carboxysomes. These ancestral traits, typically observed in cyanobacteria, have led to suggestions that glaucophytes are the earliest branching Archaeplastida lineage. Plastid phylogenomic surveys recover Glaucophyta as the earliest-diverging branch, but tree topology tests have not rejected the placement of red algae or viridiplants as the first splitting group. Resolving the branching history of the primary plastids might rely on both the implementation of phylogenetic methods that cope better with systematic errors and further expansion of the taxonomic sampling. The paucity of the Glaucophyta genome data has been a limitation when contrasting different hypotheses about the diversification of the Archaeplastida. The plastome of Cyanophora paradoxa was the only available from Glaucophyta for almost 20 years, until recently when plastomes of Glaucocystis, Cyanoptyche, and Gloeochaete and other Cyanophora species were sequenced. Comparative analyses show that the plastid gene repertoire of glaucophytes is highly conserved, and that the size and gene content of their plastomes do not differ drastically from those of other archaeplastidians. In fact, in terms of gene content, red algal plastomes are likely more similar to the repertoire of the ancestral primary plastid. Studies of plastomes have expanded our perspective about the diversity within Glaucophyta, but such studies are still based on limited taxonomic samples. The further inclusion of data from novel glaucophyte taxa will be critical to obtain more solid answers about the evolution and diversity of these rare algae.