The Genomes of Charophyte Green Algae

The charophyte green algae (sometimes called basal streptophytes) belong to the lineage that gave rise to land plants (embryophytes), but are distinguished by lacking a number of features and characteristics of land plants, most notably the land plant life cycle. Individually, many charophyte algae have been used as systems to study plant biology, but the realization that these organisms belong together as an evolutionary lineage dates only to ultrastructural studies in the 1970s, and a more detailed understanding of their relationships to each other and to land plants is still emerging. Currently recognized members of the group include Mesostigma, Chlorokybus, Klebsormidiophyceae, Charophyceae sensu stricto, Coleochaetophyceae and Zygnematophyceae (Conjugatophyceae). Contrary to earlier molecular systematic studies, Zygnematophyceae appear as the sister to land plants in current multigene phylogenetic analyses. Early studies reported chromosome numbers and morphologies, and identified polyploid series in the Zygnematophyceae and Charophyceae s. str., with less detailed chromosome counts in the other groups. Molecular studies using PCR and related methods to study individual genes and processes have been accelerated by high-throughput sequencing, particularly of transcriptomes. Both chloroplast and mitochondrial organellar genomes are available for representatives of each major subgroup, but the only near-complete nuclear genome currently available is that of Klebsormidium. There are several genome sequencing projects underway, and expansion of genomic data available should occur soon.