Comprehensive Ultrastructure of Kipferlia bialata Provides Evidence for Character Evolution within the Fornicata (Excavata)

Carpediemonas-like organisms (CLOs) are important for understanding the evolutionary history of anaerobic excavates (e.g. diplomonads and parabasalids), especially their cytoskeletal traits and the functions of their modified mitochondria (e.g., hydrogenosomes and mitosomes). Kipferlia bialata is probably the most commonly encountered CLO and has an intriguing molecular phylogenetic position within the Fornicata; however, this species has yet to be described at the ultrastructural level. This study provides a comprehensive account of the ultrastructure of this excavate using light microscopy, SEM, and serial TEM sectioning. The pattern of flagellar transformation observed with SEM confirms that the posterior basal body is the ‘eldest’, enabling us to emend the numbering system and associated terminology of the flagellar apparatus in excavates. This revised terminology is fundamental for comparing the cytoskeletons of the Excavata supergroup with other eukaryotes. Moreover, K. bialata had several unusal features, such as a hood, a distinct gutter within the ventral groove, and hairs along a single flagellar vane. The ultrastractural data reported here significantly improve our understanding of fornicate morphology, and when placed within a molecular phylogenetic context, these data shed light onto patterns of character evolution within the Excavata.