On core jakobids and excavate taxa: The ultrastructure of Jakoba incarcerata

The cellular organisation of the 'excavate' flagellate Jakoba incarcerata B ernard, Simpson and Patterson 2000 is described. Cells have one nucleus and dictyosome. The putative mitochondria lack cristae. Two flagella (anterior and posterior) insert anterior to the feeding groove. The posterior flagel lum bears a dorsal vane. An 'anterior' microtubular root arises against the anterior basal body. Two main microtubular roots, left and right, and a si nglet 'root' arise around the posterior basal body and support the groove. Non-microtubular fibres termed 'A','B', 'I', and 'composite' associate with the right root. A multilaminar 'C' fibre associates with the left root. Th e cytoskeleton of J. incarcerata indicates a common ancestry with other exc avate taxa (i.e. diplomonads, retortamonads. heteroloboseids, 'core jakobid s'. Malawimonas. Carpediemonas, and Trimastix). Overall, J. incarcerata is most similar to (other) core jakobids, namely Jakoba libera, Reclinomona, a nd Histiona. We regard J. incarcerata as a core jakobid and identify the gr oup by the synapomorphy 'varies restricted to dorsal side of the posterior flagellum'. The anterior root and position of the B fibre (and presence of dense inclusions in the cartwheels and a conscpicuous singlet root-associat ed fibre) in J. incarcerata are novel for core jakobids and argue for close relationships with Trimastix and/or Heterolobosea. The C fibre is similar in substructure to the costal fibre of parabasalids and it is possible that the structures are homologous.