The ultrastructure of Carpediemonas membranifera (Eukaryota) with reference to the "Excavate hypothesis"

The ultrastructure of Carpediemonas membranifera, a small flagellate isolated from marine intertidal sediments, is presented for the first time. This protist has two flagella inserting at the anterior end of a feeding groove. The posterior flagellum bears three vanes, The cell has an anterior nucleus, a single dictyosome and a hydrogenosome-like network. The flagellar apparatus includes a third, barren basal body. The dorsal surface is supported by a fan of microtubules associated with an anterior microtubular root. Two major microtubular roots, left and right, support the groove. A singlet microtubular root runs posteriorly between them. A suite of non-microtubular fibres support the anterior portions of the major roots. The right root splits in two and most of the individual microtubules supporting the groove are positioned between the two portions. As the groove widens and then narrows, the left root splays, then reduces to one microtubule. Much of the outer portion of the right root is also lost and the remainder becomes associated with a composite, striated and dense fibre which supports the right wall of the groove, The left side of the groove becomes bounded by a 'false left root' made up of the singlet root and a microtubule from the inner portion of the right root. All of these root remnants terminate in close association at the posterior end of the groove. In having a feeding groove Carpediemonas belongs to a collection of flagellates referred to as the 'excavate taxa', The other excavate taxa are the Heterolobosea, diplomonads, retortamonads, core jakobids (Jakoba, Reclinomonas, Histiona), Malawimonas and Trimastix. Carpediemonas is currently not assignable to any of these taxa. A comparison of various morphological characters across the excavate taxa is supplied. Carpediemonas is most similar to Malawimonas, is relatively similar to the core jakobids, but also contains some 'retortamonad-like' features, strengthening the hypothesis that at least the bulk of the excavate taxa have a common excavate ancestor. Understanding precise relationships among the excavate taxa will require a detailed analysis and more data from poorly understood taxa.