File:709 2017 1147 Fig6 HTML.webp

English: Cytoskeletal innovations during loukozoa and chromist origins. Left diagrams summarise the ancestral condition in loukozoa as represented by Malawimonas. Upper shows the whole cell seen from the right with the feeding groove tilted obliquely to show left and right mt roots (R1, R2) that support feeding groove rims and floor. Younger anterior cilium (C2) with oar-like beat and older posterior cilium (C1) undulating from base to tip simultaneously propel the cell forward (arrow) and waft food into the groove for ingestion. Lower left (loukozoa: Malawimonas) and right (ancestral chromista) diagrams view the cell apex from the ventral side (so the cell’s right is on the left) to show mt arrays (colour: mt bands R1–R3; plus a dorsal fan of diverging mts that support the cell’s dorsal surface) and associated fibrous supports (black: A–C, I). The orthogonal centrioles (anterior A, posterior P) are interconnected by asymmetric linkers and in loukozoa (left) a dorsal mt fan and anterior left mt band (R3) connect C2s to the apical dorsal plasma membrane. R3 is developmental precursor of R1. The ancestral corticate interposed novel cortical alveoli between the plasma membrane and dorsal fan, which split into a right bypassing mt band (BB) and numerous single, diverging subpellicular mts attached to alveolar inner faces. Chromists (right) initially kept all these cytoskeletal components, modifying them as centrioles moved subapically as the text explains. Their sister clade plantae lost BB, the R2 outer branch, and B fibres. A second anterior right root R4 (not shown; see text) evolved polyphyletically by heterochrony in several chromist and plant lineages as a simplified developmental precursor of R2 (1 or few mts). The text argues that developmentally and evolutionarily the singlet root (S, brown) is a specialised R2 subcomponent, not a third posterior root as traditionally assumed. Dorsal fan and apical mts are actually longitudinal (as shown for BB only); the purple line symbolises a cross section of their mt arrays.