Rebecca billardiae Véron, 2023

Véron, Benoît, Rougier, Etienne, Taylor, Anthony & Goux, Didier, 2023, New species of Pavlovophyceae (Haptophyta) and revision of the genera Exanthemachrysis, Rebecca and Pavlova, European Journal of Taxonomy 861, pp. 21-47 : 31-35

Rebecca billardiae Véron sp. nov.

PhycoBank: 103627

Figs 7–9

Diagnosis

In addition to the very pronounced angular or even cubic and non-metabolic shape that the new species can take, Rebecca billardiae sp. nov. is also distinguished by knob-scales with only one constriction instead of two, and a slightly shorter AF.

Etymology

The epithet given to the species is dedicated in honour of Professor Chantal Billard who, from the 70s to the 2010s, worked at the Université de Caen Normandie. Chantal Billard described 29 taxa including many haptophytes and she notably described D. virescens (Billard) Bendif & Véron.

Type material

Holotype FRANCE • Normandy, Baie des Veys, Canal de Carentan à la mer; 49°21′ N, 1°10′ W; alt. 0 m; 16 Feb. 2002; Fabien Jouenne leg.; Jacqueline Fresnel isol.; AC537; GenBank nos: JF714245 , JF718773 ; CN[537- 1NM] .

Description

Motile cells not phototactic often ovate, some with strongly marked angular, even cubic forms. Less angular cells highly metabolic. Cell surface irregularly covered with stalked club-shaped knob-scales with a single constriction. Single golden-brown bilobed chloroplast with parallel and helicoidal stacks of three thylakoids; pyrenoid and eyespot absent. Posterior filipodia branched and beaded. Sinusoidal anterior flagellum covered with a tomentum of knob-scales, proximal part remaining naked. Reduced posterior flagellum bearing few knob-scales. Naked bipartite haptonema with a proximal part of constant diameter and a string of pearl-like structures on distal part. Non-swimming cells gather with twisting anterior flagellum.

Microscopy and related analysis

After nearly two months of culture on agar plates, no colonies developed. Apparently only the flagellate stage exists in this species and consists of freely swimming cells. Often ovate (7.5 µm ± 0.6 × 6.1 µm ± 0.6, n = 50), cells can display very variable shapes, including cubic forms but mainly cells have flat unequal sides forming acute or obtuse angles at the circumference ( Fig. 7A ). When cells stop swimming, they gather, seeming to nestle together as closely as possible while keeping their long flagella beating ( Fig. 7A ). The appendages emerge from a sub-apical ventral pit ( Figs 7E , 9A–E ) surrounded at its base by a membrane sheath containing cytoplasm ( Fig. 8C ). There is a very clear difference between the three appendages present: the long wavy sinusoidal AF measures between 9 and 17 µm (13.6 µm ± 3.7, n = 17) and is covered, except at its base, by a thick fluff of mono-constricted KS (45 nm × 22 nm, n = 2) ( Fig. 9F–G ), at the tip it is tapered. The PF is atrophied forming a ≈ 200 nm high hump ( Figs 8H , 9B, D ). The H is well developed (3.0 µm ± 0.6, n = 8) and consists of two sections of almost equal length, the proximal part being of constant diameter, in the shape of a twisted hook ( Fig. 9A–E ), while the distal part, smaller in diameter, presents regular constrictions with the appearance of a pearl necklace ( Fig. 9B, E ). The two flagella as well as the cell body ( Fig. 9C, E–G ), but unlike the H, are covered with knobscales in the form of pedunculated and clavate structure with a single median constriction ( Figs 8E–F , 9G ). The scales are sporadically distributed on the plasmic membrane and on the diminutive PF while on the long AF they may form a thick tomentum of several layers of lines oriented perpendicular to the flagellum axis and forming regular rows ( Fig. 9F–G ). On their posterior end, the cells develop beaded and branched filipodia often of great length, as long as the cells or the AF, and appearing to trail behind the cells ( Fig. 7B–D, F ).

Taxonomic outcome: a revised description of the genus Rebecca

Although retaining the basic characters of the genus Rebecca , the species described here, Rebecca billardiae Véron sp. nov., requires emendation of the description given by J.C. Green in Edvardsen et al. (2000).

Figures

Fig. 7. Rebecca billardiae Véron sp. nov. (AC537), flagellate motile and non-motile cells, LM images. A. Gathering angular non-swimming flagellate cells with an S-shaped AF (white arrows). B. Polyhedral free-swimming cell with bilobed lateral golden-brown C, S-shaped AF (white arrow) and posterior beaded filipodium (black arrows). C–D. Cells of various shapes showing sub-apical insertion into a pit (white arrows) of the S-shaped AF. Posterior branched beaded filipodia present at the terminal part of the cell (black arrows). E. Ventral view of an angular free-swimming cell with sub-apical insertion of S-shaped AF into a pit (white arrow). F. Rounded cells with S-shaped sub-apically inserted AF (white arrow), posterior filipodium (black arrow) and bilobed golden-brown C. Scale bars: Scale bars: A = 4 μm; B–D, F = 5 μm; E = 1 μm.

Fig. 8. Rebecca billardiae Véron sp. nov. (AC537), flagella, plasma membrane and knob-scales, TEM images (A–D, G–H = Fix 1; E–F = Fix 2). A. Angular cell with bilobed C and TH lamellae grouped by 3 sometimes twisted. B. Longitudinal section of a motile cell displaying the AF bearing few KS (white arrows) and the adjacent H. C. Cross-section of the sub-apical pit of a non-swimming cell containing the bases of H and F (transverse section – right, oblique section – left) sheathed by cytoplasm and surrounded by the plasma membrane (black arrow). D. Inflated G secretory vesicles unloading their materials including KS near the flagellar insertion zone. E. Periphery of a cell showing continuity of the plasma membrane along the curved AF, both covered with pedunculate KS (white arrows). F. Isolated plasma membrane retaining pedunculate mono-constricted club-shaped KS sporadically dispersed on its surface. G. Longitudinal section of a motile cell displaying the AF and PF both bearing few KS (white arrows) and the naked H. Note discharge of KS by G cisternae. H. Reduced PF cut in median longitudinal section showing microtubules and bearing a dense pedunculate club-shaped KS (white arrow). Central pair of microtubules is absent. Scale bars: A = 2 µm; B–D, H = 0.5 µm; E–F = 50 nm; G = 1 µm.

Fig. 9. Rebecca billardiae Véron sp. nov. (AC537), flagellar apparatus details of motile cells. A–E. SEM images. A. Ventral view of a cuboid cell with the S-shaped distally tapered AF and curved H. B-C. Ventral views of angular cells with the S-shaped AF covered with KS, the vestigial PF and the curved bipartite H between both F. D-E. Details of the insertion the 3 appendages. D. Vestigial PF. E. Bipartite H with the proximal hook-shaped half and the distal half forming a pearl string structure. Note distally tapered AF. F–G. Negative staining images. F. Proximal part of the tomentose AF with multilayered KS (white arrow), extending after a bare proximal zone (black arrow) in the immediate vicinity of the cell. G. Details of mono-constricted (white arrows) KS covering the distally attenuated AF. Scale bars: A, C, E–F = 1 µm; B = 2 µm; D = 100 nm; G = 200 nm.