Further observations on the type subgenus of Pyramimonas (Prasinophyceae), with particular reference to a new species, P. chlorina, and the flagellar apparatus of P. propulsa

A new quadriflagellate species of Pyramimonas isolated from South African inshore waters is described. It has ultrastructural features most consistent with the subgenus Pyramimonas McFadden. The flagellar apparatus of this organism is basically of the 3-1 type, but a unique structure, which may be considered a homologue of the R2-associated fiber found in Pterosperma cristatum, is associated with the 1s root. It thus could be construed as an alternate primitive form of the genus to Pyramimonas mucifera Sym & Pienaar. The flagellar apparatus of Pyramimonas propulsa Moestrup & Hill has surprising differences to that of Pyramimonas octopus Moestrup. Hori, & Kristiansen, particularly with regard to the microtubular roots and the distal fibrillar bands. Features of these two species, together with those extracted from the literature, show a considerable overlap between the subgenera Pyramimonas and Punctatae but, ironically, it is considered inappropriate at this time to combine the two subgenera because of anomalies exhibited by the new species. Key words: flagellar apparatus, Pyramimonas subgen. et gen., P. chlorina sp.nov., P. longicauda, ultrastructure.

Precise flagellar configuration of the Rhizophlyctis harderi zoospore

Rhizophlyctis harder is a questionable member of the genus Rhizophlyctis, and more stable and reliable characters are needed to establish the taxonomic position of this chytrid. As a source of such characters the flagellar apparatus of zoospores of R. harderi was reconstructed from serial sections, and the precise configuration was determined. The flagellar apparatus included two major microtubular roots, each with separate points of origin near the kinetosome. Root A extended laterally from one side of the kinetosome toward and around one rumposome and then continued anteriorly. The other, root B, originated between the kinetosome and secondary centriole near fibrillar connecting material and projected anteriorly into the cytoplasm. Each of these two major roots branched into two rootlets. This system of microtubular roots is more complex than that found in the flagellar apparatus of other Chytridiomycetes presently described. The structure of the flagellar apparatus and accompanying roots clearly separates this species from others in the genus Rhizophlyctis and indicates that it can be used to establish a new genus in the Chytridiales. Key words: Rhizophlyctis harderi, Chytridiales, zoospores, flagellar apparatus, ultrastructure.

Ultrastructure and reconstruction of the flagellar apparatus in Chrysochromulina apheles sp. nov. (Prymnesiophyceae = Haptophyceae)

A new marine species of Chrysochromulina, C. apheles, is described from light and electron microscopy of a culture established from Danish coastal waters. The cells are among the smallest known in any species of Chrysochromulina, measuring ca. 4 μm in diameter. The general fine structure is illustrated and the structure of the haptonema and the flagellar apparatus is described in detail, based on serial sections. The flagellar root system, not previously examined in detail in any member of Chrysochromulina, is shown to consist of four microtubular roots, while cross-banded roots are lacking. Four cross-banded fibres were seen to interconnect the flagellar bases and the haptonema base. The haptonema belongs to the rather unusual six-stranded type. Two very similar looking types of small organic scales are present on the cell body. Unpublished data on the flagellar roots of the type species of Chrysochromulina, C. parva, are included. Chrysochromulina apheles is apparently cosmopolitan. It has presently been found in material from Denmark, Finland, England, Thailand, Australia, and New Zealand.

The ultrastructure of Ulothrix mucosa. II. The flagellar apparatus of the zoospore

The actual spatial configuration of the flagellar apparatus of the quadriflagellate zoospore of Ulothrix mucosa Thuret has been reconstructed by serial sectioning analysis. This apparatus shows an architecture quite similar to that found in related Ulvophyceae. Common characteristics are the differently leveled basal body pairs; the 180° rotational symmetry of the flagellar apparatus; the proximal overlap of the upper basal bodies which are displaced with respect to each other in the counterclockwise direction; terminal caps; four cruciately arranged microtubular roots (R2, R4); a distinctly striated distal connecting fibre that interconnects the upper basal bodies; and striated bands (SB1) that join the R4s to the lower basal bodies. Specific features are the arrangement of the R4 in a three over one configuration when entering the proximal region of the flagellar apparatus; the differently shaped proximal sheaths and their association with a proximal sheath connecting band; the presence of two system II fibres (rhizoplasts) which arise from the lower basal body pair; the striated bands (SB2) that connect the R2s to the lower basal bodies; the distinct striation of the system I fibre, which is not only intimately associated with the R2, but also with the R4 (not earlier reported for an ulvophycean alga); and, finally, the relevant displacement of the lower basal body pair in a counterclockwise direction of approximately half a basal body diameter. In light of these findings the taxonomic status of the Ulotrichales as well as of the Ulvophyceae is discussed.

Flagellar roots, mating structure and gametic fusion in the green alga Ulva lactuca (Ulvales)

The slightly anisogamous gametes of Ulva lactuca exhibit a cruciate flagellar root system consisting of 4 microtubular roots (4-2-4-2 system) and an elaborate system of fibrous roots associated with the 2-stranded microtubular roots. Two fibres (32-nm striation periodicity; system I fibres) closely underlie each of the 2-stranded roots, while different fibres (150-nm striation periodicity; system II fibres) run parallel to the root microtubules, and are 150–200 nm more internally located. Female gametes have 4 system II fibres, 3 of which are combined into a compound fibre associated with one microtubular root, while the fourth fibre is associated with the opposite root. In male gametes only 2 system II fibres are present, each underlying one of the two 2-stranded roots. A special region of the plasmalemma of both gamete types about 0.5 mum away from the basal bodies and located between 2 adjacent microtubular roots is structurally specialized and acts as a mating structure in gametic fusion. The region is oval-shaped and up to I.I mum long with a maximum diameter of 0.7 mum. A continuous electron-dense boundary layer underlies the plasmalemma at the edges of the mating structure. In both gamete types the mating structure consists of a fuzzy layer of material underlying the plasmalemma and special granules (60 nm diameter) are associated with this layer on its cytoplasmic side. In addition diffuse material overlies the mating structure, especially in male gametes. The mating structure is connected to 3 different kinds of flagellar roots: the boundary layer is linked to a 2-stranded microtubular root and its associated system I fibre; the fuzzy layer of the mating structure is connected with a system II fibre; and in female gametes this is the compound system II fibre. The ultrastructural changes which occur after mixing the 2 gamete types have been followed. Mating structure activation involves contraction of system II fibres (change of striation periodicity to 100 nm), detachment of special granules from the fuzzy layer of the mating structure and their replacement by electron-transparent vesicles at the prospective cell fusion site. Furthermore, release of electron-dense contents from Golgi-derived vesicles in the anterior part of both gamete types precedes cell fusion. Cell fusion is exclusively initiated in a region delimited by the 2 mating structures. After partial dissolution the 2 plasma membranes unite within the mating structure regions. The ultrastructure of gametic fusion in Ulva lactuca is compared to that of other green algae and the significance of flagellar roots in the mating process of green algae is discussed.

Basal Body and Flagellar Development During the Vegetative Cell Cycle and the Sexual Cycle of Chlamydomonas Reinhardii

Basal body development and flagellar regression and growth in the unicellular green alga Chlamydomonas reinhardii were studied by light and electron microscopy during the vegetative cell cycle in synchronous cultures and during the sexual life cycle. Flagella regress by gradual shortening prior to vegetative cell division and also a few hours after cell fusion in the sexual cycle. In vegetative cells basal bodies remain attached to the plasma membrane by their transitional fibres and do not act as centrioles at the spindle poles during division. In zygotes the basal bodies and associated microtubular roots and cross-striated connexions all dissolve, and by 6.5 h after mating all traces of flagellar apparatus and associated structures have disappeared. They remain absent for 6 days throughout zygospore maturation and then are reassembled during zygospore germination, after meiosis has begun. Basal body assembly in developing zygospores occurs close to the plasma membrane (in the absence of pre-existing basal bodies) via an intermediate stage consisting of nine single A-tubules surrounding a central ‘cartwheel’. Assembly is similar in vegetative cells (and occurs prior to cell division), except that new basal bodies are physically attached to old ones by amorphous material. In vegetative cells, amorphous disks, which may possibly be still earlier stages in basal-body development occur in the same location as 9-singlet developing basal bodies. After the 9-singlet structure is formed, B and C fibres are added and the basal body elongates to its mature length. Microtubular roots, striated connexions and flagella are then assembled. Both flagellar regression and growth are gradual and sequential, the transitional region at the base of the flagellum being formed first and broken down last. The presence of amorphous material at the tip of the axoneme of growing and regressing flagella suggests that the axoneme grows or shortens by the sequential assembly or disassembly at its tip. In homogenized cells basal bodies remain firmly attached to each other by their striated connexions. The flagellar transitional region, and parts of the membrane and of the 4 microtubular roots, also remain attached; so also do new developing basal bodies, if present. These structures are well preserved in homogenates and new fine-structural details can be seen. These results are discussed, and lend no support to the idea that basal bodies have genetic continuity. It is suggested that basal body development can be best understood if a distinction is made between the information needed to specify the structure of a basal body and that needed to specify its location and orientation.