Sperm ultrastructure in the diatoms Melosira and Thalassiosira and the significance of the 9 + 0 configuration

Abstract

The most complete account to date of the ultrastructure of flagellate cells in diatoms is given for the sperm of Thalassiosira lacustris and Melosira moniliformis var. octogona, based on serial sections. The sperm are uniflagellate, with no trace of a second basal body, and possess a 9 + 0 axoneme. The significance of the 9 + 0 configuration is discussed: lack of the central pair microtubules and radial spokes does not compromise the mastigoneme-bearing flagellum's capacity to perform planar beats and thrust reversal and may perhaps be related to sensory/secretory function of the sperm flagellum during plasmogamy. The basal bodies of diatoms are confirmed to contain doublets rather than triplets, which may correlate with the absence of some centriolar proteins found in most cells producing active flagella. Whereas Melosira possesses a normal cartwheel structure in the long basal body, no such structure is present in Thalassiosira, which instead possesses 'intercalary fibres' linking the basal body doublets. No transitional helices or transitional plates are present in either species studied. Cones of microtubules are associated with the basal body and partially enclose the nucleus in M. moniliformis and T. lacustris. They do not appear to be true microtubular roots and may arise through transformation of the meiosis II spindle. A close association between cone microtubules and tubules containing mastigonemes may indicate a function in intracellular mastigoneme transport. No correlation can yet be detected between methods of spermatogenesis and phylogeny in diatoms, contrary to previous suggestions.

Eukaryotic Flagella: Variations in Form, Function, and Composition during Evolution

“…For instance, in moss, land plants that do produce flagellate sperm, the '9+2' axoneme is thought to lack outer dynein arms (Merchant et al 2007). In flagellate gametes of the centric diatoms Milosira and Thalassiosira, radial spokes, central pair microtubules and inner dynein arms are absent (Idei et al 2013), and in nodal cilia, which establish left-right asymmetry during the development of mammalian embryos, the central pair and radial spokes are also absent. More extreme is the change in underlying axonemal symmetry as seen, for example, in '6+0' and '3+0' axonemes built by male gametes of gregarine protists (Prensier et al 2008, Schrevel andBesse 1975).…”

The Neomuran Revolution and Phagotrophic Origin of Eukaryotes and Cilia in the Light of Intracellular Coevolution and a Revised Tree of Life

“…Centriolar transformation temporally and physiologically differentiated the two cilia and made separate left and right ventral microtubular roots. (D) Successive dynein duplications generated inner arms (top: sufficient for bending [Heiss et al 2013] as doublet 1 -2 linker excluded arms from doublet 1, destroying ninefold symmetry; then the nexin -dynein regulatory complex [Heuser et al 2009Lin et al 2011;Bower et al 2013] for calcium regulation of beat; then outer dynein arms for greater power, and the center pair [nucleated by g-tubulin on TP, and fixed so did not rotate] with new arms [Carbajal-Gonzalez et al 2013] and kinesin-9 [Wickstead et al 2010] and spokes ] to modulate beat mechanics [not inherently needed for planar beat: Idei et al 2013]) to draw in more prey by water currents. For pictorial simplicity, the cell body is proportionally too small and ahead of the cilia, but probably at stage b/c the cytoskeleton geometrically rearranged to lift the cell body from the substratum and put the ciliated cell apex at the front, which is mechanically stabler (found in all extant posterior ciliary gliding eukaryotes), entailing a basal stable bend ([E] mechanism unknown) for the posterior gliding cilium.…”

Spermatogenesis and auxospore structure in the multipolar centric diatom Hydrosera

“…, O–Q) correspond to the rootlike bundles of microtubules found in the sperm of Thalassiosira lacustris (Grunow) Hasle (Idei et al. ), Coscinodiscus wailesii Gran et Angst (Jensen et al. ) and Pleurosira levis (Heath and Darley ).…”