Heterothallic auxosporulation, incunabula and perizonium inPinnularia(Bacillariophyceae)

Abstract

The cytology and life cycle of Pinnularia cf. gibba was examined in nine clones from three Scottish localities. This freshwater epipelic diatom is heterothallic and produces two isogametes per gametangium in type IC auxosporulation (Geitler's classification [1973]). The zygote undergoes a highly unusual metamorphosis before beginning expansion, becoming shortly linear-lanceolate; this is accompanied by formation of a complete covering of thin, oxidation-resistant strips and scale-like structures (at the poles), which are quite separate from the perizonium formed during auxospore expansion. Observations of similar incunabular structures in P. acidojaponica show that these elements are siliceous. The P. cf. gibba perizonium also has unusual features, including a remarkably wide primary band. Trikaryotic and haploid auxospores are sometimes formed and haploid 'zygotes' mature and expand like diploids, but do not develop into mature initial cells. Several phases of mucilage secretion take place, from the gametangia, zygotes and auxospores. Triplets of gametangia and polyspermy occurred with high frequency; this and the systematic significance of variation in auxospore, incunabula and perizonium structure, are discussed. Aspects of the taxonomy of the P. gibba group are treated in supplementary material provided on the European Journal of Phycology website.

Genetic characterization and life cycle of the diatomFragilariopsis kerguelensis

“…This sequence of events is similar to what has been reported for some other diatoms, including the benthic raphid diatoms Seminavis robusta (Chepurnov et al, 2002) and Pinnularia cf. gibba (Poulíčková et al, 2007), but different patterns have been reported within raphid diatoms (reviewed by Chepurnov et al, 2004). As an example, only one uninucleate gamete is formed in each gametangium of the benthic freshwater genus Sellaphora (Mann, 1989), while two uninucleate gametes are produced in each gametangium of Pseudo-nitzschia pungens (Chepurnov et al, 2005).…”

Incunabula and perizonium of Neidium (Bacillariophyta).

“…Thus, the polarity shown in the development of the incunabula, from the poles inwards, is opposite to that of the transverse perizonium. No polarity was detected in the other incunabula studied recently by SEM, in Nitzschia fonticola (trobajo et al 2006) and Pinnularia (Poulíčková et al 2007), though it would be more difficult to detect the direction of overlaps in the systems of narrow strips found in these diatoms. Mann (1984) considered that, because the caps of Neidium bore the same spatial and temporal relationship to the auxospore and perizonium as the scales of Rhabdonema, the caps were probably "homologous with the scales of Rhabdonema and centric auxospores", but that "electron microscopy will be necessary to determine to what extent the homology is reflected in structure as well as position."…”

“…Grun. (trobajo et al 2006) and Pinnularia (Poulíčková et al 2007) suggests that Rhoicosphenia and Caloneis should be reinvestigated. kaczMarSka et al (2000) showed the presence of small circular scales during auxosporulation in Pseudonitzschia multiseries (haSlE) Hasle and claimed that they were present before plasmogamy on the passive (female) gamete.…”

“…Both the caps and the plates have fimbriate edges, thus resembling not only the perizonial bands of various pennate diatoms (e.g. Mann 1982, Sato et al 2004, 2008aPoulíčková & Mann 2006, Poulíčková et al 2007) but also the incunabular scales of some other pennate diatoms (e.g. Sato et al 2004Sato et al , 2008a, and the scales and properizonial bands of centric diatoms (von StoSch 1982).…”

“…These silica elements are parts of the 'incunabula', a term (from the Latin for 'swaddling clothes') introduced by trobajo et al (2006) for the "organic and inorganic components (including silicified elements) of the auxospore wall that surround, cover, or invest the perizonium or properizonium as the auxospore expands". In Rhabdonema and Gephyria, the incunabula contain approximately circular scales, consisting of a central annulus subtending an irregular system of branching ribs (von StoSch 1982;Sato et al 2004), whereas in Nitzschia (trobajo et al 2006 and Pinnularia (Poulíčková et al 2007) there are narrow plain strips of silica, orientated approximately at right angles to the axis of the auxospore.…”