AUXOSPORE FORMATION AND THE MORPHOLOGY OF THE INITIAL CELL OF THE MARINE ARAPHID DIATOM GEPHYRIA MEDIA (BACILLARIOPHYCEAE)¹

Abstract

Recent studies have led to a rapid increase in knowledge of auxospore formation in diatoms. However, these studies have been limited to centric and raphid pennate diatoms, and there is still very little information for the araphid pennate diatoms. Using LM and SEM, we studied the development of the auxospore and the initial cell of the marine epiphytic diatom Gephyria media Arnott. Auxospores were bipolar and curved in side view, as in many other pennate diatoms. SEM revealed many transverse perizonial bands, all of which were incomplete rings. There was an elongate, sprawling, silicified structure beneath the ventral suture of the transverse perizonial bands. This structure is presumably equivalent to the longitudinal perizonial band in other pennate diatoms, although we could not determine the homologous relationship between the two features. Scales were found both in the inner wall of the perizonium and around the primary perizonial bands. The presence or absence of scales may be of phylogenetic significance in diatoms, only during the final stages of auxospore formation because scales are found in early spherical stages. The distinctive finger-like structures observed throughout all stage of G. media have not been observed before in the other diatom taxa.

Molecular data show the enigmatic cave-dwelling diatomDiprora(Bacillariophyceae) to be a raphid diatom

“…Additionally, transverse perizonial bands, as in Diprora, are also found in many raphid pennates (Mann et al, 2011). The perizonium structure for some non-raphid pennates is quite different from the condition observed in Diprora (Sato et al, 2004(Sato et al, , 2008. Finally, a close relationship between Diprora and members of the Sellaphoraceae is consistent with the presence in both of hymenate occlusions (Round et al, 1990) and a single H-shaped chloroplast lying against the epivalve (Mann, 1989).…”

Heterothallic sexual reproduction in the model diatom Cylindrotheca

“…A polar cap, presumably representing the ruptured remains of the original zygote wall, covered the auxospore endings (Figs 22,28,arrowhead). Scales (Sato et al, 2004;Kaczmarska et al, 2007) or other incunabula (e.g. the strip-like elements outside the perizonium in Nitzschia fonticola: Trobajo et al, 2006), could not be detected.…”

Congruence of morphological, reproductive and ITS rDNA sequence data in some AustralasianEunotia bilunaris(Bacillariophyta)

“…Mann, 1989). The perizonium of our E. bilunaris is thus more akin to that of the araphid pennate diatoms Rhabdonema (von Stosch, 1962) and Gephyria (Sato et al, 2004), where the central band is likewise split, than it is to most raphid diatoms. Clearly, many more araphid pennates need to be studied in detail before conclusions can be drawn, but it is unlikely a priori that the primary band would become a complete hoop until after the evolution of the raphe, since it would be difficult for essentially non-motile araphid pennate diatoms to escape from an auxospore casing constrained by a complete band around its centre.…”

“…42 (arrowhead) is homologous with the longitudinal perizonium present in some other pennate diatoms (e.g. von Stosch, 1962Stosch, , 1982Mann, 1982;Sato et al, 2004). In E. arcus Ehrenberg, Geitler (1951) described a perizonium of rings and reported that the perizonium was broken apart during formation of the initial cells (op.…”

“…The simplest explanation is that the thickened smooth element is a longitudinal perizonium (cf. von Stosch, 1962;Mann, 1982;Sato et al, 2004). The fact that the initial cell of E. arcus begins to expand before escaping from the perizonium suggests that the primary transverse band is split, as in E. bilunaris.…”