Valve Morphogenesis in the Diatom Genus Pleurosigma W. Smith(Bacillariophyceae): Nature's Alternative Sandwich

Abstract

The loculate ("chambered") valve structure of centric diatoms like Triceratium favus Ehrenberg has been mentioned time and again in the nanostructure literature. Here we draw attention to the fundamentally different alternative sandwich model nature developed in the genus Pleurosigma, where it is nonloculate. This has so far been overlooked in nanostructural studies. We suggest some mechanical aspects that would offer interesting avenues for experimental testing. The first description of the natural fabrication process ("morphogenesis") is presented. This begins with the development of the raphe sternum, which then acts as a rigid backbone. The inner layer of the sandwich-structured valve develops next, with relatively large +/- round single internal foramina not yet closed by a sieve membrane, in offset arrangement. This serves as a substrate for rows of stubby hollow pillars, also in offset arrangement. Then the outer layer of the sandwich develops and two different patterns ("coarse-mesh" and "stellate bridges") have been observed. At first, the external areolar foramina are relatively large and +/- oval, gradually filling up until the tiny slits characteristic of the genus remain. In Pleurosigma species with double internal areolar foramina, small bridges grow from the opposite margins of the single foramina until they fuse. The sieve-membranes then close the internal areolar foramina. The finished valve is a lightweight structure expected to offer excellent strength with parsimonious expenditure of the raw material-silica.

Pleurosigma hinzianum sp. nov.andP. frenguellianum sp. nov.(Pleurosigmataceae, Bacillariophyta) from Argentinean coastal waters, in comparison withP. amaraStidolph andP. elongatumW. Smith

“…The apex of the specimen pictured by Reid (2002, fig. 24) in internal view is apparently eroded, with an atypical areola pattern that might be interpreted as an early stage of the valve morphogenesis (Sterrenburg et al 2005). Specimens analysed in the present study are not eroded and show apical details that could be considered consistent with those described by Reid (2002, fig.…”

“…Specimens of the type material pictured by Reid (2002, figs 24, 26) show the areolar pores adjacent to the raphe-sternum crossed by bars whilst elsewhere they do not show bars. In Sterrenburg et al (2005), it was shown that such bars develop in the last stage of valve formation, but not necessarily in all pores.…”

Seven new species expand the morphological spectrum of Haslea. A comparison with Gyrosigma and Pleurosigma (Bacillariophyta)

“…At the margin, the two valve layers are fused, so that there is a monolayer structure. The entire process has been documented for Pleurosigma in Sterrenburg et al (2005). Exactly the same morphology is seen in H. nipkowii and will here be demonstrated in other new Haslea species, where the two valve layers are also shored by longitudinal rows of columns, the spaces in between being partially filled in, resulting in perforated longitudinal "bulkheads" we call saepes (Latin: "fence", plural: saepes) here.…”

“…These fissures can be interpreted as the result of fusion of the separate external slits present in Gyrosigma species. Because of the perforated saepes, the valve interior resembles the "parking-garage" aspect present in Pleurosigma (Sterrenburg et al 2005) and Gyrosigma.…”

Valve and girdle band morphogenesis in a bipolar centric diatomPlagiogrammopsis vanheurckii(Cymatosiraceae, Bacillariophyta)

“…Over the past 10 years, information on the valve morphogenesis of diatoms has greatly increased (e.g. Cox, 1999;Tiffany, 2002Tiffany, , 2005Tiffany, , 2008Cox & Kennaway, 2004;Sterrenburg et al, 2005;Tiffany & Hernandez-Becerril, 2005;Kaluzhnaya & Likhoshway, 2007). However, although it has become clear that some aspects of valve morphogenesis have phylogenetic significance at higher taxonomic levels (Mann, 1984;Mayama & Kuriyama, 2002), there is still insufficient information to reveal how structure and ontogenetic patterns have evolved in the major diatom groups, especially among the lineages of bipolar centrics.…”