Fottea

Fottea 9(2): 211-222, 2009 | DOI: 10.5507/fot.2009.022

Incunabula and perizonium of Neidium (Bacillariophyta).

David G. Mann¹, Aloisie Poulíčková²

¹ Royal Botanic Garden, Edinburgh EH3 5LR, Scotland, U.K.

² Department of Botany, Faculty of Science, Palacký University, CZ-771 46 Olomouc, Czech Republic

The incunabula and perizonium of a paedogamous Neidium deme (in the N. ampliatum species complex) are described in detail using light and scanning electron microscopy. The incunabula contain two types of element, both heavily silicified. At each pole of the ellipsoidal zygote there is a large cap, which almost abuts its twin on either side of the zygote, beneath the gametangial valves. The caps are deeply and widely notched on their 'girdle' sides (i.e. on the sides of the zygote that underlie the gametangial girdle) and the space created by the notches is filled by large, almost triangular incunabular plates. Thus, on each side of the equator of the zygote there is one cap element and two plate elements. The caps overlap the plates and must precede them ontogenetically, creating a polarity opposite to that of the transverse perizonium. Both elements are sparsely and irregularly porous and have fimbriate margins. During auxospore expansion, the two sets of incunabular elements separate at the equator of the cell and the transverse perizonium is formed beneath, the incunabula continuing to form a tight seal around the perizonium and apparently constraining development to form a linear tube. All transverse perizonial bands possess delicate fimbriae and are open, even the primary band, which is slightly wider than the bands on either side and bears a median ridge. The open ends of the secondary bands sweep strongly towards the centre of the auxospore, producing a very obvious and wide suture. In expanded auxospores, a robust longitudinal band is found beneath the suture, which has coarser fimbriae than the transverse bands. Possible interpretations of the incunabular elements are discussed: no structures similar to the incunabular plates have been described in other diatoms, but it is known that related genera (Biremis, Muelleria) also have large cap-like elements in their incunabula. The open nature of the primary transverse perizonial band is unusual in raphid diatoms, but auxospore coverings have been investigated ultrastructurally in too few diatoms to permit detection of evolutionary trends.

Keywords: auxospores, Bacillariophyta, diatoms, incunabula, Neidium, perizonium

Received: April 30, 2009; Accepted: May 15, 2009; Published: September 1, 2009  Show citation