Читать книгу Diatom Morphogenesis - Группа авторов - Страница 30

Centric diatom valve formation occurs from a central area to the valve margin in three general stages [2.126] and is illustrated in Figure 2.5. First, horizontal silica deposition occurs to form a basal layer. From an annulus in the central area at the site of valve formation initiation, radial rows of silica are deposited as areolae with cross extensions and connections so that a branching pattern of silica strands emerges. Gaps in this overall pattern are filled in with silica, and internal rimoportulae tubes commence [2.126]. Second, vertical silica deposition occurs so that areolae walls increase in height. Round pores are transformed to hexagonal or other shapes, and external rimoportulae are evident [2.126]. Third, areolar associated structures of cribra and cribella are completed during horizontal silica deposition so that the size and spacing of these structures occur as a response to the constraints imposed by the areolae [2.126]. Successive layers of silica are deposited, and the completion of the valve and its structures extends to the margin [2.116].

Figure 2.5 Three stages of valve formation (as defined in [2.125]) in Biddulphia. Row 1: First stage shows formation of the elongated annulus, branching virgae and vimines producing open holed areolae, and the internal tubes of rimoportulae just beginning. Row 2: Second stage shows the external emergence of rimoportulae, the filling in of areolae, and ridges are more hyaline with increasing silica deposition. Row 3: Third stage shows the beginning of pseudocelli formation, fusing of the valve margin, and the mature cribra. All photos by Mary Ann Tiffany.

Prior to valve formation, mitotic and cytokinetic processes occur [2.122]. Silica aggregation occurs within the silicalemma/SDV where new wall formation occurs [2.59, 2.116, 2.119, 2.126]. The general outline of the cell wall is molded via the plasmalemma [2.122].

As silica deposition occurs from center to the periphery, it is evident that various structures are created at various times in a sequence. Major structures common to all species of a given genus, particular valve structures that are important morphologically, and concavity or undulations associated with the valve face (perhaps due to buckling [2.52]) are associated with symmetries of the diatom valve. The species-specific differences in valve structures and their location and timing of formation are indicators of symmetry changes during valve formation.

Sexual reproduction and auxosporulation result in a cell wall that is similar to a diatom spore and takes a few generations to “restore” the vegetative shape [2.69]. The initial valve that is formed, by an unknown mechanism, morphogenetically determines vegetative valve shape via protoplast adhesion and contraction within the spherical auxospore [2.122, 2.156]. Heterovalvy is apparent even within the initial valve [2.125], having implications for symmetry.