Natural evolution uses processes of development to grow highly adapted organisms. Understanding morphogenesis, the generation of form, will therefore aid the development of well-designed, well-adapted, efﬁcient devices, such as robots, processors, and circuits. Organisms that incorpo- rate inorganic material into their morphology offer a rare and exciting opportunity to learn, from nature, efﬁcient mechanisms for the manipulation of materials for our technology.
Diatoms are single celled photosynthetic protists that thrive in many environments such as seas, lakes, and damp soils. With over 200 000 species, they are the second most diverse group of photosynthetic organisms and produce approximately 20% of the world’s carbon ﬁxation.12 Most interestingly they possess an external shell or frustule of amorphous silica that functions as a cell wall. This frus- tule is made up of two halves, each comprising a valve and a number of girdle bands. Diatom valves are often beautifully patterned, with regularly arranged pores per- forating the valves.3 As a cell wall, the frustule is struc- turally very strong and resistant to enzyme attack and also functions as a defense against grazing and infection.
However, some diatoms are susceptible to parasitism by chytrids, oomycetes, and protozoa and infraspeciﬁc vari- ation in susceptibility has been observed.4–7 Although it is unclear whether there is a consistent point of entry for the parasites. Diatom cell walls confer rigidity and precise shape to the enclosed protoplasts. However, they must also allow the transport of small molecules to and from the protoplast and allow for its expansion during the mitotic cell cycle.
Despite a variety of studies over the last few decades,9 the ﬁne control of nanometer to micrometer scale pat- tern during diatom valve morphogenesis remains poorly understood. Transmission electron microscopical (TEM) studies reveal that the cytoskeleton (the network of cyto- plasmic structural components, including actin ﬁlaments and microtubules9) is intimately involved in valve pat- terning and may also incorporate the use of cytoplasmic organelles or other inclusions as moulds for different val- var components.10–12 Schmid13 suggested that the process of using material to block the deposition of silica is com- parable to the negative technique used in batik, where the outline of an image is drawn with wax and the dye only soaks into the cloth where there is no wax. Thus color is incorporated where wax is absent.
Our model is only concerned with the morphogene- sis of raphid pennate diatoms, which are of particular biological interest to one of us (E.J.C.) and have not pre- viously been the subject of computer models. It is based on the premise that silica is deposited around organi- cally produced templates, the protoplast effectively gener- ating a negative imprint of the valve pattern.13 This paper explores the evolution of a negative space mechanism for the manipulation of silica, to produce a functional, pat- terned shape, similar in form to a raphid pennate diatom valve. Parkinson and co-workers presented a theoreti- cal model, based on diffusion-limited aggregation (DLA), which produced centriclike patterns,14 although cell biolo- gists would argue that observed patterns are not explicable by the physics of diffusion alone,81316–18 but that cyto- plasmic components and processes are modulating valve morphogenesis.
Valve Morphogenesis in Raphid Diatoms
Two major symmetry groups of diatoms can be rec- ognized, centric and pennate. Centric diatoms usually exhibit radially symmetrical valves, with an annular pat- tern center, whereas pennate diatoms have approximately bilateral symmetry and an elongate pattern center.3 Within the pennate group, raphid diatoms are characterized by the possession of a double-slit (raphe) system, which is the elongate pattern center and has an intrinsically asymmet- rical mode of development.
Diatoms reproduce predominantly by mitosis, each daughter cell producing one new valve (the hypovalve) after cytokinesis but retaining one of the parent valves as the older valve (epivalve) of each daughter. Because the new valves are formed within (and are constrained by) the parent frustule, there is often (but not invariably) a gradual decrease in average cell size over a series of mitotic divisions.1920 Within a certain critical size range, diatoms can be induced to reproduce sexually and thereby to restore the maximum size for that species.
During formation of the valves, silica is transported to the silica deposition vesicle (SDV) where it diffuses in and adheres to already consolidated silica in an accretive manner.13 Valve formation occurs in a series of stages that always occur in the same order, although taxon speciﬁc patterns are also observed. (The variation in valve morphology between species and the consistency of morphology within species together indicate that mor- phology is genetically controlled.) Siliciﬁcation begins with the raphe sternum, ﬁrst forming a longitudinal rib that curls around as it approaches the cell apices and meets the extending shorter ribs on the secondary side. The initial position of the SDV and of the raphe system seems to be controlled by the position of the microtubule center (MC), whose orientation also sets that of the valve pattern. After enclosure of the raphe slits, ribs of silica (virgae) that will ultimately lie between the striae grow out in a transapical direction, with the cross connections (vimines) developing later to deﬁne the pores (often between 0.1 and 0.5 m in diameter). As silica polymerizes onto the enclosed form- ing valve the SDV expands apically and transapically. The sequential formation of virgae, vimines, and ﬁne pore occlusions suggests that areas where siliciﬁcation is initially prevented by the presence of organic mate- rial, e.g., between forming virgae, must subsequently be opened up to allow siliciﬁcation of the vimines. Tubu- lin and actin have been implicated in pattern formation, as microtubules and microﬁlaments are variously asso- ciated with the SDV during morphogenesis, and their inhibition affects the raphe position and pore spacing respectively.