Читать книгу Diatom Morphogenesis - Группа авторов - Страница 29

Taxa were selected from a catalog of scanning electron micrographs (SEMs) in the library of one of the authors (M.A.T.). SEMs of centric diatom genera represented by selected species were used to quantify and analyze uncanny symmetry of diatom valve shape. Taxa selected for use represented circular, eccentric, and polygonal valve shapes that span the range of shapes of centric diatoms. A number of criteria were used to select images for analysis. Images were taken so that the view was determined to be perpendicular to the valve, where the valve appeared to be flat or at 0°. The whole valve had to be unbroken or in a state where breakage was qualitatively deemed to not impair the ability to discern the surface and/or shape boundary. The entire valve outline was unobstructed by debris, or if debris was present, it was qualitatively deemed to not detract from discerning the surface and/or shape boundary. Individuals were chosen to represent closely related species, and multiple species were selected from each genus when feasible.

Taxa whose images were chosen for study are given in Table 2.1. These 15 centric diatom genera represent a wide variety of genera, species, shapes, and valve patterns found in centric diatoms. Within each genus, species exhibit distinct features that are associated with the silica deposition process as evidenced by the morphology on the valve face. Morphological valve features are a record of silica deposition during valve formation and part of the morphogenetic process. The shape and position of valve features are indicators of symmetry of the valve surface.

Table 2.1 List of species considered and thumbnail, masked image examples for all of the individual diatom taxa we analyzed, all scaled to the same apparent diameter, along with individual number of rotations. Actual diameters vary with the life cycle. All thumbnails are external valves. All SEMs by Mary Ann Tiffany.

Genus species	Thumbnail images	Rotations
1. Actinoptychus
1.1. Actinoptychus senarius		3
1.2. Actinoptychus splendens		14
2. Amphitetras antediluviana		4
3. Arachnoidiscus
3.1. Arachnoidiscus ehrenbergii		14-31
3.2. Arachnoidiscus ornatus		17-24
4. Asterolampra
4.1. Asterolampra grevillei		13
4.2. Asterolampra marylandica		7
5. Asteromphalus
5.1. Asteromphalus heptactis		7
5.2. Asteromphalus imbricatus		9
5.3. Asteromphalus shadboltianus		7
5.4. Asteromphalus vanheurckii		10-11
6. Aulacodiscus
6.1. Aulacodiscus africanus		4-5
6.2. Aulacodiscus kittonii		4
6.3. Aulacodiscus oregonus		8-13
6.4. Aulacodiscus petersii		3
6.5. Aulacodiscus rogersii		3
6.6. Aulacodiscus scaber		3
7. Coscinodiscus sp.		14
8. Cyclotella meneghiniana		3-64
9. Eupodiscus radiatus		4
10. Glyphodiscus stellatus		4
11. Lampriscus shadboltianum		3
12. Spatangidium arachne		5
13. Triceratium bicorne		4
14. Triceratium
14.1. Triceratium castellatum var. fractum		3
14.2. Triceratium crenulatum		3
14.3. Triceratium favus		3
14.4. Triceratium favus var. maxima		3
14.5. Triceratium favus var. quadrata		3
14.6. Triceratium impressum		3
14.7. Triceratium pentacrinus fo. quadrata		4
14.8. Triceratium sp. [fossils]		4-15
15. Trigonium
15.1. Trigonium alternans		3
15.2. Trigonium arcticum		3
15.3. Trigonium arcticum var. kerguelense		3
15.4. Trigonium arcticum var. quadrata		4
15.5. Trigonium dubium		3
15.6. Trigonium formosum fo. quadrata		4

Aulacodiscus species can be differentiated by their tube openings, not only in the number on the valve face near the margin but also in terms of the shape and whether the tubes have external flanges or distinct internal structures. Tube openings are positioned on the valve and are indicative of symmetry. Tube opening formation occurs as valve formation proceeds. Externally flanged with tongued and ridged internal tube structures are present in circular tubed A. scaber and A. petersii in contrast to A. africanus and A. kittonii with slitted tube openings with no distinct internal structures. The fossil species, Aulacodiscus rogersii, has a flanged circular tube opening as does A. oregonus. Tube openings are formed near the valve margin, and their number and position on the valve margin are indicators of valve formation [2.144].

Areolae are loculate or poroid in Aulacodiscus, and the areas without pores that radiate from the central annulus form the rays of the valve face that extend to the tube openings. Occluded areolae occur as well at the valve margin. Cribra form from internal struts of the areolae, and as bullulae develop in early tube formation, they are covered by silica at the completion of tube formation. Cribra are finished after tube formation, with concentric rings of pores within the cribral surface and siliceous caps when mature [2.144].

Coscinodiscus has radial hexagonal loculate areolae, proximal circular rimmed foramen, and complex domed cribra [2.116, 2.126]. Areolae become higher at the margin with increased silica deposition in contrast to the center [2.116, 2.126]. The cribra are perforated and formed via differentiation of the outer velum covering each areola [2.116], and cribella fill the cribra as a small sieve plate structure [2.126]. Coscinodiscus has solid silica ribs outlining areolae radially with variably spotted and striated hyaline rays in the central area [2.126]. A central rosette from which silica strings diverge and branch form a general overall valve pattern [2.116]. There is a rimoportula at the marginal end of every two to three areolae, rimoportulae below the rimmed mantle edge, and other rimoportulae are scattered on the Coscinodiscus valve face, including the terminus of hyaline rays [2.126]. The timing of each daughter cell forming within the mother cell may be different for different species [2.116], which may have implications for symmetry during development.

Cyclotella is characterized by its mantle fultoportulae, clear central area with fultoportulae, and distinctive striations as ribs regularly placed at the valve margin covering about half of the valve face [2.56, 2.126]. At the valve margin, rimoportulae are present in varying numbers. The central area may have tubular fultoportulae and associated pores unlike mantle fultoportulae that may have a collar. Cyclotella initial cells have an unstructured central area and many valve fultoportulae but are hemispherically shaped, unlike vegetative cells which have an undulated shape. Salinity affects Cyclotella and may produce abnormal cells in terms of their internal structure [2.56].

Asterolampra valves form from a raised annulus from which siliceous rays as “spokes on a wheel” are formed, then bifurcated twice so that the spaces between the siliceous “tines” are the sites of areolae formation. Vertically, round pores form prior to the hexagonal honeycomb, and a central area fuses over the rays as a “roof” from the center to the periphery, producing a convex shape. The rays vary in number but are all of similar size and shape [2.146]. The regularity in the valve formation of Asterolampra is an indicator of valve symmetry.

Asteromphalus valves form so that a singular ray is slimmer and shorter than the rest of the rays which are uniform in shape and size but vary in number. Hexagonal areolae form between the rays prior to mantle formation, and large columnar structures form on the edges of areolae for all but the singular ray. Cribra are very complex. The central area fuses over the singular ray prior to the roof forming over the rest of the rays, and the overall surface produces an undulating shape. Internal formation of the rimoportulae is larger for the singular ray than the rest of the rays. Valve features point to asymmetry as does the eccentric annulus of Asteromphalus in contrast to Asterolampra which has a central annulus. Although Spatangidium arachne is similar to Asteromphalus, this taxon is dissimilar in having a central rimoportulae, a different cribral pattern, and a singular ray that is longer than the remaining four rays. Both genera have asymmetric valve faces unlike Asterolampra [2.146].

For species in Actinoptychus, there is a hyaline central area on external and internal valves. A narrow hyaline zone is present in A. splendens and A. senarius that extends from the central area to around halfway to the valve margin. Areolae are irregular in most species, and taxa have a variable number of sectors on the valve face [2.79]. Actinoptychus has a sectored valve face and curled rimoportulae. Areolae form a netted or reticulated pattern on the valve surface with a marginal ridge in species such as A. senarius, and other species have a loculus with a smooth valve face with no marginal ridge [2.57].

Arachnoidiscus has auxospore attachment to the hypovalve of the mother cell, while auxospore attachment to the mother cell of Amphitetras occurs on the epivalve [2.125]. The implication is that valve patterning forms differently depending on the attachment site.

Eupodiscus radiatus has marginal equally spaced ocelli with intercalated rimoportulae extending to the ocelli, a scalloped mantle edge [2.30], and loculate hexagonal areolae and cribral pores arranged in parallel rows [2.30, 2.31]. There are siliceous strips with flanges on the spines and other structures defining the valve mantle as well [2.30]. In contrast, Amphitetras has pseudoloculi with siliceous strips on the mantle without other structures present [2.30]. E. radiatus and Amphitetras symmetry may be based on the number and position of equally spaced ocelli or pseudocelli structures.

Triceratium species have elongated ocelli and poroidal areolae with domed cribra [2.31]. Unlike Triceratium, Lampriscus shadboltianum has pseudocelli [2.30], and Glyphodiscus stellatus has four-part symmetry in which the valve is concentrically undulate. Triceratium pentacrinus fo. quadrata and T. bicorne are uncommon four-part symmetrical species, as most are three-part symmetrical valves [2.31].

Pseudoloculi or loculi are variably present in Triceratium, e.g., as in pseudoloculi surrounding areolar clusters on the valve surface of Triceratium dubium [2.31]. Rimoportulae openings differentiate species, such that Triceratium dubium having an elongated tube opening with apical spinules is different from T. favus having a spatulate opening that is hemispherical internally [2.31].

Trigonium have pseudocelli, centrally located rimoportulae with oval openings, and honeycombed loculate areolae [2.143] in a radial arrangement [2.38, 2.143]. In Trigonium arcticum, areolae are covered by rota-type vela, and a straight cingular suture is present [2.38]. Toward the end of cribra formation, this structure finishes at valve surface level in Trigonium arcticum, and rotae within porelli of the pseudocelli are also being formed [2.143]. For Trigonium formosum, rimoportulae are centrally located and cribra are highly domed [2.143].