Stephen J. M. Droop

The Sellaphora pupula species complex (Bacillariophyceae): morphometric analysis, ultrastructure and mating data provide evidence for five new species

Abstract Morphometric shape analysis and ultrastructural data are provided for six genodemes of the Sellaphora pupula species complex that have been studied during the last 20 years from Blackford Pond, Edinburgh, UK. The demes have previously been shown to be separated by prezygotic reproductive barriers: cells of different demes do not pair, though they may show residual interactions when sexualized. A new morphometric method, contour segment analysis, which was recently developed for diatoms in relation to automated identification, gives a clear separation of all six genodemes and indicates no heterogeneity within each. Legendre shape analysis gives less separation in this instance. All other available data, including molecular sequence data, also support separation of the six demes at species level. Five new species are therefore described: S. auldreekie, S. blackfordensis, S. capitata, S. lanceolata and S. obesa. The identity of S. pupula sensu stricto is clarified through designation of epitypes.

SEXUALITY, INCOMPATIBILITY, SIZE VARIATION, AND PREFERENTIAL POLYANDRY IN NATURAL POPULATIONS AND CLONES OF SELLAPHORA PUPULA (BACILLARIOPHYCEAE)

The capitate and rectangular demes of the freshwater epipelic diatom Sellaphora pupula (Kütz.) Mereschk. are dioecious, the first such report for any freshwater diatom. Sexual differentiation, which is probably determined genetically, involves recognition at the cell surface as well as differences in gamete behavior (one gametangium produces an active “male” gamete, the other a passive “female” gamete). In culture, successful sexual reproduction occurs only when compatible clones are mixed. All cells of a clone behave identically in interclonal crosses, being either male or female, regardless of the stage of the life cycle, in contrast to the sequential hermaphroditism of centric diatoms. Males and females have identical frustule morphology. As in other diatoms, there is an upper size threshold for sexual reproduction, below which cells become progressively easier to sexualize. In culture, sexual interactions occur in cells much smaller than those ever seen in natural populations, so that in nature the sexual size range is effectively open. Natural populations almost always contain sexualizable cells; often, most of the cells are below the upper sexual size threshold. Male gametangia are, on average, slightly larger than females in the capitate deme, which may be produced by preferential polyandry, depleting the population of males and making them younger at mating. Rarely, selfing occurs producing zygotes, but these abort before producing initial cells. The sizes of the gametangia and initial cells are correlated but this does not invalidate the use of “cardinal points” of the life cycle in taxonomy. No interbreeding occurs between the rectangular and capitate demes. However, when males of one deme are mixed with females of the other, there is a stimulation of activity, as during the early stages of pairing in compatible intrademic crosses.

Morphological variation in Diploneis smithii and D. fusca (Bacillariophyceae)

Summary Morphological variation within the Diploneis smithii — D. fusca complex has been examined critically in a single collection from Oban, Scotland. Eleven distinct sympatric morphotypes can be identified. Each exhibits a recognizable size-reduction sequence in which all critical characters are stable. Measurements of dimensions, striation density and rectangularity are combined with two other characters (shape of central raphe endings and the arrangement of pores on the surface of the valves) in statistical analyses (including Principal Components Analysis and Hierarchical Clustering Analysis) that demonstrate objectively the validity of the groupings. The implications of such discontinuities are discussed with respect to current knowledge of patterns of reproduction in diatoms, breeding barriers between similar races, and phenotypic plasticity. The morphotypes probably represent taxa with different ecological preferences, and should in due course be given taxonomic recognition to avoid loss of information. Distinct morphotypes similar to those in D. smithii — D. fusca occur within the species of other genera, and the phenomenon may be very widespread among diatoms.

Spatial and temporal stability of demes in Diploneis smithii/D. fusca (Bacillariophyta) supports a narrow species concept

Abstract An earlier study demonstrated the presence of 11 morphologically distinct demes (‘morphotypes’) within the Diploneis smithii/D. fusca species complex in a single sample from Ganavan near Oban on Scotlands west coast. Two of these demes were subsequently shown to occur elsewhere around the British coastline and to keep their identity wherever they occurred together. However, the conclusion that the demes were sufficiently distinct and widespread for them to be described as separate species has been criticized on the grounds that no evidence exists that there are reproductive barriers between them which could provide independent evidence for their relationships. It has not been possible, thus far, to grow the species in culture (a prerequisite for controlled breeding experiments), and so independent evidence has been sought elsewhere. It is shown here that nine of the 11 demes from Ganavan also occur in Cumbrae (in the Firth of Clyde, some 80 km SSE of Oban), not only in recent samples but also in a sample collected in 1858. A detailed morphometric and ultrastructural analysis shows that the demes are stable over 80 km and 140 years. This is discussed in the context of the length of diatom life cycles, and it is concluded that at least several generations of size reduction and restitution must have occurred since 1858. In most diatoms, size reduction and restitution accompany allogamous sexual reproduction, so it is inferred that the demes within the D. smithii/D. fusca complex are probably true sexual species.

Diatom identification: a double challenge called ADIAC

This paper introduces the project ADIAC (Automatic Diatom Identification and Classification), which started in May 1998 and which is financed by the European MAST (Marine Science and Technology) programme. The main goal is to develop algorithms for an automatic identification of diatoms using image information, both valve shape (contour) and ornamentation. The paper presents the goals of the project as well as first results on shape modeling and contour extraction. Public data are available in order to create student projects beyond the ADIAC partnership.

Digital microscopy in phycological research, with special reference to microalgae

Digital imaging technology is gradually being incorporated into all areas of biological research, but there is a distinct lack of information resources targeted at scientists in their specialist areas. There is a wealth of potential applications for digital images in phycology, including morphometric or visual analysis of specimens, taxonomic databases and publication of digital micrographs in lieu of photomicrographs. Here, we provide a review of digital imaging in general and its potential for the field of microalgal research in particular. We also present a number of imaging techniques that are critical for image acquisition and optimization, which can enable beginners to build their own libraries of high quality digital images. Resolution requirements of digital cameras are explained and related to microscope resolution. The benefits of digital imaging technology are discussed and contrasted with those of traditional silver halide technology.

Diatom contour analysis using morphological curvature scale spaces

A method for shape analysis of diatoms (single-cell algae with silica shells) based on extraction of features on the contour of the cells by multi-scale mathematical morphology is presented. After building a morphological contour curvature scale space, we present a method for extracting the most prominent features by unsupervised cluster analysis. The number of extracted features matches well with those found visually in 92% of the 350 diatom images examined.

A model of diatom shape and texture for analysis, synthesis and identification

We describe tools for automatic identification and classification of diatoms that compare photographs with other photographs and drawings, via a model. Identification of diatoms, i.e. assigning a new specimen to one of the known species, has applications in many disciplines, including ecology, palaeoecology and forensic science. The model we build represents life cycle and natural variation of both shape and texture over multiple diatom species, derived automatically from photographs and/or drawings. The model can be used to automatically produce drawings of diatoms at any stage of their life cycle development. Similar drawings are traditionally used for diatom identification, and encapsulate visually salient diatom features. In this article, we describe the methods used for analysis of photographs and drawings, present our model of diatom shape and texture variation, and finish with results of identification experiments using photographs and drawings as well as a detailed evaluation.

A taxonomic database and linked iconograph for diatoms

The Royal Botanic Garden Edinburgh and the Natural History Museum are collaborating to establish a database and linked iconograph, wherever possible using TDWG (Taxonomic Databases Working Group) standards, that can handle complex nomenclature; specimen information; specimen and population descriptions that can later be assembled automatically to generate taxon descriptions and also used as a basis for the automatic or semi-automatic generation of keys and on-line identification; original descriptions of taxa; bibliography; and images. The system is at present being developed using the PANDORA floristic database system (on an AREV base) and is being designed to be compatible with other database systems suitable for use in botanic gardens and herbaria.

New methods for preparing, imaging and typifying desmids (Chlorophyta, Zygnematophyceae), including extended depth of focus and 3-D reconstruction

D.G. Mann, M.M. Bayer, S.J.M. Droop, Y.A. (Julia) Hicks, A.D. Marshall, R.R. Martin and P.L. Rosin. 2007. New methods for preparing, imaging and typifying desmids (Chlorophyta, Zygnematophyceae), including extended depth of focus and 3-D reconstruction. Phycologia 46: 29–45. DOI: 10.2216/04-60.1 Species- and genus-level taxonomy of desmids depends largely on shape and detail of the cell wall and chloroplast morphology. The depth of most desmid semicells, relative to the focal depth of conventional light microscopes, means that morphological characteristics are usually illustrated by drawings, made from material that is mounted in water to allow reorientation of specimens to different aspects of shape and pattern. Though a productive approach for two centuries, this has the disadvantages that features not initially detected or thought irrelevant are not recorded, drawing quality is variable, and individual specimens are rarely retained for further study. We describe methods for making permanent preparations of desmid cell walls and using these to produce extended depth of focus summary images and three-dimensional (3-D) reconstructions. Together with World-Wide Web dissemination of image stacks, these advances make it practical to make a desirable change from typification via drawings to typification via single or multiple preserved specimens. They will also facilitate standardization of taxon concepts and identification.