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More than 300 strains of Frustulia rhomboides sensu lato were isolated from populations across Europe to estimate its molecular diversity and geographic distribution in oligotrophic freshwater habitats. Phylogenetic analyses, based on partial large subunit (LSU) rDNA sequences, revealed six lineages that did not fully correspond with the separation of F. rhomboides into traditional varieties. Although four closely related lineages showed substantial overlaps in morphology, the characteristic morphological features could be determined using morphometric techniques. The phylogenetic lineages of F. rhomboides showed different patterns of distribution among regions. Sequence frequencies within samples from different habitats indicated that distribution of the common lineages most likely reflected their different ecological preferences rather than dispersal constraints.

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During studies of the diversity of the diatom genus Frustulia in Europe and New Zealand we recognized two species that did not fit any existing description. These species are described in this paper as F. curvata sp. nov. and F. paulii sp. nov. based on examinations of the morphology of natural populations and isolated strains. A combination of morphological features enables discrimination of F. paulii and F. curvata from previously described taxa. Frustulia curvata was found at a single locality in Ireland and is probably rare. Frustulia paulii has been identified from wetland pools in both the South and North Islands of New Zealand. The two new species are compared with similar taxa and our current knowledge of their ecology and distribution is discussed. These formal descriptions complement the results of a separate study of the molecular diversity of the genus Frustulia, which confirmed distinct lineages for both species. GenBank entries for published sequences are included in the descriptions. In the future, both frustule morphology and DNA sequences can be used to test presence of these species elsewhere in the world and examine their ecology and distribution in detail.

Irregularities in cell division can produce asymmetry in symmetric structures, such as outlines of diatom cells, which can reflect genetic, environmental, or random variability in developmental processes. This study examined 12 phylogenetic lineages of the diatom genus Frustulia using landmark-based geometric morphometrics to assess the variation between cell segments separated by apical and transapical axes. Although asymmetric variation within cells differed in some lineages, these irregularities most likely did not reflect the evolutionary history of the lineages. The intraclonal phenotypic plasticity of diatom frustules was induced rather by nongenetic factors, i.e., inherited valve abnormalities, constraints of siliceous cell walls, and random developmental instability during morphogenesis. The positive correlations between the symmetric and asymmetric components of shape variation indicated that the morphogenesis of diatom cell walls affected irregularities within cells and variability among the symmetrized cells to a similar extent.

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