Abstract
Background and aims – Mastogloia grevillei has been shown to be a polymorphic diatom species, producing frustules with morphology of Mastogloia grevillei, frustules with morphology of M. danseyi, and more rarely, heteromorphic or Janus cells with one valve of each morphology.
 Methods – We investigated a wetland population from Iowa (USA) known to produce heteromorphic valves and the type material of Mastogloia grevillei and M. danseyi to clarify the nomenclature of this taxon.
 Key results – The polymorphic shift in stria construction and density between Mastogloia grevillei and M. danseyi occurs in populations sampled decades apart, among widely separated populations, within single genotypes, and independent of sexual reproduction. Combining our observations with observations of type material for Mastogloia grevillei and M. danseyi we propose that Mastogloia danseyi f. grevillei stat. nov. be recognized as an ecophenotype of the nominate Mastogloia danseyi, as the latter taxon has nomenclatural priority. We also provide lectotypes for both taxa.
 Conclusions – Variability in stria structure and density between the two taxa is discontinuous and represents a probable polyphenism for diatoms that is likely triggered by changing total dissolved solids, conductivity, and/or solutes.
Highlights
Diatom taxa were once perceived to have constant morphologies within and among populations, environments, and over time; polymorphism and significant phenotypic variation has long been recognized in centric and pennate diatom species
While it is readily recognized that there is phenotypic variation within species, within and among populations based on environmental sampling, and within a single genotype based on laboratory results, much remains to be understood on how widespread polymorphism is, what the effects and causes of polymorphism are, and how this information can be leveraged for systematics, taxonomy, and ecological and palaeoecological assessment (Cox 2011, 2014)
Valves with the M. grevillei (n = 30; table 1) morphology were linear-lanceolate to oval, 29–57 μm long, 9–10.5 μm wide, had biseriate radiate striae at 9–10 per 10 μm that became parallel at the valve ends, 10–19 (6–7 in 10 μm) locules on each side of the partectum, a round to rhombic central area, and an undulate filiform raphe
Summary
Diatom taxa were once perceived to have constant morphologies within and among populations, environments, and over time; polymorphism and significant phenotypic variation has long been recognized in centric and pennate diatom species. Morphological variation in diatoms is, first, an expression of normal size diminution and life history (English & Potapova 2012, Kaczmarska et al 2013), but is evident as phenotypic plasticity in response to environmental stimuli noted through ecological observation, fossil sequences, and laboratory experiments (Belcher et al 1966, Stoermer et al 1989, Teubner 1995, Trobajo et al 2011). While it is readily recognized that there is phenotypic variation within species, within and among populations based on environmental sampling, and within a single genotype based on laboratory results, much remains to be understood on how widespread polymorphism is, what the effects and causes of polymorphism are, and how this information can be leveraged for systematics, taxonomy, and ecological and palaeoecological assessment (Cox 2011, 2014). Reaction norms (Stearns 1989) represent continuous variation in phenotypic characters, e.g. stria density encompasses a small range that may change
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