An insight into the cryptic diversity of Fragilariaceae (Bacillariophyta), with the description of a new Antarctic species, <i>Gedaniella antarctica</i> sp. nov.

Fragilariaceae is a paraphyletic family of araphid diatoms commonly used as bio-indicators, in environmental assessments and in paleoenvironmental reconstructions, and with various potential industrial applications. Recent molecular taxonomic research has highlighted significant limitations in traditional morphology-based investigations of these diatom species. Most descriptions of species and genera of the Fragilariaceae present broad morphological character definitions and many diagnostic characters are indistinguishable by light microscopy. In this sense, taxon misidentification is common and could have serious implications for environmental surveys and laboratory experiments. To better understand the diversity of the Fragilariaceae, we (1) performed phylogenetic analyses of the small subunit ribosomal RNA (18S rRNA), <i>rbc</i>L and <i>psb</i>C gene sequences, (2) inferred a cladogram from key morphological features used in the traditional identification of Fragilariaceae, (3) tested if the topologies of the tree recovered from the molecular phylogeny, of the tree based on morphology and of the trees constraining to monophyly the genera <i>Sarcophagodes, Pseudostaurosira</i> and <i>Nanofrustulum</i> (together and separately) were significantly different and (4) mapped morphological character states on the ML tree and inferred their evolution based on maximum parsimony. Our results supported the monophyly of a group of Fragilariaceae within small araphid diatoms including the genera <i>Cratericulifera, Plagiostriata, Castoridens, Opephora, Staurosira, Staurosirella, Punctastriata, Psammotaenia,</i> <i>Hendeyella, Stauroforma, Pseudostaurosira sensu Li</i>, <i>Nanofrustulum</i> sensu Li, <i>Serratifera</i> sensu Li and <i>Gedaniella</i> sensu Li. Molecular phylogeny and topology tests suggested that the latest circumscriptions of the genera <i>Sarcophagodes, Pseudostaurosira</i> and <i>Nanofrustulum</i> sensu Morales were not monophyletic. Analyses of the Antarctic strain IMA070A collected during the XXXIV Italian Antarctic Expedition using fine structural features of the frustule and molecular data revealed that this diatom belongs to a distinct lineage within <i>Gedaniella</i>, which we describe here as <i>Gedaniella antarctica</i> sp. nov. Fragilariaceae diversity was revealed combining molecular data and morphology.Fragilariaceae displayed cryptic diversity.<i>Gedaniella antarctica</i> represent a new araphid diatom from Antarctica. Fragilariaceae diversity was revealed combining molecular data and morphology. Fragilariaceae displayed cryptic diversity. <i>Gedaniella antarctica</i> represent a new araphid diatom from Antarctica.