Defining the core group of the genus <i>Gomphonema</i> Ehrenberg with molecular and morphological methods

In order to understand the taxonomy and phylogeny of the genus <i>Gomphonema</i>, the type of the name of the genus, <i>G. acuminatum</i>, and related species complexes are studied with an integrative taxonomic approach. Sixty-seven unialgal <i>Gomphonema</i> strains were established from 34 different water bodies in Croatia, Faroe Islands, Finland, Iceland, Germany, Mexico, Spain, Switzerland and Korea, which provided molecular (18S and <i>rbc</i>L), morphometric and ultrastructural data. Our study was complemented by molecular data from INSDC, summing up the studied data to 82 freshwater <i>Gomphonema</i> strains from the <i>G. acuminatum</i> and related species complexes which are here defined as the <i>Gomphonema</i> core group. Except for outline variations which occur even within one clone, their micro-morphologies are indistinguishable. The molecular data of the two markers clustered the taxa of the core group into one clade, well differentiated from other <i>Gomphonema</i> taxa such as <i>G. augur</i> and <i>G. parvulum</i>. For differentiation within the clade, the plastid marker <i>rbc</i>L provided better species resolution comparable to traditionally recognized taxa. The combined analysis of the molecular as well as morphological data resulted in the defining of only 7 subclades in contrast to 20 currently assigned names for the taxa of the <i>Gomphonema</i> core group in this study. The <i>Gomphonema</i> core group has been overdescribed due to the use of outline as the main criterion for species delimitation. A polyphasic approach, combining molecular and micromorphological data for taxonomy, nomenclatural evaluation, and observations from clonal cultures can reveal the full intricacies of evolutionary relations.