A new concept for Dictyostelium sphaerocephalum based on morphology and phylogenetic analysis of nuclear ribosomal internal transcribed spacer region sequences

Three dictyostelid isolates were found in Spain and Argentina that are morphologically different from known species. These isolates have some features similar to Dictyostelium sphaerocephalum (Oudem.) Sacc., Marchal & É.J. Marchal, but differ in size and sorocarp branching pattern. We sequenced the nuclear ribosomal internal transcribed spacer region to explore phylogenetic relationships among this group of species, including the three new isolates and their closest relatives. In all phylogenetic analyses performed, sequences of all three isolates group together with sequences from ‘‘typical’’ D. sphaerocephalum samples. This result supports previous observations of the morphological plasticity in dictyostelids, especially D. sphaerocephalum, leading us to broaden the classical concept of this species.