Abstract
The biological species concept defines species based on sexual incompatibility between strains and the F1 sub-viability or inviability. However, to date, there is only a limited number of studies that formally deal with sexual incompatibility in unicellular protists and hence the rigorous application of biological species concept is fuzzy in these organisms. Here, we investigated interbreeding between two species of the planktonic pennate diatom Pseudo-nitzschia, P. arenysensis and P. pseudodelicatissima. We observed hybridization between these two species in controlled laboratory condition. The F1 generation showed: i) low viability; ii) morphological, ultrastructural, and morphometric features that resembled those of one of the parental strains (P. pseudodelicatissima); iii) intermediate maximum cell size to the one observed in intraspecific sexual crosses in the parental species. Our results may suggest that interbreeding between these two species is possible although likely rare. We invite a larger body of experimental evidence in unicellular protists to assess the applicability of the biological species concept to these organisms.