Reproductive Compatibility of European Populations of Two Species of Ulnaria (Bacillariophyta)

— Clones of two morphotypes corresponding to two species of pennate araphid diatoms, Ulnaria ulna and U. acus , were isolated from samples taken in Europe. Clonal cultures entered heterothallic sexual reproduction in combinations appropriate to their mating types. Following the concept of biological species, we tested reproductive compatibility between all representatives of geographically distant populations. Isolated clones proved to be reproductively compatible within the limits of each of the species; the progeny (generation F1) were viable and fertile and freely entered into backcrossing. No cases of interspecies hybridization were found. The absence of intraspecific reproductive isolation between distant populations in these two species suggests that the populations of both U. ulna and U. acus are conspecific.