A combination of genetics with inter- and intra-strain crosses and RAPD-fingerprints reveals different population structures within the Paramecium aurelia species complex

Abstract

Summary A combination of classical methods with modern molecular biological techniques has been used to detect differences in geographical population structure between two species of the Paramecium aurelia complex. We combined interand intra-strain crosses via mating type reactions and random amplified polymorphic DNA-Polymerase chain reaction (RAPD-PCR). This RAPD-PCR method is quite favourable to identify sister species of the P. aurelia species complex and to detect different population specific genotypes within sister species. A comparison of the DNA fingerprints of several P. tri-aurelia strains as well as P. sexaurelia strains with the percentage of surviving hybrid strains of strain crosses in Fl and F2 generations revealed surprisingly results. A high percentage of offspring survive of Fl and F2 generations was typically observed in crosses with different P. triaurelia strains. The fingerprints distinguished three different genotypes within the investigated P. triaurelia strains. All three genotypes were distributed throughout the European regions where these strains were collected. P. sexaurelia, in contrast, showed low survival in crosses between regions and in some cases the stocks from different regions would not mate. Within P. sexaurelia four different fingerprint genotypes were found, but stocks from a single region had only a single genotype. We interpret these results as evidence for genetic gene flow and genetic isolation, respectively. The differences indicate two different life history strategies as a consequence of different degrees of inbreeding. P. triaurelia was confirmed as a moderate inbreeder and P. sexaurelia was discovered to be an extreme inbreeder, with breeding restricted to individuals belonging to the same genotype. We argue that not only different species complexes within the genus Paramecium have different life history and evolution strategies but remarkable ecogenetic differences are also true for sibling species within the same species complex.