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

A combination of classical methods with modern molecular biological techniq ues has been used to detect differences in geographical population structur e between two species of the Paramecium aurelia complex. We combined inter- and intra-strain crosses via mating type reactions and random amplified pol ymorphic DNA-Polymerase chain reaction (RAPD-PCR). This RAPD-PCR method is quite favourable to identify sister species of the P. aurelia species ;comp lex and to detect different population specific genotypes within sister spe cies. A comparison of the DNA fingerprints of several P. triaurelia strains as we ll as P. sexaurelia strains with the percentage of surviving hybrid strains of strain crosses in F1 and F2 generations revealed surprisingly results. A high percentage of offspring survive of F1 and F2 generations was typical ly observed in crosses with different P. triaurelia strains. The fingerprin ts distinguished three different genotypes within the investigated P. triau relia strains. All three genotypes were distributed throughout the European regions where these strains were collected. P. sexaurelia, in contrast, sh owed low survival in crosses between regions and in some cases the stocks f rom different regions would not mate. Within P. sexaurelia four different f ingerprint genotypes were found, but stocks from a single region had only a single genotype. We interpret these results as evidence for genetic gene f low and genetic isolation, respectively. The differences indicate two diffe rent life history strategies as a consequence of different degrees of inbre eding. P. triaurelia was confirmed as a moderate inbreeder and P. sexaureli a was discovered to be an extreme inbreeder, with breeding restricted to in dividuals belonging to the same genotype. We argue that not only different species complexes within the genus Paramecium have different life history a nd evolution strategies but remarkable ecogenetic differences are also true for sibling species within the same species complex.