SPECIES INTRODUCTION IN FRESH-WATER ENVIR ONMENTS - GENETIC IMPACTS

Species introductions have three kinds of consequences that were discu ssed during the congress ''Species introductions'', Paris February 199 6 : consequences in ecology, pathology, and genetics. The latter only occurs when an introduced taxon hybridises with a resident taxon and i f their offspring (F1) is fertile. In this paper, the different types of interactions are classified and illustrated by examples that are no t limited to France. Type 0 = no interaction; type 1 = non-viable hybr ids; type 2 = production of sterile F1; type 3 = production of weakly fertile F1; type 4 = production of highly fertile F1 and easy backcros ses that can lead to the homogenisation of the two taxa. For the popul ation, the main genetic risk when hybridisation occurs is an outbreedi ng depression, which can be explained by the breakdown of coadapted ge ne complexes and/or the breakdown of local adaptation. For the taxon, the risk is a loss of genetic diversity at the population level and/or at the species (between-population) level. Altogether, genetic introg ression is considered nearly irreversible, and the final consequence c an be the extinction of the taxon from a genetic point of view. To avo id such a dramatic outcome, some precautions may be proposed: - severa l examples developed in the text clearly explain that the specific lev el is not always the good one for investigation, The subspecific level must often be taken into account. However, a good genetic knowledge i s necessary for handling that level of organisation because morphology is generally inefficient in distinguishing local forms. So a prelimin ary genetic description of the taxa to be introduced is encouraged; - when no genetic information on the species to be introduced is availab le, which is almost the general case, precaution must be accepted as a matter of principle. The principle especially adapted to freshwater o rganisms is to allow introductions, in the absence of genetic knowledg e, only if the introduced individuals are taken from the same catchmen t.