Preliminary data on the genetic characterization of twaite shad (Alosa fallax rhodanensis Roule, 1924) population in the river Rhone.

Two species of the genus Alosa (A. alosa Linne, 1758 -allis shad- and A. fa llax Lacepede, 1803 -twaite shad-) colonized the East Atlantic and the Medi terranean Sea. Their systematics is still confused without genetic characte rization. Indeed, six subspecies of A. fallax are actually identified only on the basis of morphometric and meristic characters. One of these subspeci es (A. fallax rhodanensis Roule, 1924) is endemic of the Rhone River. Altho ugh this taxon has the same morphometric and meristic characters as A. fall ax, it is closer to A. alosa for the size and the ecological features. So a genetic study, with electrophoretic analysis of liver (electrophoresis in starch gel) and blood (isoelectric focusing in polyacrylamid gel) proteins, has been carried out to elucidate the taxonomic status of this population. We used two of the most discriminating loci between the two Atlantic shad s pecies : MPI (Mannose Phospho Isomerase) and HBA (alpha haemoglobin chain). Allelic frequency puts in conspicuous the appartenance of A. fallax rhodan ensis to the taxon A. fallax. Then, we used a locus known to discriminate t he populations of A. fallax in Atlantic (ADH Alcohol Deshydrogenase). Throu gh ADH allelic frequency, twaite shad from the Rhone River is close to popu lations of the north of Portugal. Results are discussed according to recent genetic studies made on Atlantic shads. They showed that further information had to be regarded on the genet ic characteristics of this allis shad in relation to its particular status.