On the issue of the state of study of the <i>Pelophylax esculentus</i> complex (Berger, 1970) on the northern border of its range within the Upper Volga

This article analyzes the current state of knowledge of the complex of Central European green frogs in the Upper Volga basin. In addition, a summary of the results of herpetological studies, characteristics and distribution features of the Pelophylax esculentus complex population systems in the Volga region are presented. The batrachocomplexes of the Middle and Lower Volga regions turned out to be the most studied, while the northwestern and northern parts of the Volga basin remain poorly explored. The results of molecular genetic analysis of the verification of Pelophylax esculentus complex species in the Upper Volga region confirmed the presence of P. lessonae and P. ridibundus species. The presence of a cryptic species of marsh frog (P. cf. bedriagae) was also revealed on the southern coast of the Rybinsk Reservoir. This fact indicates the existence of population systems of RL-type green frogs in the studied territories and also expands the zone of sympatry and the probability of hybridization of the «western» and «eastern» forms of the lake frog in the Volga region to 58° N. The obtained data can serve as a basis for understanding the biological characteristics of the Pelophylax esculentus complex, as well as for deepen and detail the evolutionary and genetic processes in peripheral populations.