Is Salix fursaevii Mavrodiev (Salicaceae) a palaeoendemic of the Volga River valley or an ecotype of S. triandra L.?

Abstract

Salix fursaevii, a species endemic to the Volga River flood plain, was described by E. Mavrodiev and co-authors in Byull. Moskovsk. Obshch. Isp. Prir. in 2012. It differs from the closely related and widespread S. triandra by a few quantitative morphological characters and, most of all by its much later flowering time. This late-flowering willow was known and studied before as an ecotype of S. triandra. Our study of a large sample of S. triandra s. l. revealed that 1) S. triandra s. str. and S. fursaevii could not be clearly distinguished using several morphological characters and the time of flowering; 2) specimens morphologically corresponding to S. fursaevii are with a single exception strictly confined to the Volga River flood plain; 3) the populations of S. triandra and S. fursaevii do not differ by nrITS sequences; 4) the populations of S. triandra and S. fursaevii are poorly differentiated by chloroplast atpB-rbcL sequences; 5) representatives of both taxa are tetraploids 2n = 4x = 76. Poor differentiation of populations by chloroplast sequences is indicative of the intraspecific variability, the geographic pattern of which suggests that the Volga River valley was a probable glacial refugium and a source of post glacial spread of S. triandra.