Differentiation within and between river basins of Podostemum irgangii (Podostemaceae), a rapid-water macrophyte of southern Brazil

► High levels of genetic diversity between and within-populations were observed in Podostemum irgangii. ► High within-population genetic diversity suggests that sexual reproduction is common. ► Genetic differentiation exists even between populations located within same river. ► The pervasive genetic differentiation at local and regional scales likely represent failure or diminution of gene-flow mechanisms.