Implications of absence of seawater-type mitochondria-rich cells and results of molecular analyses for derivation of the non-parasitic Ukrainian brook lamprey Eudontomyzon mariae

The Ukrainian brook lamprey Eudontomyzon mariae is the most widespread lamprey species in eastern Europe. Although E. mariae is generally considered a derivative of Eudontomyzon danfordi, an exclusively freshwater parasitic species, it has alternatively been suggested that it was recently derived from a now extinct anadromous Black Sea ancestor. Several non-parasitic lampreys and the landlocked sea lamprey, which have recently evolved from anadromous ancestors, still develop a seawater-type mitochondria-rich cell (SW-MRC) in their gills. In contrast, this cell type is not present in the gills of either Lampetra aepyptera, a non-parasitic lamprey of ancient origin, or the parasitic Ichthyomyzon unicuspis and I. castaneus that likewise have long evolutionary histories in fresh water. Eudontomyzon mariae from the Vistula River in the Baltic River basin does not possess SW-MRC, which is inconsistent with a recent origin from an anadromous ancestor. Mitochondrial DNA sequence data were thus used to infer the relationship between different populations of E. mariae and E. danfordi, and to reconstruct the transition from anadromy to freshwater residency. The results suggest that E. mariae evolved independently in the Baltic, Black, and Caspian Sea basins, and not recently from an anadromous ancestor. Although E. mariae in the Danube River may have arisen relatively recently from E. danfordi (differing by 0.7-1.1% in cytochrome b gene sequence), other E. mariae populations (including in the Vistula River) are genetically closer (0.6%) to the hypothetical ancestor of both E. mariae and E. danfordi. That ancestor was probably a freshwater resident, since SW-MRCs are not rapidly lost following confinement in fresh water.