  In this paper, the authors report the results of an experimental study of
effective mass, electron mobility and phase shift of Shubnikov de Haas
oscillations of transverse magnetoresistance in an extended electron
concentration region from 8.8*10^15 cm^-3 to 4.3*10^18 cm^-3 in single crystals
of mercury selenide. The revealed features confirm the existence of a Weyl
semimetal phase in HgSe at low electron density, which has been indicated by
previous magnetotransport studies. However, the most significant result is the
discovery of an abrupt change of Berry phase of Pi at electron concentration
2*10^18 cm^-3, which we explain in terms of a manifestation of topological
Lifshitz transition in HgSe that occurs by tuning Fermi energy via doping.
