The results of investigation of sexual reproduction and life cycles of two new for the flora of Ukraine and two rare species of green flagellate algae are presented. Sexual reproduction of Chlamydomonas sphagnicola (F.E. Fritsch) F.E. Fritsch et Takeda is found for the first time. The details of structure of some stages of life cycle and behavior of sexual reproduction of Ch. suboogama Tscherm.-Woess, Ch. coccifera Gorozh. and Oogamochlamys gigantea (O. Dill) Pröschold et al. are specified. The data of morphology, ecology, geographical distribution of these species are summarized and completed; original drawings and micrographs are provided as well. The materials concerned taxonomy of studied species in the light of peculiarities of their sexual reproduction as well as data of molecular phylogeny are discussed.

Keywords: sexual reproduction, green algae, Chlamydomonas, Oogamochlamys, Volvocales, new for the flora of Ukraine species, rare species, taxonomical significance of characters

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