sect. : new boundaries, extended biogeography and unexpected diversity unravelled by transatlantic studies.

As currently delineated, sect. is a polyphyletic assembly within subg. , encompassing glutinous and pigmented taxa. According to available literature, between a dozen and twenty species may belong in the section, mostly represented in continental and boreal forests of Europe and North America. However, the limited phylogenetic and biogeographic coverage of the genus does not presently allow for a reliable assessment of its taxonomic boundaries, nor does it provide a complete picture of species diversity within sect. . In an ongoing effort to confer an evolutionary backbone to systematics, we assembled and analysed a dataset comprising 268 intercontinental sequences, including holotypes of 7 taxa previously not positioned phylogenetically, and enriched with collections from largely unexplored Mediterranean and Anatolian ecosystems. Overall, 30 clades are identified within 5 distinct lineages, including 11 species putatively new to science. Seven of these are formally described here as , , , , , and spp. nov. This enriched coverage of section s.lat. calls for a re-evaluation of its natural boundaries into a core monophyletic clade, including and five closely related lookalikes, as well as the assignment of the section rank to the four remaining lineages: sect. sect. nov., sect. sect. nov., sect. sect. nov. and sect. , respectively. We also stabilize the usage of six historical names, , , , , and , through designation of two neotypes, three lectotypes and four epitypes. : Bellanger J-M, Lebeuf R, Sesli E, et al. 2021. Hygrophorus sect. Olivaceoumbrini: new boundaries, extended biogeography and unexpected diversity unravelled by transatlantic studies. Persoonia 46: 272-312. https://doi.org/10.3767/persoonia.2021.46.10.