Russulaceae of the Pakaraima Mountains of Guyana. IV. New species forming a distinct lineage of Lactarius subg. Plinthogalus .

The Neotropics have recently emerged as an important region for studies of tropical ectomycorrhizal (ECM) fungi. Specific neotropical areas with high ECM host tree densities have ECM fungal diversities rivaling those of higher-latitude forests. Some forests of the Guiana Shield are dominated by endemic ECM trees of the Fabaceae, including species of Dicymbe (subfam. Detarioideae), Aldina (subfam. Papilionoideae), and Pakaraimaea (Cistaceae). One of the most species-rich ECM fungal families present in each of these systems is Russulaceae. Long-term sampling in forests in Guyana's Pakaraima Mountains has revealed a number of species of the Russulaceae genera Lactarius, Lactifluus , and Russula . In this study, we document a previously unknown, distinct lineage of Lactarius subg. Plinthogalus containing eight species from the Guiana Shield. Here, we describe five of these species from Guyana as new to science: Lactarius humiphilus, Lactarius mycenoides, Lactarius guyanensis, Lactarius dicymbophilus , and Lactarius aurantiolamellatus . Morphological descriptions, habit, habitat, and known distribution are provided for each new species. Sequence data for the barcode internal transcribed spacer (ITS) locus are provided for types and most other collections of the new taxa, and a molecular phylogenetic analysis based on the ITS, 28S, and RPB2 (second-largest subunit of the RNA polymerase II) loci across the genus Lactarius corroborates their morphology-based infrageneric placement. The discovery of this lineage changes our insights into the biogeography and evolutionary history of Lactarius subg. Plinthogalus .