Lactarius volemus sensu lato (Russulales) from northern Thailand: morphological and phylogenetic species concepts explored

Lactarius volemus (Fr.: Fr.) Fr. is a well known and morphologically easily recognizable milkcap of the Northern hemisphere, forming ectomycorrhiza with both deciduous and coniferous trees. It was originally described from Europe, but is also reported in other continents. Although it is characterized by several unique macro- and micromorphological features, substantial variation in colour, lamellae spacing and changing and staining of the latex has been recorded and it is therefore considered as a putatively unresolved species complex. This study explores the concordance between morphological and phylogenetic species concepts within L. volemus sensu lato of northern Thailand, combining a critical morphological scrutiny with a multiple gene genealogy based on LSU, ITS and rpb2 nuclear sequences. Twelve strongly supported monophyletic clades and six terminal branches are discernable in all phylogenetic trees and represent 18 phylogenetic species. Six of the monophyletic clades can be morphologically distinguished and are described as new species: L. acicularis, L. crocatus, L. distantifolius, L. longipilus, L. pinguis and L. vitellinus. Five other clades also show some morphological differences, but these are too subtle and do not allow for a clear-cut species delimitation without the corroboration of molecular data. Lactarius volemus sensu lato of northern Thailand is therefore still considered as a partially cryptic species complex. Pleurolamprocystidia, pileipellis hairs and to a lesser degree also pileus colour are important diagnostic characteristics. Spore morphology, latex discoloration and pileus surface texture are less useful as diagnostic features. Whether this rich diversity is the result of in situ Pleistocene survival or post-glacial expansion and subsequent radiation, has yet to be revealed.