Infrageneric relations of the genetically diverse milkcap genus Lactifluus (Russulales, Basidiomycota) are poorly known. Currently used classification systems still largely reflect the traditional, mainly morphological, characters used for infrageneric delimitations of milkcaps. Increased sampling, combined with small-scale molecular studies, show that this genus is underexplored and in need of revision. For this study, we assembled an extensive dataset of the genus Lactifluus, comprising 80 % of all known species and 30 % of the type collections. To unravel the infrageneric relationships within this genus, we combined a multi-gene molecular phylogeny, based on nuclear ITS, LSU, RPB2 and RPB1, with a morphological study, focussing on five important characteristics (fruit body type, presence of a secondary velum, colour reaction of the latex/context, pileipellis type and presence of true cystidia). Lactifluus comprises four supported subgenera, each containing several supported clades. With extensive sampling, ten new clades and at least 17 new species were discovered, which highlight the high diversity in this genus. The traditional infrageneric classification is only partly maintained and nomenclatural changes are proposed. Our morphological study shows that the five featured characteristics are important at different evolutionary levels, but further characteristics need to be studied to find morphological support for each clade. This study paves the way for a more detailed investigation of biogeographical history and character evolution within Lactifluus.
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
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