The fishy milkcaps (Lactifluus volemus sensu lato), cryptic species with a long and pandemic history
Fungal species have been mainly diagnosed by their morphological characteristics up to the middle of the 20th century. Morphology-based taxonomy, however, can lead to an unnatural classification for several reasons. As fungal morphology is relatively simple, few diagnostic characters can be used to identify species. As a consequence, differences between species are often subtle and may easily escape the notice of the observer, or can be regarded as intraspecific morphological variation. Second, speciation is simply not always accompanied by morphological changes.
Starting from the 1960s and 1970s, the application of DNA-based methods ushered in a new era of species delimitation. Numerous reproductively and genetically isolated clusters within many of the traditional morphological species were now revealed, and as surveys of DNA sequence variation are on the rise, so are the records of cryptic species (different species which are morphologically indistinguishable).
Lactifluus volemus (Fr.: Fr.) Kuntze is such a traditional morphological species. It can be easily recognized in the field by its dry, often minutely velvety, clay-buff to orange-brown, reddish-brown cap, fish-like smell, greenish reaction with FeSO4 and the extremely copious, white milk which stains surrounding tissues conspicuously brown. Key microscopical diagnostics are the conspicuously large, fusiform, very thick walled and abundant lamprocystidia, subglobose to globose reticulate ornamented spores, the lampropalisade structure of pilei- and stipitipellis and the presence of numerous rosettes of sphaerocytes in the trama of the lamellae. Nevertheless, substantial variation in colour, lamellae spacing and changing and staining of the latex has been observed. This variation has led to the description of different varieties to which, in general, no taxonomical importance seems to be attributed.
Since its description by Fries in 1821, the name has been applied for morphological look-alikes in Guatemala, Mexico, eastern North America and Asia, where it forms ectomycorrhiza in both deciduous and coniferous forests. International conspecificity has not been evaluated and L. volemus gained the reputation of being a widespread species.
Is Fries’ L. volemus indeed a single widespread species, or is it in fact a complex of species with limited distributions? Are these species cryptic, or can we identify them based on morphology? How are these species related to each other?
When using the right markers, DNA-based phylogeny can unmask look-a-like species. Moreover, it has the potential to reveal new species and to guide us towards a more accurate estimate of evolutionary history and biodiversity. We believe, however, that morphology-based taxonomy should not be abandoned. We therefore aim to pinpoint species boundaries based on DNA-sequences, but also to identify the morphological characters diagnosing those species.
So far, extensive sampling revealed the presence of at least 18 phylogenetic species in Northern Thailand (Van de Putte et al. 2010), 6 phylogenetic species in Sikkim Himalaya (India) (unpublished results), and three phylogenetic species in Europe (unpublished results). Six Thai species and three Sikkimese species could be morphologically identified and have been described as new species.
Van de Putte K., Nuytinck J., Stubbe D., Le H.T., Verbeken A. (2010) Lactarius volemus sensu lato (Russulales) from northern Thailand: morphological and phylogenetic species concepts explored. Fungal Diversity 45: 99-130.