Diversity and phylogeny of Lactarius subgenus Russularia in South-East Asia
The genus Lactarius Pers. is one of the major groups of macrofungi, with more than 500 species actually known, but the real number estimated to be higher than 650. It is well-recognized by the presence of latex. Since 2008 the genus is subdivided into three monophyletic genera: Lactifluus, Lactarius and Multifurca (Buyck et al. 2008). The group fulfills an import economical (many species are edible, Boa 2004) and ecological role. Forming ectomycorrhizae, it is one of the dominant genera in many ecosystems (e.g. temperate and boreal forests, subtropical miomboforest and tropical rainforest, Rivière et al. 2007) and especially also in South-East Asia, where milkcaps are dominant in many vegetation types (moist evergreen forest, deciduous dipterocarp-oak forest, pine forest, mixed deciduous forest) growing with mainly Fagaceae (Castanopsis, Nothofagus, Lithocarpus), Dicterocarpaceae (Dipterocarpus, Shorea) and Pinaceae.
After splitting the original genus Lactarius, Lactarius sensu novo remains the largest group. It contains three large subgenera: L. subg. Piperites, L. subg. Russularia and L. subg. Plinthogali. The second largest genus, Lactifluus, consists of about 25% of the former Lactarius species in a higher number (at least 6!) of very divergent subgenera.
Interestingly, we have a smaller genus Lactifluus with a main distribution in the Southern hemisphere, with large phylogenetic distances between the subgroups but where, as to species level, the molecular variation seems to be higher than the morphological one, thus leaving us with some species complexes (or complex species?). On the other hand, we have the large genus Lactarius with a lower genetic variability, but where we have species that seem morphologically well-recognizable. Species delimitation and relationships between species have in most groups not been tested using molecular data. This is the case for several representatives of Lactarius subgenus Russularia. Research based on European material suggests that the morphologically recognized species are not always supported by molecular data.
L. subgenus Russularia is characterized by dry to sticky basidiocarps, with often orange to brownish or reddish colours. The milk is white or watery and sometimes changing to yellow. There is a large variation in pileipellis structures which makes this a very important microscopical feature. The same is true for the spore ornamentation.
The subgenus shows a remarkable distribution: dominant in all temperate zones, completely absent in tropical Africa, poorly represented in South America but with an important distribution in South-East Asia. 35 species have been described from South-East Asia so far (mainly from China, Papua New Guinea, India and Japan). Many of these species are only known from the type locality, since the area is seriously underexplored.
A relatively well-explored area is Northern Thailand where a recent revision of Lactarius has been made (Le 2007, Le et al. 2007a, b, c). In this study, subgenus Russularia is represented by 11 species (after the main subgenus Piperites, with 12 species) of which only 2 species have a name, 2 are attributed a provisional name, and the others remain undescribed.
We could state that the group illustrates a well-known and general fact in mycology: the knowledge of the biodiversity is extremely low – an estimated 1.5 million species present of which only 100.000 actually known (Hawksworth 1991).
The occurrence of intercontinental conspecificity in many genera of ectomycorrhizal macrofungi, but especially in Lactarius, remains a big question mark and there are still too few mycological studies tackling worldwide taxonomical problems. In L. subgenus Russularia, L. obscuratus and L. tabidus were first described from Europe where they are common species, but these names have been used for records in e.g. Thailand, China and India without molecular confirmation of the supposed conspecificity.
- How big is the biodiversity of L. subgenus Russularia in South-East Asia? How many taxa can be distinguished?
- Is it possible to distinguish the taxa in the field and develop user-friendly identification tools that can be used by mycologists and ecologists in South-East Asia?
- Do Russularia species that are known from temperate regions also occur in South-East Asia or is the resemblance only superficial?
- Can the hypothesis that Lactarius has higher morphological variation than molecular variation (as opposed to Lactifluus) be confirmed? Studying subgenus Russularia in a tropical region is an ideal opportunity to test this, since we already have European data that seem to confirm this hypothesis.
- Do cryptic species occur here or are species sufficiently different macro- and microscopically to be identified? Some European Russularia species show rather variable macromorphological and micromorpholgical characters and are considered to be species-complexes. This would be a first tropical case study on that matter.
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