Introduction
Russulaceae is one of the largest families of ectomycorrhizal fungi that plays a dominant role in many vegetations worldwide. The core of this family consists of two genera, Lactarius and Russula, both described from Europe two centuries ago as clearly recognizable agaricoid genera. However, recent research (Buyck et al., 2008) demonstrated that the traditional classification of Russulaceae into these two genera is not supported. Instead, the family consists of four mainly agaricoid genera and includes some corticioid species. In one of these genera (Multifurca), some rare representatives of the former Lactarius and Russula are united. The rest of Russula is supported as a monophyletic group. Lactarius, notwithstanding its striking feature of exuding milk or latex (thus: the milkcaps) which always seemed a good synapomorphic character, is paraphyletic and is split in two genera: Lactarius and Lactifluus.
Based on morphological and molecular characteristics, Lactifluus seems the most variable out of the four Russulaceae genera. Lactifluus mainly consists of tropical species and recent studies discovered large cryptic diversity within the genus (Stubbe et al., 2010; Van de Putte et al., 2010). Morphologically, this genus is characterized by the complete absence of zonate and viscose to glutinose caps, and it contains many species with veiled and velvety caps and all known annulate species (Buyck et al., 2008). Lactarius on the other hand, has a more temperate distribution, with many cold-adapted species and contains the typical zonate and viscid to glutinose milkcaps. Our present knowledge results in a large genus Lactarius with about 75% of the milkcaps and a smaller genus Lactifluus, but the real number of species in Lactifluus might be higher due to cryptic diversity, which seems to be absent in Lactarius. Our hypothesis is that Lactifluus evolved genetically faster but is morphologically more stable, while morphological variation is more common in the genetically slower evolving genus Lactarius. The speciation-extinction dynamics of both genera are clearly different. These discrepancies have strongly influenced the evolutionary history of both genera.
Objectives and approach
In current phylogenies, the genus Lactifluus is strongly underrepresented. We want to meet this deficit by collecting more species of this genus. Recent biodiversity studies and expeditions to unexplored areas provided surprising new insights into the phylogeny of Lactifluus (e.g. Van de Putte et al., 2009). Previous research indicated that many valuable sites are highly endangered remnants of original forest. Some of these sites are protected and studied by the Center for Tropical Forest Science (CTFS, part of the Smithsonian Tropical Research Institute). One important benefit of CTFS woodlands is that all tree species are inventoried, so the ectomycorrhizal trees are known, which makes targeted searches possible. We plan expeditions to such selected areas, for searching phylogenetic relevant material (missing links) and to contribute to an inventory of the biodiversity. This might also contribute to the protection of these relict areas, as prioritizing habitats for conservation often relies on estimation of species richness and endemism (Bickford et al., 2007).
A phylogeny of Russulaceae, focusing strongly on Lactifluus, will be constructed based on the following nuclear and mitochondrial genes: nLSU (nuclear ribosomal large subunit 28S region), tef1 (codes for the translation elongation factor 1α), rpb1 and rpb2 (code for the largest and second largest subunit of the RNA polymerase II gene) and mSSU (mitochondrial ribosomal small subunit 16S region).
- Constructing a comprehensive phylogeny is a first step towards obtaining insight into any existing phylogenetic predispositions to certain morphological and ecological character states within these genera. In this second step we will use phylogenies to address general trends and broad historical patterns in the evolution of Russulaceae. We will collect and integrate existing data concerning distribution, morphology and ecology of the different species. Subsequently, computational techniques will be used to study the correlation of the collected data and to compare the evolution of the Lactarius and Lactifluus diversity through time. These phylogenetic comparative analyses will be carried out with the use of stochastic mapping techniques, applying Bayesian approaches. First of all we will check if, besides the L. gerardii and L. volemus species complexes, L. piperatus and L. gymnocarpoides are also cryptic species complexes with a large genetic diversity, and whether cryptic speciation is thus a widespread phenomenon in Lactifluus. The diversification consequences of particular character states will be explored by testing key-innovative hypotheses, in which stochastic models of character evolution and cladogenesis are employed, integrated into a Bayesian framework in which uncertainty in historical inferences (such as phylogenetic relationships) is allowed (Ree, 2005). With these tests we can investigate whether or not a morphological novelty or a switch in host plant family results in a different rate of speciation. We will assess directionality in character state transformations using ancestral state reconstruction methods, and we will investigate the possible correlation of characters (Schmitt et al., 2009). The outcome of these techniques can provide an idea on how the ancestor of Lactarius and Lactifluus may have looked like and how their morphological and ecological characters have evolved. Russulaceae is an ideal model group to apply these new techniques to, because these techniques require comprehensive phylogenies, which will be available for the four genera (Lactarius and Lactifluus in Ghent, Russula and Multifurca in Paris). Furthermore it is a group which apparently unites genera with a very different evolutionary history, a world-wide distribution, host plant specialists and generalists.
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