Abstract
BackgroundCortinarius species in section Calochroi display local, clinal and circumboreal patterns of distribution across the Northern Hemisphere where these ectomycorrhizal fungi occur with host trees throughout their geographical range within a continent, or have disjunct intercontinental distributions, the origins of which are not understood. We inferred evolutionary histories of four species, 1) C. arcuatorum, 2) C. aureofulvus, 3) C. elegantior and 4) C. napus, from populations distributed throughout the Old World, and portions of the New World (Central- and North America) based on genetic variation of 154 haplotype internal transcribed spacer (ITS) sequences from 83 population samples. By describing the population structure of these species across their geographical distribution, we attempt to identify their historical migration and patterns of diversification.ResultsModels of population structure from nested clade, demographic and coalescent-based analyses revealed genetically differentiated and geographically structured haplotypes in C. arcuatorum and C. elegantior, while C. aureofulvus showed considerably less population structure and C. napus lacked sufficient genetic differentiation to resolve any population structure. Disjunct populations within C. arcuatorum, C. aureofulvus and C. elegantior show little or no morphological differentiation, whereas in C. napus there is a high level of homoplasy and phenotypic plasticity for veil and lamellae colour. The ITS sequences of the type specimens of C. albobrunnoides and C. albobrunnoides var. violaceovelatus were identical to one another and are treated as one species with a wider range of geographic distribution under C. napus.ConclusionsOur results indicate that each of the Calochroi species has undergone a relatively independent evolutionary history, hypothesised as follows: 1) a widely distributed ancestral population of C. arcuatorum diverged into distinctive sympatric populations in the New World; 2) two divergent lineages in C. elegantior gave rise to the New World and Old World haplotypes, respectively; and 3) the low levels of genetic divergence within C. aureofulvus and C. napus may be the result of more recent demographic population expansions. The scenario of migration via the Bering Land Bridge provides the most probable explanation for contemporaneous disjunct geographic distributions of these species, but it does not offer an explanation for the low degree of genetic divergence between populations of C. aureofulvus and C. napus. Our findings are mostly consistent with the designation of New World allopatric populations as separate species from the European counterpart species C. arcuatorum and C. elegantior. We propose the synonymy of C. albobrunnoides, C. albobrunnoides var. violaceovelatus and C. subpurpureophyllus var. sulphureovelatus with C. napus. The results also reinforce previous observations that linked C. arcuatorum and C. aureofulvus displaying distributions in parts of North America and Europe. Interpretations of the population structure of these fungi suggest that host tree history has heavily influenced their modern distributions; however, the complex issues related to co-migration of these fungi with their tree hosts remain unclear at this time.
Highlights
Cortinarius species in section Calochroi display local, clinal and circumboreal patterns of distribution across the Northern Hemisphere where these ectomycorrhizal fungi occur with host trees throughout their geographical range within a continent, or have disjunct intercontinental distributions, the origins of which are not understood
Analysis of ITS sequences supported the phylogenetic placement of a total of 83 population samples identified as belonging to C. arcuatorum, C. aureofulvus, C. elegantior and C. napus lineages
The internal phylogeny of C. arcuatorum is composed of four major lineages: a clade including population samples from Costa Rica (I); a clade comprising population samples from Mendocino (Pacific USA) (II); a clade composed of samples from Del Norte Co (Pacific USA) (III); and a large clade containing mostly European and two sample populations from Wyoming (IV), respectively
Summary
Cortinarius species in section Calochroi display local, clinal and circumboreal patterns of distribution across the Northern Hemisphere where these ectomycorrhizal fungi occur with host trees throughout their geographical range within a continent, or have disjunct intercontinental distributions, the origins of which are not understood. Mushrooms in the genus Cortinarius, section Calochroi (calochroid clade), with over 100 species, represent one of the most conspicuous ectomycorrhizal members of contemporary Northern Hemisphere forest ecosystems [14]. Previous studies of species in Calochroi show various patterns of geographic distributions, including local (C. cisticola), clinal (C. arcuatorum) or circumboreal (C. aureofulvus, C. aureopulverulentus, C. cupreorufus, C. elegantior) distributions. In most instances fungus species distributions correspond to that of their host trees within continents and often disjunct distributions across the Northern Hemisphere show a similar pattern [15,16,17,18]. Allopatric populations/ species on an intercontinental scale display relatively high host fidelities at the host genus level, suggesting potential co-evolutionary/co-migratory trends [14]
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