Evolution and host specificity in the ectomycorrhizal genus Leccinum

Area of Science:

• Mycology
• Evolutionary Biology
• Ecology

Background:

• Ectomycorrhizal fungi, particularly the genus Leccinum, are generally characterized by their host specificity.
• Understanding the evolutionary dynamics of host associations is crucial for comprehending fungal speciation.

Purpose of the Study:

• To investigate the phylogenetic relationships among Leccinum species from Europe and North America.
• To reconstruct the evolutionary history of host associations within the Leccinum genus.
• To elucidate the mechanisms driving speciation in ectomycorrhizal fungi.

Main Methods:

• Phylogenetic analysis using the second internal transcribed spacer (ITS2) and glyceraldehyde 3-phosphate dehydrogenase (Gapdh) gene sequences.
• Reconstruction of ancestral host associations using maximum likelihood and parsimony approaches.
• Application of a molecular clock to the Gapdh gene for dating host specificity evolution.

Main Results:

• The Gapdh gene provided higher phylogenetic resolution than ITS2, and the resulting phylogenies were incongruent.
• Most Leccinum species exhibit high host tree specificity, with Leccinum aurantiacum being a notable exception, associating with diverse hosts.
• Ancestral host association reconstructions indicated that L. aurantiacum evolved from a specialist ancestor.

Conclusions:

• Phylogenetic incongruence suggests complex evolutionary histories, potentially involving horizontal gene transfer or incomplete lineage sorting.
• Host switches, particularly associated with geographic isolation and expansion into ecologically equivalent niches, appear to be significant drivers of rapid speciation in Leccinum.
• The study proposes a model where cycles of host niche contraction and expansion facilitate speciation, with implications for understanding host race formation and incipient speciation in symbiotic fungi.