Geographic Structure in a Symbiotic Mutualism

Susanne Altermann

University of California Santa Cruz

December 2009

ABSTRACT: An increasing number of studies analyze the geographic structure of species interactions, including symbioses, but few address mutualistic symbioses, systems where phylogenetic congruence can be expected when there is vertical inheritance of symbiotic partners. In this study, I examine fungal-algal partnership patterns and geographic structure of the fungus Letharia vulpina s. lat. (Ascomycota) and its partners, multiple clades of Trebouxia jamesii (Chlorophyta), across Western North America. Letharia lichens are unusual in that multiple gene genealogies delimiting taxa on both sides of the symbiosis are available from a prior study. This previous work provides a robust phylogenetic context for the study of a multi-species interaction network that occurs at a large geographic scale. The primary methodology employed is DNA sequencing of seven fungal loci and two algal loci. The study provides: 1) Formal description of a lichenized fungus, Letharia lupina sp. nov., that is distinct from Letharia vulpina s. str., based on morphology, geographic distribution, fungal DNA sequences, and algal partner. L. lupina and its algal partners form “Mountain Wolf” lichens. 2) A map of the geographic distribution of five clades of Mountain Wolf algae in Western North America. 3) A comparison of geographic structure of Mountain Wolf fungi with Mountain Wolf algae. 4) A comparison of the geographic structure of Mountain Wolf algae with co-distributed plant and animal taxa in Western North America.

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