The Evolution of a Lichen Symbiosis: A Case Study Investigating Menegazzia (Ascomycota) and Trebouxia (Chlorophyta)

Abstract

The mechanisms driving lichen symbiont association are in the early years of being understood, and it is with the intention of furthering our knowledge of these mechanisms that this case study was devised. The genera used here were the lichenised ascomycete genus Menegazzia, and its photobiont, the lichenised chlorophyte genus Trebouxia. Accessions were collected from throughout New Zealand and eastern Australia, and a symmetrical DNA dataset of ITS+ef1α was generated for the fungi, and ITS+rbcL for the algae. This data, along with the addition of sequences from GenBank, were then analysed in molecular phylogenetic and phylogeographic frameworks. First, each genus was analysed separately, independent from its lichen symbiont. Second, all of the data was brought together and analysed using tanglegram and co-phylogeographic techniques, with the intent of elucidating the drivers shaping the symbiotic relationship between these two genera.

The results of the independent Menegazzia/Trebouxia analyses, when investigated using Bayesian coalescent phylogenetics, revealed well-resolved hypotheses of the evolutionary histories of both groups. The results of the combined symbiont analyses were in agreement with much of the modern lichen symbiosis literature: genus-wide co-evolution was not supported, and there was some evidence for algal switching and photobiont guild structuring. However, in contrast to most other studies, there was also evidence for multiple cases of highly specific fungal-algal partnerships. These patterns are best-explained in light of asexual joint-dispersal strategies vs. sexual non-joint-dispersal strategies, ecological factors (elevation, substrate, ecotype), and historical biogeographic influences. Future research directed at community level symbiont pairings, across multiple Trebouxia containing mycobiont genera, should prove to be highly informative.