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Lichens are present in most land ecosystems, frequently occupying habitats where few other organisms are able to survive. Their contribution to the ecosystems in terms of biomass and ground cover increases with latitude and altitude, being, together with bryophytes, the most conspicuous component of alpine and polar landscapes. Whereas some polar lichens have reduced distributions and are restricted to high latitudes, most of them have very wide distributional ranges, which oven extend over several climatic regions. Many of them are common to Polar Regions of both hemispheres, a distributional pattern that has been denominated as bipolar, antitropical or amphitropical. Bipolar distributions are not exclusive to lichens, but common to many groups of organisms. The bipolar element in lichens is exceptional as it includes a large number of species, while in most other land organisms it includes genera or families but very seldom species.
In this dissertation I use the bipolar lichen Cetraria aculeata to give a first insight into the phylogeography of this biogeographic element in lichens. I discuss how and when the disjunct distribution of C. aculeata came to be, and try to partial out the roles that historical and ecological processes played in shaping its distribution.
Sampling was designed to cover a wide geographic extension. The main e"ort was made to collect in boreal, temperate and tropical mountain ranges in North and South America, as well to include Mediterranean populations in which specimens with deviant morphologies are observed.
I found that Cetraria aculeata forms a genetically congruent taxon. Although whether it should include C. muricata remains unsolved, I excluded all specimens identified as the latter from our analyses. Thee populations of both algal and fungal symbionts have a strong geographic structure. The study of the lichen fungus suggested that the species originated in the Eurasian continent and later expanded to acquire its current distribution during the Pleistocene. The results showed that all American populations originated from an ancestral population, more similar to the extant Arctic populations than to the Mediterranean ones.
The comparison between the structure of fungal and algal populations showed a high degree of coherence between them. However, the similarity in photobiont use between Arctic and Antarctic populations suggests that photobiont use responds not only to a history of codispersal in vegetative propagula, but it is also a result of a selective process related to climate. Since this climatic pattern of similarity is also found in the community of Alphaproteobacteria associated with C. aculeata, we concluded that lichens might be able to accommodate or to respond to different environmental conditions by selectively associating with different symbiotic partners.
Lastly, we found the Mediterranean populations of C. aculeata to be genetically differentiated in algal and fungal symbionts from the rest of the populations. While we found no grounds to believe that the overgrown morphs encountered in the region are due to the association with different algal lineages, I believe that a switch in photobiont use might be responsible for the pattern of genetic isolation encountered. Furthermore, I suggest that the Mediterranean and bipolar C. aculeata could be two different species, since both are ecologically, genetically and at least in part morphologically divergent.
This manuscript-based thesis is divided into four chapters. Chapter one is an introduction to lichens and the Antarctic. It introduces the goal of the thesis and the problems related with lichen systematics and the lack of knowledge about Antarctic lichens. The Antarctic is one of the last wildernesses, isolated from the other continents by the Antarctic Circumpolar Current, the Subantarctic Front, the Antarctic Polar Front, and the Drake Passage. Terrestrial life in Antarctica is restricted to widely separated and small ice-free areas that cover only 0.3% of the continent. Colonization of the Antarctic is a challenge for many taxa and is related to their ability for long-range dispersal and their adaptation to the harsh climate. Antarctic terrestrial ecosystems are significantly threatened by climate change, invasive species, and their interactions. Glacial retreat caused by higher than average temperatures exposes new habitats that can be easily colonized from local biota, but non-native species can also be favored by the new climatic conditions. In addition, propagule movement mediated by humans can introduce new species or change the population structure of many taxa. The terrestrial biota is comprised almost exclusively by “lower organisms” (invertebrates, bryophytes, algae, lichenized fungi, and microorganisms). Lichens are the dominant component, and the most important primary producers. Lichens are symbiotic associations consisting of a fungus (mycobiont) and one or more photosynthetic (photobiont) partners. They can disperse sexually or vegetatively. There are several problems related to the symbiotic nature of lichens that do not facilitate easy identification; although molecular data offers additional evidence, species delimitation in lichens is still not straightforward. The true number of species is underestimated due to the presence of cryptic species and species pairs. Recommended universal fungal barcode sequences (e. g. ITS) sometimes fail to delimit species pairs. Thus, it is necessary to identify fast-evolving markers that allow for the delimitation of closely related species before proceeding with the analysis of lichen populations. The goal of this thesis is to elucidate the so far unknown genetic structure among Antarctic lichen populations because of the immediate consequences for conservation strategies. The thesis focuses not only on patterns of differentiation and gene flow, but also investigates the question of human-mediated propagule transfer into Antarctica and among Antarctic sites. This project provides data on the genetic structure of Antarctic lichens that is urgently needed to develop conservation strategies in the face of global warming and increased human activities in the region. Due to the fact that it is not possible to apply all of the unspecific fingerprinting methods to lichens, microsatellites or simple sequence repeats (SSRs) are one of the best tools to investigate the genetic structure of lichen populations. SSRs offer the possibility to discriminate the lichen partners, but species-specific microsatellites have been developed for only a few species. Regarding the Antarctic, only one species has been studied with SSRs.
The second chapter describes new methods and tools to delimit closely related species of lichens and provides fast evolving markers to characterize their genetic structure. The chapter introduces the lichen species analysed in this thesis and the problems related to their correct identification by morphological methods and molecular data. Chapter two explains the sampling methods for lichen populations and the localities from small areas in which the species pairs occur together. Then the methods used to generate and validate fungal specific microsatellites that cross-amplify species pairs are described. This chapter focuses on the species pair Usnea antarctica and U. aurantiacoatra because they are the most common lichens in the Maritime Antarctic. An internal transcribed spacer (ITS) marker do not discriminate between these species, and some authors have suggested to synonymize them. Unpublished results from another Antarctic species pair, Placopsis antarctica and P. contortuplicata, are included to confirm the capability of SSRs to discriminate closely related lichen species. This thesis is the first study to generate SSRs that cross amplify species pairs, using BLAST to compare one genome against the other to obtain markers with the same length in flanking regions. The de novo developed SSRs are able to discriminate the two closely related species, and can detect variability at the population level. In the end of the chapter, ITS sequences, microsatellites, and SNPs are used to delimit the species of Usnea antarctica and U. aurantiacoatra. The chapter exposes the importance of a correct species delimitation and the ability of SSRs and SNPs to delimit the Antarctic Usnea species pair compared with the recommended universal fungal barcode sequence ITS. ...