Using historical herbarium specimens to elucidate the evolutionary genomics of plant invasion
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The world’s herbarium collections contain a vast number of specimens that were collected up to 400 years ago. Due to recent advances in DNA extraction and sequencing, these specimens are now readily available for use in genomic studies. This allows us to directly sample past diversity and reveal evolutionary and population histories that are difficult or even impossible to infer from modern data alone. Due to globalization, increasing numbers of species are introduced into locations they could not reach through natural dispersal. Some of these introduced species are able to establish a stable population in the introduced range and eventually become invasive. These species may be able to outcompete native species and thus can drive them to local or even global extinction. Therefore, invasive species are a threat to global biodiversity. Despite this, invasive species are good study systems for evolutionary processes. Each introduction to a new environment can be viewed as a natural experiment that is often running for more generations than can be studied e.g. in experimental evolution studies. Thus, parallel adaptation to similar environments can be studied. In combination with archaeobotanical samples or historic herbarium records, these changes can be observed directly using genetic evidence. One hypothesis as to why invasive species are successful in the introduced range is the Evolution of Increased Competitive Ability (EICA) hypothesis. It says that due to the release from native enemies, plants are able to allocate resources away from defense mechanisms and towards increased growth and reproduction which makes them better invaders. Ambrosia artemisiifolia (common ragweed) is a very successful invasive annual weed native to North America that was introduced to Europe in the late 19th century. In this thesis, I investigate the genomic basis of this invasion using 297 historic herbarium specimens and 350 contemporary samples from both the native and the invasive range in what is thus far the largest study of whole genomes of a non-model, non-crop plant. I found that the population structure in the native range contains three main genetic clusters and one admixed cluster, and, using this information, I was able to identify the most likely source population for the European invasion. Unlike in the native range, population structure changed drastically over time in Europe. Moreover, I found selection on traits in Europe which are consistent with the EICA hypothesis. In addition, I used the herbarium specimens as well as the contemporary samples in the first study of its kind to look at differences of the metagenomic community between ranges and over time. I found that some taxa are less common in Europe, providing evidence that enemy release might have played a role in the plants’ invasive success. This thesis demonstrates how genomic data from herbarium specimens can be used in a variety of studies and how they add value to the study of invasion.