Samenvatting

Climate change is a major contributor to the range shifts of several organisms such as plant species. A range is the geographical area where a species can be found and it is possible for plant species to undergo a range shift while still being present in their original one. These plants are called range-expanders. They interact with a huge range of organisms both above-and belowground, including a wide range of fungi and oomycetes which are often pathogens. Plant species are often suggested to have a faster expansion rate than their related belowground organisms and thereby outrun pathogens, which is called the enemy release hypothesis (ERH) and it is believed to attribute to the success of successful range-expanders.
The suggested main drivers of the ERH were cultivated, fungi and oomycetes, from the seeds of Centaurea stoebe and Centaurea jacea, commonly known as knapweeds, to see what potential pathogens they inherently carried with them and if root endophytes are seed derived. Similarly, those organisms were also cultivated from the roots to see if C. stoebe and C. jacea selected for different endophytes. To test whether the community of C. stoebe differed between ranges all plants were grown in both Slovenian and Dutch soil with a sterile soil as control. The DNA of all isolated cultures was extracted and sequenced to identify individual cultures. Three genera of fungi were identified to be dominantly present; Chaetomium spp., Fusarium spp. and Alternaria spp. These three genera were divided based on sequence information combined with morphological information of non-sequenced isolates that resembled sequenced cultures to look into their diversity. The abundance of the cultures was also distinguished to define infection rate.
From the results it would seem that C. jacea and C. stoebe select for different endophytes, since the diversity among C. jacea was significantly higher compared to C. stoebe (p<0.001). The infection rate is also significantly higher among C. jacea when compared to C. stoebe (p<0.001). The community of the range-expander C. stoebe does not seem to differ between ranges, suggesting that the enemy release hypothesis might not be involved in its successful range expansion. However, C. stoebe might be carrying Alternaria spp. within its seeds to gain a competitive advantage by accumilating local pathogens. This observation is based on the infection rate among the seeds of C. stoebe being significantly higher compared to C. jacea (p<0.01) while no Alternaria spp. cultures were isolated from its roots. The dominant presence of Chaetomium spp. in the roots of C. stoebe might play a role here because of its biological control properties, shielding it from the negative impact Alternaria spp. would have on it. Because of the similar morphogroups of Fusarium spp. and Alternaria spp. between the seeds and the roots, the fungi that the seeds of plants inherently carry with them might play a role after germination. Especially if the accumulation of pathogens does indeed drive the succes of C. stoebe as a range-expander.