qPCR-based relative quantification of the brown algal endophyte in : variation and dynamics of host-endophyte interactions.

Morphological changes-such as dark spots, twisted stipes and deformed blades-have been observed in wild and cultivated . The putative cause for the disease symptoms is the filamentous endophytic brown alga , which is known to invade stipes and fronds of its hosts. Little is known about this interaction and its occurrence in the field, although former studies indicated high endophyte prevalence in kelp populations. Previous epidemiological studies on kelp endophytes were mainly based on the examination of microscopic sections, followed by time-consuming isolation and cultivation steps in order to identify the endophyte and a reliable method to quantify endophyte infections was missing. As a novel approach, we established and validated a qPCR assay for relative quantification of the endophyte within its host , which allows to examine both, the prevalence of endophytic algae and the severity of infections. The assay was shown to be highly specific and suitable to reliably detect small amounts of endophyte DNA in the host. Using this method, we detected very high endophyte prevalence in the investigated kelp populations, up to 100% in young sporophytes in Brittany during spring. Furthermore, our results suggest that sporophytes are infected early in their life and that seasonality and environmental factors have a significant impact on infection rates. In the future, this approach could also be applied to study other host-endophyte pairs using specific primers.