SymBirth: Development of an artificial symbiont’s birth

Symbiosis, a phenomenon in which dissimilar organisms live together, can be found all around us, ranging from beneficial associations to harmful ones. If persistent for an extended period of evolutionary time, the symbiotic partners can become highly intimate and inseparable, resulting in drastic modifications of genetic and observable features. The study of symbiotic associations dates back over 100 years. However, only in the last decades have we developed comprehensive tools to characterise and study them deeply. Nutritional-based symbiosis, where the symbiont provides nutrients lacking from its host`s diet, has received particular attention. Namely, animals with nutrient- restricted diets, such as plant-sap and blood, have an obligate need for these nutritional symbionts. Thus, specialised inheritance mechanisms have evolved among these organisms to ensure the transmission of the symbiont from mother to offspring. While many of these obligate associations have now been studied, little is still known about the factors behind their establishment. This knowledge gap is due to the very few examples of nascent obligate associations and the inherent limitations of comparative studies. This project will contribute to filling this gap by promoting the artificial birth of a novel symbiosis in the blood-feeding European pond turtle leech Placobdella costata. Similarly to other blood-feeding organisms, the European turtle leech has evolved specialised organs, called bacteriomes, to house nutrient-supplying symbiotic bacteria. Recent investigations have revealed that different Placobdella species harbour similar symbionts. Most importantly, the symbionts are closely related to a free-living bacteria (Gellertiella hungarica) isolated from the Gellért baths (Hungary). In fact, they belong to the same species, with the leech symbionts exhibiting small to large genetic and observable modifications when compared to their free-living relative. This rarely observed feature makes the European turtle leech and its symbiont a promising biological system to study the early onset of obligate symbiosis, namely through the experimental replacement of its naturally occurring symbiont with the free-living G. hungarica. Through the combination of genomic, microscopic, and experimental techniques, we will determine the route the symbionts take when transmitting from mother to offspring. Secondly, we will explore different routes to symbiont clearing. Lastly, we will attempt to trigger the establishment of the free-living G. hungarica as a novel obligate symbiont, thus effectively replacing its naturally occurring symbiont. This project will not only clarify the hitherto unknown route for symbiont transmission in strict blood-feeding leeches but will also result in the development of a tractable, modifiable, and highly promising experimental system that will enable the discovery of key factors driving the establishment of beneficial microbial associations.