The project continues to address the establishment and functional consequences of the microbiota in the two basal metazoan animals, the Scyphozoan jellyfish Aurelia aurita (Cnidaria) (B2.1, PI Schmitz-Streit) and the comb jelly Mnemiopsis leidyi (Ctenophora) (B2.2, PI Reusch) in a highly comparative approach.
Analysis of the associated microbial community composition of these two metazoan animals during the first funding phase, led to the identification of 16 bacterial key species responsible for community differences. Out of 16 operational taxonomic units (OTUs), eleven were present in both communities, in different ratios, with four restricted to M. leidyi and one to A. aurita. Moreover, 25 representatives of these indicator OTUs are already isolated, which can be used for recolonization experiments planned in the second phase.
We further demonstrated that (i) life stage-specific bacterial community patterns of A. aurita undergo significant restructuring during metamorphosis, and obtained first experimental evidence that the associated microbes are crucial for reproductive output of the host. (ii) A. aurita-derived Quorum Quenching (QQ) activities are enhanced upon challenging with pathogens and interfere with bacterial Quorum Sensing; in case of M. leidyi screening EST libraries demonstrated even higher numbers of host-derived QQ activities. (iii) A two-generation (F2) evolution experiment using two different base populations of M. leidyi demonstrated rapid adaptation of growth and reproduction to contrasting salinity levels, which coincided with significant changes in the microbiota, suggesting an important role of the microbiota for rapid responses of M. leidyi to environmental change. Under low salinity challenge, we identified ten indicator OTUs, several of which have been isolated during the first funding phase and will serve as critical resource during the second phase, when evolution experiments are aimed to assess the functional role of microbes in the invasion process.
Based on this work, the following key questions will be addressed in the second funding phase to elucidate the functional consequences of the microbiota in the two basal metazoan animals:
(i) What is the basis of the observed microbial contribution to A. aurita reproduction? The underlying molecular mechanism will be illuminated with re-colonization experiments of germ-free animals combined with metatranscriptomic and biochemical approaches.
(ii) What is the functionality of QQ activities between microbes for establishing the associated microbiota? Functional assays will be performed using simplified and genetically engineered microbial communities (Mock community) for both hosts to elucidate the importance of the identified QQ open reading frames (ORFs) in respect to a healthy metaorganism.
(iii) Are there conserved host-derived QQ patterns in different metaorganisms (lower metazoan)? This will be addressed by a combination of homology and domain based comparative genome and gene expression pattern analyses.
(iv) What is the function of the associated microbiota for M. leidyi host fitness? Targeted experiments will address the fitness effects of germ-free animals versus animals reconstituted with Mock communities of varying degree of simplification.
(v) What is the importance of microbiota for rapid adaptation during the invasion process of M. leidyi? Using evolution experiments under low salinity conditions versus controls, the role of microbiota for adaptation of the M. leidyi metaorganism will be assessed. By altering the co-evolved microbiota at the end of the selection phase, we can directly address how the associated microbiota influences major fitness traits during invasion.
The overarching questions on the establishment and functional consequences of microbiota will be addressed in a highly comparative way for both hosts combining the complementary expertise of the two PIs in different biological fields. Elucidating these questions will not only contribute to the understanding of host interactions with microbial communities, but also aims to provide conceptual insights into the complexity of host-microbe interactions in general. The identification of underlying principles in basal metazoan-host interactions is imperative to under-stand how metazoan life emerged in a “soup of microbes” in the ocean.