Posch, T. | bioRxiv Science

bioRxiv2025

An early evolutionary stage of mutualistic endosymbiosis reveals a parasitic aspect of endosymbionts

Mutualistic endosymbiosis is a cornerstone of evolutionary innovation, enabling organisms to exploit diverse niches unavailable to individual species. However, our knowledge about the early evolutionary stage of this relationship remains limited. The association between the ciliate Tetrahymena utriculariae and its algal endosymbiont Micractinium tetrahymenae indicates an incipient stage of endosymbiosis. Although T. utriculariae cells rely on endosymbiotic algae to grow in low-oxygen conditions, they gradually lose the endosymbionts in aerobic conditions. Our comparative genomics reveals that the mitochondria-related genes of T. utriculariae are fast-evolving. Symbiotic cells display elongated mitochondria, which interact intimately with endosymbionts. Moreover, inhibiting mitochondrial fatty acid oxidation reduces host fitness but increases the endosymbiont population. Time-series transcriptomics reveal physiological fine-tuning of the host during day and night, underscoring adaptations for nutrient exchange and regulation. Notably, endosymbiotic algae downregulate photosynthesis-related genes compared with free-living cells, correlated with a substantially reduced chlorophyll content. These findings indicate that the endosymbionts exploit host metabolites to supplement reduced photosynthesis. Consistently, symbiotic Tetrahymena cells exhibit lower fitness under aerobic conditions than aposymbiotic cells. Our results support that mutualistic and parasitic relationships between symbiotic organisms are condition-dependent, especially at an early evolutionary stage.

evolutionary biology↗

bioRxiv2017

Hunters or gardeners?Linking community structure and function of trap-associated microbes to the nutrient acquisition strategy of a carnivorous plant

All higher eukaryotes live in a relationship with diverse microorganisms which colonize their bodily surfaces; plants are no exception. However, we still lack a satisfactory understanding of how these loosely associated microbiomes with immense diversity and functional potential interact with their hosts or how these interactions shape processes within populations and ecosystems. There is considerable similarity between microbial communities colonizing plant surfaces such as roots, and those of the animal gut. This often overlooked parallel allows us to look at microbial as well as host ecophysiology from a fresh perspective. The traps of carnivorous plants are sophisticated digestive organs and interface environments between the supply and the demand for nutrients. We selected the miniature ecosystem in the traps of aquatic carnivorous Utricularia plants as our model system. By assessing the trap-associated microbial community structure, diversity, function, as well as the nutrient recycling potential of bacterivory, we gained insight into the nutrient acquisition strategies of the Utricularia hosts. We conclude that trap ecophysiological function is in many aspects highly analogous to that of the herbivore gut and centers around complex microbial consortia, which act synergistically to covert complex organic matter, often of algal origin, into a source of nutrients for the plants.

microbiology↗