GIANT CILIATE – THIOTROPHIC SYMBIONT MUTUALISM MAINTENANCE AND PERSISTENCE

Knowledge of the identity of the partners in mutualism, their taxonomic placement and their potential functional repertoire is crucial basic information in mutualism research. The mutualism between the giant ciliate Zoothamnium niveum and its chemoautotrophic ectosymbiont Cand. Thiobios zoothamnicoli from shallow-water wood falls is a dual species partnership based on 16S and 18S rRNA gene sequences but genomic information lacks so far. The symbiont is transmitted vertically. Because the symbiosis is not found in winter due to lack of sulfide emitting from wood, but the host can be cultivated aposymbiotically under oxic conditions survival of both partners separated from each other with horizontal transmissions each spring may guarantee persistence of the association. Population genomic studies can reveal which transmission mode is dominant by assessment of the co-diversification between both partners. Evolutionary explanations of mutualism remain controversial. How do partners provide benefits to the other without guarantee of reciprocation? The dominant narrative is based on sanctions, the punishment of the cheating partner. Empirical research with fitness characterizations of all partners involved in a relationship under variable environmental conditions, however, are rare but can be highly informative on the mechanisms facilitating fitness coupling. The giant ciliate mutualism is highly suitable for such studies because it can be cultivated.

Aims: We aim to determine the reliable in-depth phylogenetic placement of the giant ciliate symbiont, and to establish the pattern of co-diversification with the host further informing the transmission mode of this symbiosis. Its potential metabolic capabilities and population genomics will be described. Fitness coupling can be shown with an experimental setup that addresses the performance of both partners facing changing environmental controlled conditions.

Approach: We apply high throughput sequencing genomics to obtain the symbiont metagenome assembled genome and the host mitochondrial genome and bioinformatic tools in the comparative analyses. Fitness parameters of both partners can be obtained by cultivation in flow-through aquaria under controlled conditions combined with light and electron microscopy.

Relevance: The giant ciliate mutualism model system is especially suited for experimental approaches. It is found in coastal marine areas, it is easy to collect and its short generation time allows to obtain fast information on relevant fitness parameters. Evolutionary insight can be drawn for the maintenance of the mutualism. Genomic functional information on the symbiont side can provide important insights facilitating further research. Population genomics of both partners can shed light on the transmission mode.

Student: Salvador Espada-Hinojosa

Faculty: Bright (PI), Horn

Funding: FWF project "TIOCIM"

Selected publications:

Bright M., Espada-Hinojosa S., Lagkouvardos I. & Volland J.-M. The giant ciliate Zoothamnium niveum and its thiotrophic epibiont Candidatus Thiobios zoothamnicoli: a model system to study interspecies cooperation. Frontiers in Microbiology. 5, 145 (2014).

Archetti M., Scheuring I., Hoffman M., Frederickson M. E., Pierce N. E. & Yu D. W. Economic game theory for mutualisms and cooperation. Ecology Letters. 14, 1300-12 (2011).

Genkai-Kato M. & Yamamura N. Evolution of mutualistic symbiosis without vertical transmission. Theoretical Population Biology. 55, 309-23 (1999).