Abstract
In obligate symbioses, partner fidelity plays a central role in maintaining the stability of the association across multiple host generations. Fidelity has been well studied in hosts with a very restricted diversity of symbionts, but little is known about how fidelity is maintained in hosts with multiple co-occurring symbionts. The marine annelid Olavius algarvensis lives in an obligate association with at least five co-occurring bacterial symbionts that are inherited vertically. The symbionts so efficiently supply their hosts with nutrition that these worms have completely reduced their mouth and digestive tract. Here, we investigated partner fidelity in the O. algarvensis symbiosis by sequencing the metagenomes of 80 host individuals from two mitochondrial lineages and two locations in the Mediterranean. Comparative phylogenetic analyses of mitochondrial and symbiont genotypes based on single nucleotide polymorphisms revealed high fidelity for the primary symbiont that dominated the microbial consortium of all 80 O. algarvensis individuals. In contrast, the secondary symbionts of O. algarvensis, which occurred in lower abundance and were not always present in all host individuals, showed only intermediate to low fidelity. We hypothesize that harbouring symbionts with variable levels of fidelity ensures faithful transmission of the most abundant and nutritionally important symbiont, while flexibility in the acquisition of secondary symbionts enhances genetic exchange and retains ecological and evolutionary adaptability.
Competing Interest Statement
The authors have declared no competing interest.
