MCCOY, A.M.; Harvard University: Host-Symbiont Specificity in a Microalgal-Invertebrate Symbiosis

One of the key issues in microalgal-invertebrate symbiosis is the mechanisms by which host-symbiont specificity is determined and maintained. This includes both the recognition of appropriate symbionts and post-ingestion regulation of symbionts. The mechanisms remain unknown for many associations in part because only a limited number of associations are amenable to manipulation. The invasive marine acoel Convoluta convoluta is a recently introduced species in the Gulf of Maine. It is now highly abundant in its new habitat despite maintaining a highly specific obligate relationship with environmentally acquired symbionts. Similar to other symbiotic associations the acoel can form symbioses with several related species in laboratory experiments, but has only the preferred species in the field. This is typically attributed to low specificity of initial uptake of symbiosis competent algae with subsequent post ingestion regulation. Indeed, juvenile C. convoluta show no difference in uptake of two potential symbiont species with a 24 hour exposure to cultured algae as is seen in other systems, and also show post ingestion regulation of symbiont division; however, by looking at very short time intervals of minutes to a few hours highly significant differential uptake becomes apparent suggesting differential uptake may play an important role in natural specificity. Additionally, the post ingestion regulation observed was not consistent with the paradigm of symbiont competition suggesting differential uptake may be the primary determinant of specificity in this association.