2022

Hiillos, Anna-Lotta

Symbiotic interactions (antagonistic, synergistic, or neutral) have been of fundamental importance in shaping evolution of their hosts as well as other symbionts infecting the same host. Understanding the diversity, drivers and outcomes of these interactions is important in resolving species capability to adapt in changing environments. Apicomplexans are known to infect a wide variety of marine invertebrates, but their diversity and how they affect their hosts’ fitness is unclear. Are they parasites or mutualists? In this thesis, I aimed to disentangle these interactions between two marine apicomplexans, Rhytidocystis sp. and Selenidium pygospionis, infecting a marine polychaete. Using molecular methods, I surveyed natural host populations for infection dynamics, coinfection dynamics and how they are affected by different host characteristic. In addition, I described apicomplexan richness in relation to their host species richness on a large spatial scale, the Baltic Sea salinity gradient. I found that infection patters vary spatially and temporally and are affected by host size, but independent of host population density and genetic diversity. The spatial differences are likely due to differences in local environmental factors affecting symbiont transmission. I also found signs of synergistic (beneficial) interactions between the two symbionts while coinfecting the same host. In addition, I found indications that the richness of host communities might inhibit infection success (dilution effect). Overall, this thesis describes the infection patterns of the understudied apicomplexans, gives an indication of factors affecting the interactions between them and their host, as well as how other factors might affect their infection success in ecologically important benthic animals. However, this thesis also emphasizes that resolving the nature of symbiotic interactions is difficult with only direct observations from nature and controlled experimental approaches are required to gain a deeper understanding of these relationships.