ALBERT

Description: 1.Parasite spillover from invasive aliens to native species increases the risk of disease emergence within native biota - either by direct harm to the new host or by indirect effects like increased risks of secondary infection. 2.One example for such a detrimental effect is the parasitic copepod Mytilicola intestinalis that infected blue mussels Mytilus edulis after being introduced into the North Sea in the early 20(th) century. Since 1949, the parasite was blamed for multiple mass mortalities of infested blue mussels but evidence for a direct causal involvement of M. intestinalis remained circumstantial. 3.Here, we now examine the potential effects of primary infections by the invasive parasite on the susceptibility to secondary infections with virulent bacteria (Vibrio spp.) in a full factorial infection experiment combining parasite infection (control vs. infected) with different Vibrio infection treatments (control, bath challenge, injection) in environmental conditions that either favored the host (ambient temperature) or the bacterium (elevated temperature). The influence of primary and secondary infections on cellular immunity (phagocytosis) and Vibrio load in the hemolymph was used to correlate these results to host survival. 4.Our results suggest that the rate of secondary Vibrio-infection is increased due to lower efficiency of the cellular immune response. As a consequence, the failure of clearing Vibrio from the hemolymph might increase mortality of mussels infected by M. intestinalis. 5.This demonstrates that indirect effects of parasite invasions can outweigh direct effects of the infection highlighting the need for a more integrative approach to understand and predict the consequences of parasite invasions.

Description: Within parasite communities infecting the same host, ecological theory predicts that two species occupying the same niche should evolve distinct niche use to avoid direct competition. Biological invasions can however create situations, where competition could not select for different niche occupancy and closely related parasites species find themselves competing for the same host resources for the first time since their lineages split. Such novel interactions cannot only alter the evolutionary trajectories of both parasite species, but will also feed back on the immune response of the host. Here, we show how the invasion of the parasitic copepod Mytilicola orientalis creates competition with the established congeneric parasite Mytilicola intestinalis, and how this novel menage a trois feeds back on the immune response of the blue mussel host Mytilus edulis. From a series of controlled infection experiments that manipulate competition among the parasites we can show that, although both species occur in the same section of the mussel gut, competition between the parasites is weak and shows similar impact on host condition in either simultaneous or sequential infections. Triplet transcriptomics of matching host (M. edulis) and parasite samples (M. intestinalis and M. orientalis) however revealed that the novel interaction of the invader with the host changes the transcriptional activity of many more genes and processes than the interaction with the established and coevolved parasite. Our results therefore not only show the utility of biological invasions of parasites to study coevolutionary processes, but also shows that responses to novel host-parasite interactions can lead to massive reactions on the molecular level that are not reflected in host or parasite phenotypes.

Description: Infection with ostreid herpes viruses (OsHv-1) have been causing mass mortalities and reeking havoc on production areas world wide. The resulting production losses have sparked several selective breeding programmes trying to improve resistance to infection. Recent studies showed however that there is genetic variation in OsHv-1 strains between different outbreak areas. Furthermore, it has been suggested that secondary infection by pathogenic bacteria contribute substantially to the observed mortalities. Together this means that the disease differs between regions suggesting that also the genetic determinants of resistance differ them. To test assess the role of environmental variation in disease we employ whole genome sequencing of pools of individuals (poolseq) pairing before and after mortality samples collected during mass mortalities across Europe (France, Germany, Ireland, Norway). We identified 67-109 outlier loci for all pairwise comparisons that showed either signatures of selective sweeps (fixation of major alleles) or allelic pull-ups (increase of minor alle frequency). While we could identify single nucleotide polymorphisms (SNPs) located in close genomic proximity for most pairwise comparisons, that also matched previous QTL-studies, the majority of outlier loci were unique to each location. This supports the polygenic architecture of OsHV-resistance, but also highlights the substantial selective differences between locations. Since the shared outlier loci probably only explained a relatively small variation in resistance, breeding programs should aim at location specific resistance rather than general resistance.

Description: Bacteria of the Vibrio genus are the most predominant infectious agents threatening marine wildlife and aquaculture. Due to the large genetic diversity of these pathogens, the molecular determinants of Vibrio virulence are only poorly understood. Furthermore, studies tend to ignore co-evolutionary interactions between different host populations and their locally encountered Vibrio communities. Here, we explore the molecular targets of such co-evolutionary interactions by analyzing the genomes of nine Vibrio strains from the Splendidus-clade showing opposite virulence patterns towards two populations of Pacific oysters introduced into European Wadden Sea. By contrasting Vibrio phylogeny to their host specific virulence patterns, we could identify two core genome genes (OG1907 and OG 3159) that determine the genotype by genotype (G × G) interactions between oyster larvae and their sympatric Vibrio communities. Both genes show positive selection between locations targeting only few amino acid positions. Deletion of each gene led to a loss of the host specific virulence patterns while complementation with OG3159 alleles from both locations could recreate the wild type phenotypes matching the origin of the allele. This indicates that both genes can act as a genetic switch for Vibrio-oyster coevolution demonstrating that local adaptation in distinct Vibrio lineages can rely on only few genes independent of larger pathogenicity islands or plasmids.

Description: The parasitic copepod Mytilicola orientalis infesting mussels and oysters was so far only described in saline waters – such as the North Sea. In April 2018, it was recorded for the first time at a low salinity location in the Kiel Bight, Baltic Sea. Two mature females of M. orientalis were found in two separate individuals of Baltic blue mussels (Mytilus spp.). Prevalence of parasites in the whole sample was low (3.6%), and no males or eggs were detected. In a second sample from October 2018, another adult female was found indicating spread over larger areas and longer time periods. The findings of this study further indicated that larvae from introduced M. orientalis adults can hatch under low saline conditions of Kiel Fjord and are able to infest and to develop within tissues of Baltic blue mussels. It therefore may just be a matter of time before the establishment of the full life cycle of M. orientalis in the Baltic Sea.

Description: Biological invasions often have negative impacts on native biota. This is particularly true if the invasive species is a parasite. Blue mussels Mytilus edulis in the North Sea were invaded as a new host of the parasitic copepod Mytilicola intestinalis in the 1930ies starting a new coevolutionary arms race. Here, we explore the evolution of parasite and host traits along separate fronts of the invasion and how infection with the new parasite affects host physiology directly. However, next to direct effects, this host-parasite interaction can also have profound indirect effects that feed back on mussel fitness. These include changes of gut microbiota, resistance to secondary infections but also the interactions with epibionts as well as predators indicating that indirect effects can outweigh the direct effects.