Print PageEcologists are increasingly aware that the outcomes of symbioses are more flexible and far-reaching than previously assumed. Our previous research has described a cleaning symbiosis between a keystone species/ecosystem engineer (crayfish) and ectosymbiotic annelid worms (branchiobdellids). Branchiobdellids benefit from their relationship with the crayfish through food acquisition, protection, and reproductive dependence. However, the reciprocal effects of worm on crayfish are variable and more complex. At lower densities, worms appear to clean the gills of their host, likely improving gas exchange and excretion. However, at higher densities, branchiobdellids will actually begin to consume the gills of their hosts. Given that many changes in physiology are mediated by gene expression, we hypothesized that the worms might alter gene expression in their hosts. We designed an experiment to examine the interaction between crayfish and branchiobdellids at the molecular level by manipulating branchiobdellid density on crayfish hosts (0 worms vs. high worm density) and measuring the expression of 5 candidate immune genes in the crayfish Cambarus chasmodactylus twice during a 6d experiment. We isolated RNA from the crayfish hepatopancreas and used semi-quantitative RT-PCR to characterize expression of the 5 candidate immune genes.
Results/Conclusions
Branchiobdellids affected expression of 2 of the 5 candidate genes associated with crayfish immune function: astacidin and prophenoloxidase (PPO). In both cases, branchiobdellid presence was associated with a down regulation of expression. For prophenoloxidase (PPO), the effect persisted throughout the experiment. However, for astacidin, there was an interaction with time such that a strong effect of symbionts on host gene expression on day 2 had disappeared by day 6. These results strongly suggest one of two possibilities: 1) branchiobdellids induce down regulation of specific immune pathways in their hosts to increase host suitability, or 2) the higher levels of gene expression in the 0 worm treatment is indicative of environmental stress on the crayfish host which is ameliorated by branchiobdellid cleaning activity, resulting in reduced immune and stress gene expression in the presence of branchiobdellids. However, there was also significant evidence of physical damage by branchiobdellids to the gills of their hosts, suggesting that explanation #1 may be correct. These results are part of our ongoing effort to use the crayfish/branchiobdellid system as a model for partner regulation in symbiotic systems, and suggest a surprising mechanism by which branchiobdellids may affect their hosts.
