Never too old to acquire some symbionts

A recent study by Wentrup et al (2014), looked at how and if species of deep sea clams and mussels were able to continue obtaining new symbionts throughout their life. Bivalves show continuous gill growth which raised the question as to how these new areas of gills are colonised the two possible ways were from the surrounding environment or from self-infection from symbionts already occurring on the bivalves. As with much of deep sea research these questions have previously been hard to answer due to limitations in cultivating deep-sea bivalves. This study overcame these limitations by using the method fluorescence insitu hybridization (FISH) with probes specific to the symbionts, along with transmission electron microscopy to examine mussel and clam gill tissues.

In total 4 different species of clams and mussels were investigated both of which showed either horizontal or vertical transmission. I believe it was important to represent both transmission types as colonization could be different between horizontally and vertically transmitted symbionts.

The finding found that sulfur and methane oxidising symbionts co-occurred intracellularly in gill bacteriocytes, the symbionts were also found to change the morphology of the host cells which became flatter and wider with the reduction of microvilli on their surface, however they were absent from the ends of the filaments at the frontal gill surfaces. The first newly formed filaments to show symbionts present evident by FISH signals were the seventh to ninth filaments. A gradient was also found to occur with the youngest gills filaments containing less symbionts then slightly older gill filaments.

Two factors were found to be important in symbiont colonization, firstly developmental stage, the findings indicated that colonization can only occur after the host cell has reach a certain differentiation stage (seventh to ninth filaments).
Secondly symbiont source was important there were two different ways symbionts could be transmitted either from the environment which would include free living symbionts or symbionts from co-occurring individuals, or alternately from within the host via self-infection. The results suggest that self-infection is the best explanation, the first gill cells to become colonized were always located closest to already colonized gill tissue. Both the anterior side and the posterior side of the filament are formed at the same time however the anterior side which was closest to already colonized gill tissue was always colonized first if symbionts were transmitted from the environment the distribution would be expected to be random between areas of gill tissue of the same age.
 Within the different species examined there was found to be different explanations for colonization of gill filaments via self-infection, for Bathymodiolus spp. it is possible that symbionts become extracellular and invaded surrounding gill cells through their apical surface However in C. ponderosa symbionts were found in the gill tissue that connects the filaments allowing symbionts to move between host cells.

One limitation of the study is although it shows that self-infection is most likely the source of the symbionts the addition of environmental symbionts might also be possible especially under certain conditions such as during early development stages, or where uptake of locally adapted free living symbionts would produce a strong selection advantage, or if the internal population reduces due to internal stress. Future work hopes to address this by looking at the genetics of symbionts within the host and how related the population within the host are. I think taking into account different environmental condition between areas is a good focus for further research because the environment could be driving some of the changes in symbiont colonization.

references 

Wentrup, C., Wendeberg, A., Schimak, M., Borowski, C. and Dubilier, N. (2014). Forever competent: deep-sea bivalves are colonized by their chemosynthetic symbionts throughout their lifetime. Environmental Microbiology, 16(12), pp.3699-3713.