Inside the armoured snail = insights into evolution?

Crysomalon squamiferum also called the armoured snail, due to the scales covering the side of its foot, lives on hydrothermal vents in the deep sea. The scales are thought to be a defence mechanism to protect the snail against environmental threats in such extreme habitats. Luckily it does not have to fight on its own. Epsilon-proteobacteria and delta-proteobacteria, known to reduce sulfate to sulfides, have been previously associated to be living in symbiosis with them. Next to these two episymbionts, gamma-proteobacteria an endosymbiont has a more metabolic task and is found within the cells of the snails enlarged oesophageal gland where they synthesise organics from CO₂ by oxidising reduced sulfur compounds. As collection and maintenance of this deep sea gastropod are so difficult, there is not much know about their symbiosis with gamma- proteobacteria. However, technology has improved so a recent study by Nakagawa et al. (2014) managed to take a closer look at the genome of gamma-proteobacterial symbionts for a better understanding of their eco-physiological characteristics. The large genome size of gamma-proteobacteria also suggests a very recent association with the host, which makes them ideal to learn more about early stages of genome reduction during symbiont evolution in general. A complete genome sequence analysis indicated a high genetic clonality of the endosymbionts within a host gastropods. ¹³C tracer experiments and hemolymph glycan analysis provided information about the physiology of the gastropod. 

In my opinion the interesting part about this deep-sea symbiosis is how symbionts get into their hosts. This can either happen by horizontal or vertical transmission of the symbionts. As gamma-proteobacteria play such an important role in the metabolism for the scaly-foot gastropod, we would assume that the bacteria were to be transferred vertically, which means the larvae would already contain a bacterial population, such as previously found in Vesicomyid clams. This differs to what was found in Vestimentiferan tube worms, where the larvae must acquire bacterial symbionts from the surrounding water itself. 

Previous studies showed that genomic plasticity may give chemoautotrophs an advantage in such highly changing environments like deep-sea vents, however extreme genetic homogeneity was found in endosymbiont populations in armoured snails. I  wonder if this is due to the sampling method where no different strains of the bacteria have been described. Extreme genetic homogeneity also suggest the same kind of horizontal gene transfer in armoured snail such as found in tube worms. So what is the advantage of requiring symbionts from scratch in each generation? 

The problem that occurs with vertical transmission is that symbiont populations experience a bottleneck effect by a host mediated reduction during transmission from one host generation to the next. This can causes random genetic drifts of bacterial populations into a direction that might be advantageous for the environmental conditions the host parent lives in but may be completely different for the next generation, due to the vast changing hydrothermal vent habitat. Vertical symbiont transmission would therefore have consequences for the evolution of the bacteria. In horizontal transmission where new symbiont population are specifically selected in each host generation, this problematic effect of random genetic drift in the symbiont population can be overcome. This study not only gives insights into symbiosis and symbiont evolution but also shows the potential of micro-organisms to add towards our understanding of general speciation and evolutionary events.

Nakagawa S., Shimamura S., Takaki Y., Suzuki Y., Murakami S., Watanabe T., Fujiyoshi S., Mino S., Sawabe T., Maeda T., Makita H., Nemoto S., Nishimura S., Watanabe H., Watsuji T. & Takai K. (2014) Allying with armored snails: the complete genome of gammaproteobacterial endosymbiont. The ISME Journal. 8: 40-51.