Phylogenetic divergence between the obligate luminous symbionts of flashlight fishes demonstrates specificity of bacteria to host genera.

Many facultative symbiotic bacteria are known to exist in the ocean and one such is a bioluminescent bacteria Vibrio fischerii with its squid host, Euprymna scolopes. Although they do infect the host for certain portions of their life, the host will expel most of them on a diurnal cycle, thus the bacteria maintain a free-living state allowing them to survive without protection and that also allows for further infection of new host individuals. This enables for horizontal gene transfer to occur between different bacteria populations thus promoting divergent evolution from the host. Obligate symbionts cannot undergo horizontal gene transfer due to their closed environment and most of these types of bacteria are dependent on their host to provide a suitable environment allowing for growth and development. This, in turn with purely vertical gene transfer, prevents diversification and oftentimes leads to a reduction in genomic growth.

Hendry & Dunlap (2014) used phylogenetic analysis to understand the relation between different obligate symbionts and their anomalopid host species (flashlight fish). Their theory was that if the bacteria was environmentally acquired at the host larval stage and if they were in similar spatial ranges, then different host species could acquire the same bacterial symbionts. However, Photoblepharon palpebratus and Photoblepharon steinitzi were noted to have identical symbionts (99.6% identity using 16S rRNA sequence analysis) despite their spatial separation and contrastingly, the fish species Anomalops katoptron and P. palpebratus have different bacterial symbiont species despite inhabiting the same geographical location of southern Pacific Ocean, Philippines to Vanuatu. Alongside this analysis of previous species, they found what they predicted was a new species of symbiont in the Photoblepharon genus from P. palpebratus and proposed the name “Candidatus Photodesmus blepharus”. They noted it had a 94.8% identity with the P. palpebratus and P. steinitzi symbiont and 94.3% for Kryptophanaron alfredi symbiont. They used Stackebrandt and Goebel’s (1994) cut off point for species difference of 95% to determine that they had found a new species of symbiont.

This is a prime example of the specificity that hosts and bacterial symbionts can have with each other. It might be a method of eliminating competition for spatially co-occurring hosts or bacteria, by preventing those within the same geographical range from having to compete with one another. This could explain why spatially separate hosts can acquire the same symbionts, such as with P. palpebratus and P. steinitzi, although the authors commented that this could be due to a recent speciation event for the hosts themselves and there has been inadequate time for symbiont specificity to occur.

It is interesting to note that, although the P. palpebratus and P. steinitzi symbionts are likely to be identical species, they show different patterns of host-symbiont interactions. P. palpebratus expels its symbiont bacteria from its light organs but the authors did not comment as to whether this was a similar strategy for P. steinitzi. Inferring from this, P. steinitzi could be host to an obligate symbiont and despite their 99.6% identity, their symbionts exhibit highly varied life strategies. Obligate symbionts display very different evolution by way of genetic drift and vertical transmission in a static environment. Therefore, their genome can become severely reduced so it can limit the amount of energy required for growth and development. In contrast, facultative symbionts could evolve a much more varied genome due to requirements of maintaining its free living stage and by way of evolutionary tactics such as lateral gene transfer from other bacteria. Could these two identical symbionts eventually become different species by way of evolutionary divergence according to their survival strategies? Furthermore, could a single host evolve to have multiple symbionts that work together to achieve the same goal, or does the specificity prevent such a relationship from forming?

Due to the relatively few fish species collected and examined, the data could be predicting an inaccurate trend simply from there not being enough of it. Although, I think this paper does provide a stepping stone into the examination of these fish species different symbiotic bacteria but further sampling and analysis could provide a more complete picture. There is considerable difficulty in obtaining flashlight fish species as they occur at depth and the unculturable nature of their symbiont bacteria also needs to be taken into account. 

Primary reference:

Hendry, T. A. and Dunlap, P. V. (2014), Phylogenetic divergence between the obligate luminous symbionts of flashlight fishes demonstrates specificity of bacteria to host genera. Environmental Microbiology Reports, 6: 331–338. doi: 10.1111/1758-2229.12135

Secondary reference:

Stackebrandt, E., and Goebel, B.M. (1994) Taxonomic note: a place for DNA–DNA reassociation and 16S rRNA sequence analysis in the present species definition in bacteriology. Int J Syst Bacteriol 44: 846–849.