Rise of the Other Kind(s): Part III

Maliha Tanjum Chowdhury

Freshman

School of Life Sciences

Independent University, Bangladesh

July 13th, 2017

This the concluding part of  a three-part series of articles that aim to introduce the study of evolution using microbes as model systems while focusing on a recent study on speciation in bacteriophages.

After looking at infectivity of the bacteria by the differently evolved bacteriophages, the next factor studied was the adsorption rate – how well the phages can bind to either receptor protein. Surely enough, every specialist bacteriophage that had evolved had a binding affinity greater than the original or ancestral bacteriophage to their preferred host. Additionally, the scientists have also inferred that the gains in adsorption rate for the preferred receptor were greater than the losses on the alternative.

Now, to dig even deeper, the researchers meticulously sequenced phage alleles and measured the differences in gene sequences. All the specialists were found to have mutations in the host recognition gene J (the gene that codes for the protein that is responsible for binding to the bacterial host) and all individual mutations were seen to be non-synonymous – that is, the mutations resulted in changes in protein structure (which can be expected to ultimately affect binding). Interestingly, regardless of evolving by allopatry or sympatry, specialists for either type of receptor showed stronger genetic relatedness between themselves than with specialists of the other type.

Finally, the scientists resorted to cross-checking their findings to verify that their inferences were indeed true. They created new bacteriophages by artificially constructing the mutated alleles of either specialist, the ancestor EvoC strain, and a hybrid with all the mutations from the specialists for both receptors. Reality met expectations as the mutations found in either evolved strain did prove to be responsible for their respective host specializations and the genetic configuration of the ancestral EvoC was indeed seen to be expressing the generalist trait. The hybrid-child, sadly, did not prove to be viable. These observations not only confirm the J allele mutations as the cause of diversification, but also show how species may begin to emerge through mutations that result in reproductively incompatibility, proven by the production of non-sustainable hybrid progeny.

While I know all you science-mad kids are getting totally dizzy and starry-eyed marveling this experiment’s successes, it is always sensible to remember that even the coolest experiments are, to some degree, chained down by assumptions and reality. For instance, in this study, even with the substantial dissimilarities between them, the specialists are still much more similar than the cut-off for different species, that is, less than 70% sequence similarity. Not ones to be disheartened, the scientists argued that they did indeed observe the trademark processes that lead to eventual speciation, but did not let it run long enough for actual, classifiable species to emerge. 

Aside from this, there also surfaced some confusion over the incidence of genetic reversion – did the EvoC phage simply “go back” to being its ancestor, the predominant LamB-specialist? In depth analysis, however, put this matter to rest as among the 12 sequenced alleles, only 1 was seen to have reverted at a single site of the 5 mutations that set EvoC apart from its earlier ancestor. The LamB specialist can thus be described as much an independently evolved phage as the OmpF specialist. Another experimental drawback was in that Lambda phages are not completely sexual; also, phages need only a few mutations in a single gene to become reproductively isolated. As a consequence, it’s logical to think that these conclusions may not apply to speciation that requires more genetic change.

Apart from the specific constraints of this particular experiment, there always remain some basic unanswered questions – how likely is this effect to be a noteworthy and widespread one in nature? What key factors are responsible in speeding up or slowing down speciation? And, most importantly, what parameters can be measured to directly test the limits of speciation? These will be important open questions in evolutionary for a long time.

It’s been a riveting journey observing how life-forms function in response to the winds of change over the course of this experiment. It is no doubt that we owe our heartfelt gratitude to these brilliant minds as they have been able to physically show us the baby steps of a transition as complex and mind-boggling as evolution. Sure, saying that, in 50 more years, we might be able to “see” the monstrous Triceratops transform into the ethereal, enchanting peacock (for example) is a stretch. But because of such breakthroughs occurring ever so often in this millennium of miracles, I dare to dream that we understand the process much more clearly.

Maliha is a weirdo who somehow believes she's from a different planet. But she likes Earth just fine, and is fascinated by the science and beauty of life and has made it her purpose to explore it. Besides this, her most burning desires include becoming a synthetic biologist/ genetic engineer and running away with a heavy metal band.