by Elio & Merry
Two Greek philosophers, Heraclitus and Democritus, found the human condition vain and ridiculous, but Democritus always appeared with a jeering and laughing countenance, whereas Heraclitus commiserated our condition with a sorrowful look and tears in his eyes.
Michel De Montaigne (1533–1592)
While philosophizing about what is a species, should one laugh or cry? Maybe it matters how you feel at the moment. For me (Elio), that’s the case. In my bilious mood, I despair about the chances for a useful, encompassing, and consensual definition. But when I feel more jovial, I applaud the continuous efforts of erudite biologists to grapple with this, possibly the most intractable of all biological predicaments. Month by month, new approaches are put forward, many borne out of newly acquired genomic and ecological information, and their accompanying mindsets. Perhaps the Holy Grail will one day be found. Meanwhile, we recently offered some mirth.
We have thought about this issue often, and what brought it up just now is a paper showing that two bacterial species (allow us to call them that) can not only diverge from one another, but can, in fact, converge, possibly to become one again. Regardless of how you define species, the phenomenon is interesting. The creatures in question are Campylobacter jejuni and C. coli, two distinct gastroenteritis-causing pathogens whose housekeeping genes have only an 85% sequence identity, well below the 94–97% cut-off point often used to delineate bacterial species. (Salmonella and E. coli, thought to have diverged 120 million years ago, also display about the same percent identity.) It looks like C. coli acquired genes from C. jejuni on multiple occasions, which is blurring the species difference between the two.
The research reported is based on the multi locus sequence typing (MLST) method widely used to type bacterial strains. MLST involves PCR amplification and sequencing of fragments of seven unlinked housekeeping genes, followed by assembly of the sequences into 3309 base pair haplotypes. The MLST sequences from several thousand isolates were already available in the Campylobacter database. These researchers set to work analyzing this data. They found that more than 99% of the haplotypes fell neatly into one or the other of the two species. However, there were some mixed "hybrid" haplotypes, typically C. coli that contained sequences from C. jejuni. Most of these showed up within one of the three C. coli clades (clade 1).
More genes moved from C. jejuni to C. coli than went the other direction. Since most of the gene acquisitions were found in C. coli clade 1, the transfer occurred after it diverged from clades 2 and 3. Some of acquisitions may have been quite recent, as many of the imported sequences are identical to those in the donor population. Swaps of this sort leading to “despeciation” happened on multiple occasions, and at least four times more frequently than those leading to divergence. As the authors state:"if maintained over time, these rates would lead to progressive genetic convergence. Well, if so, then at just what point should the recipient C. coli’s be considered "hybrids" and no longer the original species?"
An interesting point is that the imported C. jejuni genes are found more often in C. coli strains isolated from domesticated cattle and birds, thus suggesting that humans may have played a role in reversing the usual pattern of divergence by providing new hosts that are shared by both species. Maybe. Whatever the impetus, these two bacteria do not behave like good God-fearing species because the genetic barriers between them are quite porous.
What to make of this? In principle, lateral gene transfer should be a two-way street, with genes able to travel in both directions—sometimes even back to the organism they came from. So, are we witnessing something new? Perhaps not in kind, but in an altered pattern due to our altering the selective pressures that operate on “species” selection. The authors lament:"Humans continue to cause rapid and extensive environmental change. Preexisting environments have been degraded, and novel ones, such as intensive farms or acid mine drainage areas, have been created that provide a new adaptive landscape and opportunities for hybrid forms to evolve."
Nature seems to be able to put Humpty Dumpty back together again, a lively demonstration of the plasticity of the prokaryotic (and probably also eukaryotic) species. In this game, nothing stays the same. As Heraclitus, the "weeping philosopher" of the painting, said: Пάντα ρεΐ (panta rhei, everything is in flux). Is it ever!