Selective Sweep - Relevance To Disease

Relevance To Disease

Because selective sweeps allow for rapid adaptation, they have been cited as a key factor in the ability of pathogenic bacteria and viruses to attack their hosts and survive the medicines we use to treat them. In such systems, the competition between host and parasite is often characterized as an evolutionary “arms race”, so the more rapidly one organism can change its method of attack or defense, the better. This has elsewhere been described by the Red Queen Hypothesis. Needless to say, a more effective pathogen or a more resistant host will have an adaptive advantage over its conspecifics, providing the fuel for a selective sweep.

One example comes from the human influenza virus, which has been involved in an adaptive contest with humans for hundreds of years. While antigenic drift (the gradual change of surface antigens) is considered the traditional model for changes in the viral genotype, recent evidence suggests that selective sweeps play an important role as well. In several flu populations, the time to the most recent common ancestor (TMRCA) of “sister” strains, an indication of relatedness, suggested that they had all evolved from a common progenitor within just a few years. Periods of low genetic diversity, presumably resultant from genetic sweeps, gave way to increasing diversity as different strains adapted to their own locales.

A similar case can be found in Toxoplasma gondii, a remarkably potent protozoan parasite capable of infecting warm-blooded animals. Interestingly, T. gondii was recently discovered to exist in only three clonal lineages in all of Europe and North America. In other words, there are only three genetically distinct strains of this parasite in all of the Old World and much of the New World. These three strains are characterized by a single monomorphic version of the gene Chr1a, which emerged at approximately the same time as the three modern clones. It appears then, that a novel genotype emerged containing this form of Chr1a and swept the entire European and North American population of Toxoplasma gondii, bringing with it the rest of its genome via genetic hitchhiking. The South American strains of T. gondii, of which there are far more than exist elsewhere, also carry this allele of Chr1a.