Saturday, August 13, 2011

Sex and the Red Queen

You knew sex was good. Now it is shown to fight disease and extinction too.

As studies of evolution progress, we often find papers dotting i's and crossing t's more often than opening new vistas of theory. This is one of those papers, showing with elegant experiments that reproduction via sex has significant advantages for a population of animals faced with pathogens that evolve quickly- as most of them do, and as most populations are.

In alchemy, the queen is typically white, and marries the red king to bring about the union of opposite natures, resulting in the birth of a unified hermaphrodite, the diamond body, the rubido, the golden flower, or the philosopher's stone. In psychological terms, enlightenment is achieved through such unifications when one has found the Jungian Self, or turned into the Nietzschean √úbermensch. Given all this, I have no idea why Lewis Carroll chose the Red Queen as his character who runs and runs, but never gets anywhere- he was apparently more familiar with card play than with mystical esoterica.

At any rate, Carroll's Red Queen symbolizes an important feature of evolution, which is that a great deal may be going on genetically and physiologically over evolutionary time, with little changing on the surface (i.e, in  the fossils or other visible features). Even in the absence of selection and adaptation, evolution continues apace through neutral change, as mutations and other alterations accumulate, leading to the many forensic tools we have today to identify people and trace their lineages.

The most pressing source of non-neutral change, i.e. selection, is typically from microbial pathogens, which we may not think about very much in the developed world, but which pervade the evolutionary setting. Such pathogens evolve quickly, as we have learned from spectacular feats of antibiotic resistance. So, while we as animals possess a sophisticated multi-layer defense with an adaptive immune system, it isn't enough. Plagues of many kinds, and more chronic infections have wrought havok, keeping lifespans low, infant mortality high, and life generally perilous. Our salvation is genetic diversity, accumulated through mutation, kept in ready reserve in our diploid genomes where recessive genes are frequently not expressed, and shuffled continually by sexual reproduction.

For example, some people are resistant to AIDS. They may be infected by HIV, but do not become ill. They have a mutation that, when present in two copies, denies HIV access to the receptor it binds on the cell surface, and that is that.. complete protection. Were the AIDS epidemic to run to completion, those people might be the only ones left.

After several epidemics of this kind, (which cause genetic "sweeps" in the human population), a lot of evolution has happened, raising the proportion of previously rare gene alleles, but for little obvious gain- just to escape the next pathogen, and the next, and the next, in a never-ending arms race with those evil micro-terrorists who would do us ill. That is basically the Red Queen hypothesis.

The role of sex in this scenario is manifold. First, it keeps shuffling the genes around in the population, so that those animals that survive a genetic sweep should still harbor a fair proportion of the population's genetic variation at other loci, (genes), in preparation for the next pathogenic assault or other adaptive crisis.

Second, the shuffling uses the diploid nature of our genomes to "hide" recessive alleles of genes, so that even if such alleles have slightly deleterious effects most of the time, they persist in the population and can come to the rescue when they represent a key solution to an adaptive challenge.

Third, sex recombines genes that may in combination represent adaptive solutions that they do not in isolation. Fourth, the shuffling process deals out especially "bad hands" to a few organisms, which concentrates the bad genetic material and presumably kills it off by selection, counteracting the ever-rising level of mutations in the population at large, the large majority of which are deleterious- a process that goes by the name of Muller's Ratchet.

Very well- the theory behind all this is solid enough, biologists have deduced instances many times, and have tested it explicitly where it is easy to do- among microbes. But the current paper demonstrates the benefit of sex in particularly clear-cut fashon, using the nematode worm C. elegans. This tiny worm has a sexual choice- hermaphroditism and self-fertization, or maleness and obligatory sex, which can be enforced in the lab with appropriate genetic mutations when desired. In the wild, C. elegans are mostly hermaphroditic, with about 20% choosing the single-sex (male) lifestyle which carries the risk of not finding a partner and not reproducing at all. The choice is stochastic, but also a matter of genetics, so worm populations can evolve different rates of hermaphroditism when the trait matters.

The experimenters subjected worms to persistent infection with a bacterial species they typically contend with at much lower levels in the wild. In various experiments, the bacteria were either held constant or allowed to evolve along with the worms, in which case they evolved against each other through thirty worm generations. To maximize bacterial evolution, the bacteria for the next generation were taken from the dead carcasses of worms they had killed in the current generation. To help the worms along, they were mutagenized lightly before beginning, so their populations would have increased genetic variation.

One question was- if worms are genetically prevented from cross-fertilizing, can they keep up with an evolving pathogen? The answer turned out to be... no they can't. Such strains went extinct within about ten generations.

The metric for this work was bacterially-induced mortality rate of worms at the end of the experiment, at either generation ten (to accommodate those worm strains that went extinct shortly thereafter), or at generation thirty. The orginal mortality rate was 20% to 40% for all strains. For the obligately selfing (hermaphroditic) worms, this rose slightly against the non-evolving bacteria (to 40%), and rose dramatically- to 80%- against bacteria allowed to co-evolve, after which these worm strains went extinct.

On the other hand, wild-type worm strains ended up just where they began, no matter what the bacterial regimen- at about 30% mortality. And the obligately outcrossing worms succeeded by generation thirty in lowering their mortality from infection to about 15%, even in the face of co-evolving bacteria. The message is that sex strongly facilitates the rapid evolution that is required to outrun pathogens which have short generation times and rapid rates of evolution themselves.

An interesting extra analysis showed that among the wild-type strains put through this process, their rate of outcrossing increased markedly in response to bacterial infection, ending up at 90% in the face of co-evolving bacteria (see graph). This indicates that not only is sex helpful in staving off infection, but is itself a target of selection in organisms that have a choice in the matter, and is thus a sensitive gauge of sex's benefits.

Rate of outcrossing (sex) among wild-type C. elegans subjected to co-evolution with pathogenic bacteria (solid line), or to non-evolving bacteria (dashed line), or to no bacteria at all (dotted line).


"This increasing political pressure to destroy the foundations of the New Deal is bizarrely paradoxical. The right-wing coalition is on the verge of succeeding in its eighty-year quest to defeat the New Deal, not in spite of, but because it produced three-decades of economic failure and exploding deficits. It is the huge rise in government debt generated by the right-wing model that created the recent financial and political crisis that in turn spawned a wide-spread demand for austerity."

3 comments:

  1. "...it [sex] is shown to fight...and extinction too."

    Headline: Scientists now show a direct link between reproduction and preservation of life.

    Really.

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  2. really hair-splitting - thanks :) don't listen to all the above, there are talking bs.

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  3. Darrell-

    Ha ha.. very good.

    But if you read a bit more closely, you would find that these scientists were able to isolate their variables, by virtue of worms that don't need to have sex to reproduce. Then they can ask what role sex has by itself.

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