Averting the Approaching Apocalypse

This post is a guest contribution by Dr. Levi Morran, NIH postdoctoral fellow at Indiana University. Levi studies the role that both coevolutionary relationships and mating systems play in shaping evolutionary trajectories. His research using experimental coevolution to test the Red Queen hypothesis recently appeared in Science and was featured on NPR and the BBC.

electron micrograph of the aerobic soil bacterium Pseudomonas fluorescens(photo credit http://bacmap.wishartlab.com/organisms/500)

Electron micrograph of the aerobic soil bacterium Pseudomonas fluorescens
(credit BacMap)

I’ll begin by acknowledging that the title of this entry is probably a bit more dramatic than it needs to be. Nonetheless it’s pretty catchy isn’t it?

Given that the human population seems to have survived that whole 2012 Mayan calendar thing without incident, I know several of my friends (I won’t name names, but you know I love you) that would immediately think about zombies upon reading this title.  However, I am not particularly concerned about the extinction of the human race at the hands of zombies. For one thing, I need more evidence (or in fact any evidence whatsoever) before I buy the whole “zombies will rise up and end us all” fear. Further, Max Brooks (son of Mel Brooks) has given us a hilarious and potentially mildly effective guide to surviving the zombie apocalypse. Ultimately I am far more concerned about bacteria. To avoid inducing mass panic, I’m not talking about a terrified level of concern here, but certainly concerned enough to give it some thought.

Why bacteria? Well, the human population is currently in an evolutionary arms race with many of the bacterial species that infect us.  We continue to hurl scores of antibiotics at bacterial infections, imposing very strong natural selection, with little regard for the evolution of antibiotic resistance in those bacterial populations. Using current strategies in medicine, we are forced to administer greater doses of drugs or develop novel antibiotics to combat infections as the bacteria evolve greater levels of resistance (Levy and Marshall 2004, Martinez et al. 2007). This is a vicious cycle. I believe it is time to develop new strategies of managing our pathogens and treating infections. Thankfully there are many people that agree and are conducting ground-breaking research in this area, like Andrew Read’s group at Penn State University.

A paper by Quan-Guo Zhang and Angus Buckling (2012) takes an experimental evolution approach to begin addressing this issue empirically. In search of a different strategy for curbing the evolution of antibiotic resistance in their experimental populations of the bacterial species Pseudomons flourences, Zhang and Buckling treated their bacterial populations with either antibiotics, a bacteriophage or “phage” (a virus that attacks bacteria), or a combination of the antibiotic and phage. Zhang and Buckling predicted that the combination treatment might be more effective than either antibiotics or phage alone because the combination treatments should better reduce bacterial population sizes and limit their response to selection (Alisky et al. 1998, Chanishvili 2001, Comeau 2007). Additionally, bacterial mutations that confer resistance to antibiotics generally do not also confer resistance to phage, so evolution of resistance to the combination treatments would likely require at least two mutations, and thus require more time to evolve resistance than the other treatments (Chanishvili 2001, Kutateladze 2010). Continue reading

When mummies attack! Why specificity matters for coevolution

Evolutionary change by means of Natural Selection needs a couple of things in order to happen: heritability and variation in fitness. That is, offspring need to resemble their parents at least a little (heritability) and individuals need to differ in their survival and offspring production (fitness). WORDLE Rouchet Vorburger 2012We’ll worry about heritability in another post, but variation is something that seems like it might be hard to maintain. Some forms of Natural Selection will reduce variation as more fit individuals become frequent and all the different kinds of less fit individuals are eliminated from the population. However, there is a force, common in nature, which may maintain variation, parasites.

Interactions between hosts and parasites can generate strong selective pressures on each player, especially if your life depends on infecting a host. Often, biologists make an analogy to an arms race where players are developing bigger and better defenses or weapons. Antagonistic interactions may also generate negative frequency dependence where a rare host type is favored because the parasites are adapted to a common type. You can learn more by checking out CJ’s post on the Red Queen Hypothesis or Jeremy’s post on a different coevolutionary puzzle. A key component for maintaining variation via negative frequency dependent selection is specificity. There must variation in the interaction among different host genotypes and parasite genotypes. This is sometimes referred to as a GxG interaction. If parasites can infect all the hosts, there is no specificity. Specificity allows different hosts to be favored over time depending on the composition of the parasite population.

Theoreticians love to use different models of interactions between hosts and parasites, but without empirical evidence, there seems little point. In a recent paper by Rouchet and Vorburger (2012), the authors looked for evidence of just the kind of genetic specificity would result in the maintenance of genetic variation.

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Introducing the Black Queen Hypothesis

A paper by Morris and colleagues (2012) has generated some stir among biologists. The authors are proposing the Black Queen hypothesis to explain genomic reductions among free living interacting microbes. Rather than rehash arguments that have been made more eloquently, I’d like to just point out some informative ones

Quick summary over at the New Scientist

In depth critique by Robert T. Gonzalez

Tommy Leung also reminded me of a great review paper by Sachs et al (2011) over at TREE that is highly relevant to this debate.

In flour beetles, coevolution mixes things up

A red flour beetle. (Image via Wikimedia Commons.)

When evolutionary biologists think about sex, we often think of parasites, too. That’s not because we’re paranoid about sexually transmitted infections—though I’d like to think that biologists are more rigorous users of safer sex practices than the general population. It’s because coevolution with parasites is thought to be a major evolutionary reason for sexual reproduction.

ResearchBlogging.orgThis is the Red Queen hypothesis, named for the character in Lewis Carroll’s Through the Looking Glass who declares that “it takes all the running you can do to keep in the same place.” Parasite populations are constantly evolving new ways to infest and infect their hosts, the thinking goes. This means that a host individual who mixes her genes with another member of her species is more likely to give birth to offspring that carry new combinations of anti-parasite genes.

But although sex is the, er, sexiest prediction of the Red Queen, it’s not the whole story. What matters to the Red Queen is mixing up genetic material—and there’s more to that than the act of making the beast with two genomes. For instance, in the course of meiosis, the process by which sex cells are formed, chromosomes carrying different alleles for the same genes can “cross over,” breaking up and re-assembling new combinations of those genes. Recombination like this can re-mix the genes of species that reproduce mostly without sex; and the Red Queen implies that coevolution should favor higher rates of recombination even in sexual species.

That’s the case for the red flour beetle, the subject of a study just released online by the open-access journal BMC Evolutionary Biology. In an coevolutionary experiment that pits this worldwide household pest against deadly parasites, the authors show that parasites prompt higher rates of recombination in the beetles, just as the Red Queen predicts.

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How to celebrate Valentine’s Day: A note on the Red Queen and maintaining sexual reproduction

This year's Valentine's card of choice

HAPPY VALENTINE’S DAY! As a perpetually single girl this is my favorite holiday of the year. The first and second half of those statements may appear conflicted. However, every year on Valentine’s Day, I send out glorious amounts of Valentine’s to all my single friends (See below for this years!), eat chocolate and drink red wine while ordering myself sexy lingerie on the internet. Yeah, it’s a pretty awesome holiday. This year, one of my favorite evolutionary biologists has upped the excitement by publishing a paper on what conditions favor sex! Perfect for Valentine’s Day!

Why organisms reproduce sexually is one of the great mysteries in evolutionary biology (I’d like to note that here I’m talking about sex in terms of reproduction. It isn’t a mystery to me why organisms copulate, the differences being if that sex comes with offspring while copulation is just good old fashioned fun). There are a number of theoretical reasons that they shouldn’t! One of the strongest arguments was first laid out by John Maynard Smith (1978) who noted that an asexual female can produce twice as many offspring per individual than a sexual female, who wastes half of her effort producing males. This almost immediately results in sexuals being driven to extinction and is termed “the two-fold cost of males.”

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Notes From the Field: Going to where the snails are, because they are not going to come to me

Lake Gunn in the fiordlands. Lots of tetraploids and triploids in there!

Hello from the land of Kiwis (the fruit, the bird and the people)! As I mentioned in my last post, I’m a coevolution nut, and down here with all the kiwis there is also an excellent system for studying coevolutionary interactions between hosts and parasites. So during the most frigid part of the terrible winter in Washington state, I take off to the sunshine and summer of the southern hemisphere to do my field work! It’s a rough job, I know.

A little over a quarter century ago, Curt Lively, noted this adorable little New Zealand snail (Potamopyrgus antipodarum) has sexual and asexual forms that coexist at varying frequencies in lakes across New Zealand. This variation suggests that there are some environments where it is advantageous to reproduce asexually and some environments where it is better to be sexual.

From then on P. antipodarumhas become an excellent system to study the evolution and maintanence of sexual reproduction, a long standing debate in evolutionary biology (See Maynard Smith 1978, Williams 1975, Bell 1982, Kondershov 1988).

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