Sunday, April 24, 2016

Ebola, evolution, and the role of phylogenies

   As an evolutionary biologist I love phylogenies. Or, I love phylogenies so I became an evolutionary biologist. Except I did not always know what a phylogeny was. A phylogeny is similar to a family tree except we usually have to infer who the ancestors were. To do this, we collect and study present-day organisms and use the data to reconstruct what happened millions of years ago. In the end we are left with an illustration of relationships, a phylogeny, which gives us an idea of who is more closely related to whom. Are beetles more closely related to ants or cockroaches? Ants. Are pine trees more closely related to kelp or moss? Moss. In fact, there is a huge push to understand what is called the Tree of Life – a phylogeny that includes all the major extant (living) organismal lineages. An updated version was recently published and did a nice job of putting humans in our place. We are not as big of a deal as we like to think, evolutionarily speaking - we are merely a small nubbin on the twig of ‘Eukaryotes’.

Recently published phylogeny of the Tree of Life (Hug et al., 2016)


   Understanding phylogenetic relationships gives us a foundation to test hypotheses and tease out answers about how our world works. The information we retrieve tells us that birds are actually dinosaurs, neanderthals exchanged genes with humans, and that immune systems can evolve under pressure from viruses and other pathogens.
 
   That's where we are going today - using phylogenies to unpack the evolutionary dynamics of one-up-man-ship between a virus and the host immune system. The plan is to delve deep into stuff that will make you say, wow. First, a little background. I am not going into great detail about Ebola as this has been extensively covered. Briefly, Ebola is a filamentous virus, categorized as a filovirus, which infects humans (and other mammals) resulting in viral hemorrhagic fevers that can be fatal. In addition to researching treatment and vaccine options for Ebola, scientists are conducting research to understand how the virus ‘jumps’ from a non-human host to humans (a.k.a a spillover event). To do that, we first need to figure out what that non-human host is. Based on data showing remnants of filoviruses in fruit-eating bats and insect-eating bats, many researchers are focusing on bats as a likely reservoir. Of course there are many questions making this is an active area of research. One of those questions, investigated by Ng et al. (2015), is: how come bats do not seem susceptible to Ebola and other filoviruses? The short, brief, and tractable answer of the Ng et al. study is that some bats have a genetic mutation in the protein filoviruses use to bind to and infect the bat’s cells. It would be like playing a game of Pac-Man where the ghosts have gone wild and the only way Pac-Man can stay alive is by having a mutation that prevents the ghosts from recognizing him. Did I just date myself? Or, let’s try this analogy. Hackers recently found a way to generate ‘universal keys’ allowing them to break into cars with keyless entry systems. This would be our virus hacker, breaking into cars using sophisticated biochemical tactics. But, let’s say there is one type of car (the new Tesla Model 3?) that has changed its keyless entry system so it is no longer susceptible to hacking. The virus hacker tries its key against the lock but it doesn’t work. Now, of course, people with the resistant car are more likely to keep their cars in this hacking environment, and the desire to have this car spreads and they become more and more frequent. Tough luck for the hacker.

   And this is what Ng et al. (2015) found. The mutation that prevents Ebola virus from attaching to the bat’s cells is under positive selection, meaning that its frequency in the population increased rapidly relative to other genes. When researchers find evidence of positive selection it lends further support to the idea that something important is happening. Interestingly, Ng et al. found that not all bat species had the mutation.

   In walks the bat phylogeny. Bats are a diverse group of flying mammals with over 1200 species that live in many different habitats all over the world. 


Townsend's big-eared bat (Corynorhinus townsendii), a nice picture of a bat but not one included in the study by Ng et al. (Image Credit)


   If we want to understand disease dynamics, and how and why some bats have the mutation and others don’t, we can use the phylogeny as a forensic tool to reconstruct how resistance evolved. We can ask questions like: are bats with the mutation close relatives? Did they inherit it from an ancestor? Or, did the mutation evolve multiple times in different bat species? And, due to the innate curiosity of humans, there are phylogenies for lots of different animal groups, including bats. In 2007, Miller-Butterworth and colleagues put together an impressive genetic data set (over 11,000 base pairs of DNA) to essentially go back in time and figure out how different types of bats are related to each other. It is important to point out here that our estimates of phylogenetic relationships are just that, estimates. The reason is that any given phylogeny is a hypothesis, our best estimate given the data available. For a long time, biologists relied on morphological data to figure out who was most closely related to whom. Evidence of close relationship was inferred if two species shared a trait that was presumably inherited from a common ancestor. But you can imagine how this might lead to spurious relationships. Just because both bats and birds have wings, it doesn’t mean they are close relatives. We know from studying the morphology underlying bird and bat wings that these animals did not inherit wings from a common ancestor, they each evolved them independently; thus, it would be a mistake to infer they are close relatives because they both have wings. In the past few decades, researchers have been able to rely more heavily on genetic data to infer relationships. And while these data have their own set of problems, they provide an additional perspective to consider alongside morphological data. As new technologies are developed and our ability to accumulate more data improves, our phylogenies are continuously updated and remain dynamic. This may frustrate many students in introductory biology classes, but underscores the importance of understanding the process and history of scientific data collection and hypothesis testing.
  
   Back to bats. What Ng and colleagues found when they combined their data with the bat phylogeny was that the mutation preventing filovirus infection evolved once about 25 million years ago, and was passed down to descendant lineages. The researchers interpreted these results as evidence that bats have been evolving alongside filoviruses, and for much longer than previously thought. But what do we know about hackers that we should be thinking about here? They evolve too. Just because the Model 3 may be resistant to hackers now, the hackers are not going to sit idle and find another means of making a living. The hacker tactics will shift and evolve to overcome new car-locking technology. And Ng et al. found evidence of this happening between bats and Ebola too. What they see is multiple mutations with a signature of positive selection, which they suggest results from counter-attacks by the virus. In other words, the virus sees the bat mutation and raises it a mutation of its own. It is an arms race. Between the virus and its host. Between the hacker and the car company. It is known as the Red Queen Hypothesis (coined by Leigh Van Valen in 1973) and that is a story for another day.


“Now, HERE, you see, it takes all the running YOU can do, to keep in the same place.” (Lewis Carroll, Through the Looking Glass)


  P.S. There is a nice podcast on TWiEVO discussing the Ng et al. (2015) paper with some of the authors of the paper.

Monday, April 11, 2016

Schrank update

 
“Franz von Paula Schrank was a polymath in the true sense of the word.” (Edmund Launert)1

   We first met Schrank back in February during our discussion of parasites and swellable microneedle adhesives. I promised to come back to Schrank, and I think about him often enough that I did not forget. Instead of going to the Stanford library, I submitted an interlibrary loan request for Annette Zimmerman’s biography on Schrank (Franz von Paula Schrank (1747-1835): Naturforscher zwischen Aufklärung und romantic) and got it over a month ago. 
 
Title page and table of contents of Zimmerman's biography on Schrank

And there it is, sitting on my kitchen table, filled with rich information on Schrank that I cannot read because I don’t speak German. It calls out to me on a daily basis (in English), taunting me to read it and learn about Schrank. I will have to return it soon, unread. (I am open to ideas for efficient translation to English if you have them.) Several of his publications have also been scanned by the Biodiversity Heritage Library and are available to read. But not only are these also in German, the intricate script makes it difficult to decipher the letters.

   I did track down a review of Zimmerman’s biography, written in English by Edmund Launert, by means of a second Interlibrary Loan (thank you UCSD library).1 It is not a substitute by any means, but in combination with what is available from the Catholic Encyclopedia2, it gives us a little information to fly with. Schrank was by all means a polymath, someone who is skilled and experienced in a variety of different subjects (not just math – the Greek translation of polymath is “having learned much”, derived from the Greek work ‘manthánein’). Schrank is on the list of Roman Catholic cleric-scientists and fulfilled the duties of priest, and professor of botany and zoology. While this may sound unusual to us given current perspectives of an oft-assumed divide between science and religion, this was very common historically, especially in 1774 when Schrank was ordained as a priest. Other notables on the list you may be familiar with are Nicolas Copernicus (mathematics and astronomy - a few hundred years before Schrank) and Gregor Mendel (genetics – a hundred years after Schrank). At age 15, Schrank became a Jesuit, a Catholic religious order well known for many things (good and bad) including its contributions to science. He was born into a well-respected family in Munich, Germany and started his studies at the Jesuit College of Passau when he was 9. 

   His interest in the natural world was inspired by Father Sluha (Szluha is the native spelling) when Schrank was a budding Jesuit student at Oedenburg collegium in Hungary. Father Sluha had been a Jesuit missionary in Brazil until he was put in a Portuguese jail when the Jesuits were expunged from Brazil in 1759.2 There is very little known about Sluha, and another source (McKinley3) (who, unlike me, could read German) reported that Zimmerman was not able to find much out about Sluha when she was working on her biography of Schrank. Presumably, the idea is that as a missionary in Amazonian Brazil (1753-1760) Sluha acquired or refined his skills as a naturalist (probably focusing on botany), and opened the door of natural history to the young Schrank. Just to be clear, I am speculating on this, but the idea is not that far-fetched.

   Schrank was a Jesuit teacher until 1773 when the Jesuit religious order was repressed and abolished by Pope Clement XIV, the year before Schrank was ordained and received his doctorate in Theology. He started his academic career as a professor of mathematics and physics in 1776 at Amberg, and was a professor of botany, agriculture, mining, forestry, and zoology at the University of Ingolstadt starting in 1784 (with a subsequent move to Landshut). Schrank published his first scientific study (of over 40) during this time (Beiträge zur Naturgeschichte - Contributions to Natural History - in 1776). While many of Schrank’s publications were focused on natural history (botanical, zoological, and entomological), he also published papers in the fields of physics, chemistry, agriculture, travel, geology, mineralogy, theology, and poetry. Are you kidding me? And, all of this is inaccessible to me because it is written in German. An unfortunate outcome of growing up monolingual.

   In 1809, Schrank was appointed to be the first Director of the München Botanical Garden. His influence there was described by Stafleu & Cowan4 as follows: “The present state of the garden bears out Schrank’s early and wise judgment” (p. 325). In the same text, he was attributed with the quote: “Man würde weniger deräsonniren, wenn mann über nichts räsonnirete, was man nicht versteht.” When I type this into Google translate I get “One would less deräsonniren when man räsonnirete about anything you do not understand”, which is not super helpful. So I asked a German colleague of mine for help and he responded: “Hehe, ‘deräsonieren’ appears to be a word creation by the author. It means: the opposite of ‘reasoning’, maybe unreasoning, or reasoning in an unreasonable way.” I love this, he made up a word for something I can barely wrap my head around. But, essentially, he seems to be saying: One would have fewer mental issues if one did not speculate on things one does not know about. Seems like a good idea. My German colleague referred me to a similar quote from a German comedian, Dieter Nuhr: "Wenn man keine Ahnung hat, einfach mal die Fresse halten". Consider to shut up if you don’t know what’s going on. I don't think there is a moral here but thought it was interesting.......


München Botanical Garden (Image credit: Diego Delso)


1E.E. Launert (1983) Natruforscher zwischen Aufklarun und romantic. Archives of Natural History 11: 362-363.
3 J. Stein (1912) Franz Paula von Schrank. In The Catholic Encyclopedia. New York: Appleton Company. Retrieved April 7, 2016 from New Advent:
2 D. McKinley (1992) Adrien Lebreton, S.J. (1662-1736): A search for the identity of a neglected botanist in early Martinique. Huntia 8: 155-162.