Why Should You Care How Bacteria Fight Viruses?

Regular readers have been learning a great deal about the human immune system thanks to our ongoing series on allergies by Julia van Rensburg. But did you know that bacteria have an immune system of sorts, too? Yes, even germs get germs!* Bacteria are susceptible to a group of viruses called bacteriophages, or phages for short. Phages resemble early spacecraft and “land” on the surface of bacteria in order to inject their DNA/RNA, much like a syringe ejects its contents.

Houston, we have a problem! A phage has just injected its DNA into our cell!

Bacteria, which have been on Earth for some 3.5 billion years, have had plenty of time to evolve defense mechanisms against predatory phages. Just like human viruses, phages are a most unwelcomed guest. They barge into the cell unannounced, “borrow” cellular components without asking, and then use them to make baby viruses until the cell becomes so engorged with viral progeny that it explodes, releasing the huge viral family so that it can invade more bacteria and repeat the process all over again. Phages that burst the bacterium like this are called “lytic”, but there are other types that don’t blow the house up. These are referred to as “lysogenic” phages and can insert their genetic material into the bacterial genome, becoming a permanent resident of that bacterium. Even more sinister, the incorporated viral genome is copied like all the other bacterial genes when the bacterium divides, so it is inherited by the daughter cell!

Lytic phages will replicate until they blow the infected bacteria apart. In contrast, lysogenic phages can stick around forever, even getting passed on to future generations since the viral genome was inserted into the bacterial genome.

So that sucks – imagine if you had uninvited viral DNA shoved into your DNA – such viruses basically transform you into a GMO. Sorry to inform you, but up to 8% of your genome is already littered with lots of viral DNA. If you oppose GMOs, I hope you can still stand to be in your own skin!

Presently, we don’t know how to remove foreign DNA from our own. But bacteria have figured out a way to get rid of incoming phage DNA, which provides the basis for a type of bacterial immune system.

 

Some combinations work great together, like chocolate and peanut butter. But getting viral DNA stuck into your own DNA, a strategy used by many viruses including HIV, is not a welcome combination.

In 1987, scientists uncovered unusual repeat sequences in the genome of E. coli bacteria, which were later named “clustered regularly interspaced short palindromic repeats”, or CRISPR. In the early 2000s, scientists identified bacterial proteins interacting with CRISPR sequences (now called CRISPR-associated (Cas) proteins) and discovered that they provide resistance to phage infection. Through the efforts of many laboratories, it is now known that bacteria can use a phage invasion as a vaccination by incorporating some of the foreign DNA between CRISPR repeat sequences. This provides the bacteria with a “catalogue” – a memory system, if you will – of foreign DNA that it can pass along to future generations.

But CRISPR is not just a storage system. The bacteria can retrieve these sequences and hook them to Cas9, a nuclease enzyme that can cut DNA. When foreign DNA enters that bacteria, its CRISPR-Cas9 system can specifically target the invasive element and neutralize it.


Foreign DNA, such as that injected by a phage, can be neutralized by CRISPR/Cas9, which serves as a type of bacterial immune system. Bacteria can store foreign DNA sequences in its genome and express them as crRNAs that bind to Cas9. If the bacterium encounters foreign DNA that matches any of the sequences stored in its CRISPR array, the crRNA will deliver Cas9 to that invading sequence to chop it up.

Pretty clever for tiny bacteria, huh? But here is where things get really interesting, or worrisome, depending on your appetite for paranoia. Scientists have adapted CRISPR/Cas9 to work in all sorts of cell types, including human. Cas9 acts as DNA shears that can cut wherever we tell it to by directing it with a “guide RNA” (analogous to how a crRNA operates in bacteria). This provides us with an unprecedented means to easily “edit” the genome of virtually any living thing, including stem cells and embryos. Furthermore, Cas9 has been modified to do more than just cut DNA; versions exist now that can insert new DNA sequences or switch out bad (mutated) DNA with good DNA.

In the hit TV show, Orphan Black, a group of clones discover that their DNA has been “barcoded” to designate them as intellectual property by their maker. Theoretically, CRISPR technology could have been used to tag DNA in this fashion.

The power of genome editing can be used for good. Several diseases, such as cystic fibrosis and sickle-cell anemia, are caused by a single mutation in one gene. CRISPR/Cas9 is a plausible tool that may be able to repair this defect. However, tinkering with one gene can have unforeseen repercussions on other genes, so this exciting technology could have adverse effects. In March, 2015, a group of scientists proposed a ban on editing the human genome, arguing that a greater understanding of how CRISPR/Cas9 works is required before we even consider applying it clinically.

Gene editing using CRISPR/Cas9 can be used to modify the genome of virtually any creature. One recent application is the creation of wheat that is resistant to a fungus that causes mildew.

Here is a video that shows how CRISPR/Cas9 works and some of the applications it may have down the road:

 

 

Contributed by:  Bill Sullivan

Follow Bill on Twitter.

*It should be noted that not all bacteria are “germs”; in fact, many species of bacteria inhabit our bodies to constitute our “microbiome” and provide important services to us. Learn more about your microbiome here.
 

Sander JD, & Joung JK (2014). CRISPR-Cas systems for editing, regulating and targeting genomes. Nature biotechnology, 32 (4), 347-55 PMID: 24584096

Garneau, J., Dupuis, M., Villion, M., Romero, D., Barrangou, R., Boyaval, P., Fremaux, C., Horvath, P., Magadán, A., & Moineau, S. (2010). The CRISPR/Cas bacterial immune system cleaves bacteriophage and plasmid DNA Nature, 468 (7320), 67-71 DOI: 10.1038/nature09523

Horie, M., Honda, T., Suzuki, Y., Kobayashi, Y., Daito, T., Oshida, T., Ikuta, K., Jern, P., Gojobori, T., Coffin, J., & Tomonaga, K. (2010). Endogenous non-retroviral RNA virus elements in mammalian genomes Nature, 463 (7277), 84-87 DOI: 10.1038/nature08695

Horvath, P., & Barrangou, R. (2010). CRISPR/Cas, the Immune System of Bacteria and Archaea Science, 327 (5962), 167-170 DOI: 10.1126/science.1179555

Baltimore, D., Berg, P., Botchan, M., Carroll, D., Charo, R., Church, G., Corn, J., Daley, G., Doudna, J., Fenner, M., Greely, H., Jinek, M., Martin, G., Penhoet, E., Puck, J., Sternberg, S., Weissman, J., & Yamamoto, K. (2015). A prudent path forward for genomic engineering and germline gene modification Science, 348 (6230), 36-38 DOI: 10.1126/science.aab1028

The Avengers: Is It Possible Someone Could Turn Into A Hulk?

"The Avengers: Age of Ultron" has finally arrived, reuniting fans with their favorite superheroes and introducing them to new ones to cheer on, like Quicksilver.  

 

 

Judging from the never-ending string of successful superhero films, it seems safe to say that we're obsessed with champions of justice who harbor extraordinary abilities. No doubt we've shared this fascination with superpowers since the beginning. Some of us are born faster, stronger, smarter - causing the rest of us to wonder whether we can tap into some hidden superpower within ourselves. We love hearing stories of genius and watching talent shows, just to catch a glimpse of someone crossing the threshold of what we thought was the boundary of human capability.

Stanford biologist Sebastian Alvarado is no exception, but he is endeavoring to put some scientific plausibility behind some of our favorite superheroes. Take the Hulk, for instance. The Hulk is the muscular green beast that scrawny scientist Bruce Banner transforms into whenever he gets enraged.

A lot of scientists can identify with Dr. Banner’s plight, and I have seen many undergo an analogous transformation while reading their grant reviews.

 

How did Dr. Banner gain this blessing and curse? As a scientist, he was researching how people summoned these unusual bursts of strength. Using himself as a guinea pig, he exposed himself to gamma radiation in an attempt to become stronger. There was no noticeable effect at first, but when Dr. Banner got angry, his skin turned green and his muscles burst out of his shirt. Since Dr. Banner is a good guy, the Hulk is generally a good beast, although somewhat messy. When the anger subsides, Dr. Banner returns to his modest, wimpy self and heads to the store to buy new clothes.

The comic book tale prompted Dr. Alvarado to wonder:  is this even remotely possible? He addresses the question in the video below.

 

Let’s clarify a few of these points for those who might be less familiar with the concepts. First, gamma radiation blasts your DNA (chromosomes) apart. As Dr. Alvarado mentioned, there are enzymes that will “heal” the DNA, but it doesn’t always heal correctly, which might result in new genes (and the loss of other genes). Second, we are learning more and more that genes are regulated in a surprising number of ways. They are not merely binary switches that turn on and off, but rather they are controlled more like volume knobs. Epigenetics refers to the factors in your cells that have their fingers on those volume knobs.

We discussed epigenetics in a previous article covering Ozzy Osbourne’s genome; in the case of the Hulk, epigenetics provides an attractive means to account for how Dr. Banner can switch between Hulk and normal guy. Between transformations, Dr. Banner’s genes are not changing, but which ones are active – and the degree they are active – is changing. For example, epigenetic factors can crank up genes controlling muscle development when they receive a signal in the form of a stress hormone that increases during temper tantrums. As this hormone subsides, other epigenetic factors return the volume of those genes to their normal level. You can think of genes as the selection of music, but epigenetic factors are the DJs.

 

So what kinds of epigenetic factors are there? We are discovering a dizzying array of cellular components that can alter gene expression, which can result in changes in physical appearance, behavior, mental abilities, and more. It has long been known that chemical modification (i.e. methylation, delivered by enzymes called DNMTs – DNA methyltransferases) of DNA itself can shut down genes. DNA methylation marks are like orange construction cones blocking the highway. Scientists then discovered that histone proteins, which congregate in bundles of 8 to form nucleosomes, could also be chemically modified in several different ways. The nucleosomes give DNA the “beads on a string" appearance shown below.

This is your DNA, not a pearl necklace! The DNA "string" wraps around the "beads", which are nucleosomes composed of 8 histone proteins. Once thought to merely help package DNA, we now know these nucleosomes are major contributors to the regulation of genes on the DNA.

These proteins were long thought to be just scaffolding components for the DNA, but now we know they play a major role in directing the activity level of nearby genes. Numerous chemical modifications, such as acetylation, methylation, phosphorylation (and more), can take place on multiple places of each histone protein. These may alter the binding between nucleosomes and DNA, making certain genes more accessible, or these modifications may form a cellular “code” that can affect gene expression levels.

Histones can also be moved, replaced, or evicted by epigenetic factors called SWI/SNF ATPases. As the name implies, these enzymes require energy from ATP to affect gene expression. More recently, it has also been found that small non-coding RNA molecules can regulate genes.

A summary of the major epigenetic factors that can regulate the "volume" of gene expression.

While these complex methods a cell employs to influence gene expression offer a potential explanation for how someone could temporarily become a Hulk, it is by no means probable. Most massive gamma radiation doses would destroy genes that are essential to survival. But it is fun to use cutting-edge science to put just a tiny hint of credence behind the superpowers. And even more fun to think that with enough knowledge we may be able to modulate epigenetic factors to treat disease or maximize human potential.

Contributed by:  Bill Sullivan

Follow Bill on Twitter.

Falkenberg KJ, & Johnstone RW (2014). Histone deacetylases and their inhibitors in cancer, neurological diseases and immune disorders. Nature reviews. Drug discovery PMID: 25131830

Haggarty P, Hoad G, Harris SE, Starr JM, Fox HC, Deary IJ, & Whalley LJ (2010). Human intelligence and polymorphisms in the DNA methyltransferase genes involved in epigenetic marking. PloS one, 5 (6) PMID: 20593030

The Last Man And Woman On Earth – Can Two People Repopulate The Planet?

Imagine a virus wipes out everyone on the planet except you. You are free to roam the world and do whatever you please, all in the comfort of your pajamas. No more rules and regulations. No more 9 to 5. You can pick whatever house you want, fill it with priceless artwork, and drive your favorite sports car as fast as you want. That is the concept behind the new television show, “The Last Man on Earth”.

The novelty of being so free does wear off for our protagonist, who soon suffers a level of loneliness that drives him to a suicide attempt. But just before he extinguishes the last XY chromosomes on the planet, he finds the last woman on Earth. A woman who wastes no time in eroding his freedoms, insisting that they use correct grammar and still stop at stop signs.

The lone pair faces the inevitable question:  can they repopulate the Earth? To do so, their children would have to mate with one another, or mom and dad, in order to rebuild the human race. All the incestuous taboos aside, is this even genetically possible?

If just one man and one woman are left to repopulate Earth, then their “family tree” would look more like a family pole.

Inbreeding has unfortunate genetic consequences due to the increased inheritance of recessive genes, which can result in neonatal death. Inbred children that survive are at increased risk of congenital birth defects, reduced fertility, smaller size, immune deficiencies, cystic fibrosis, and more. These defects are also likely to be passed on to their children as well.



If you’ve ever seen The Jerry Springer Show, you know what happens when two closely related individuals start dating. A whole bunch of pushing and shoving! While the show frequently pokes fun at incestuous relationships, it doesn’t emphasize the catastrophic consequences that may befall inbred children.

Some real-life examples of the consequences of inbreeding can be found in places where there are restricted breeding opportunities – for example, within monarchies, islanders, or closed societies. Hemophilia was notoriously prevalent in European royal families. Some Amish societies have a larger number of children born with extra digits on their hands or feet. Jews of Eastern European descent tend to have higher rates of a number of genetic diseases, including cystic fibrosis.

To understand why children of incestuous mating are often plagued by these rare diseases and disorders, we need to review some genetics. For each gene in our 46 chromosomes, we actually possess two copies called alleles – one came from mom, the other from dad. Alleles can be dominant or recessive, the former being expressed while the latter is not. So if you have a bad gene, it could be masked if you have a dominant allele; in other words, you would not exhibit that trait but you would be a carrier. If you mate with someone who also has a recessive allele for that gene, there is a chance your child will be born with two copies of the recessive allele. Such a child would exhibit that gene defect.

Dominant and recessive alleles at work. As a simple example, pretend the trait under study here is lactose intolerance and the bad allele is shown in yellow (the good allele is green). In this example, mom and dad are heterozygous for this lactose intolerance gene – they have one good allele and one bad. Consequently, they can enjoy all the dairy they want because they are only carriers. Their children get one allele from mom and one from dad and can be unaffected (hitting the lottery and getting two good alleles), carriers like mom and dad, or lactose intolerant (losing the lottery and getting both bad alleles).

 The net result of inbreeding is that the resulting population loses a diverse genetic portfolio, which means they are less resistant to rare diseases and deformities. The smaller the gene pool, the faster it gets dirty. Such individuals would also have less diverse immune systems, making it much easier for a single germ to wipe them all out. That would be an ironic twist of fate since there was something peculiar in the genomes of the last man and woman that kept them alive during the mass extinction!

In addition to the genetic landmines, the family would likely have a very difficult time overcoming the innate resistance most species have against inbreeding. Evolution knows that inbreeding is not good for the species, so it engineered a built-in “incest taboo” that creates a strong aversion to such behavior. A devil’s advocate, however, could argue that the biological barrier to familial sex could be overcome through artificial insemination.

What about using a sperm bank? Sperm is stored in liquid nitrogen, so it would stay frozen for a short time after the power goes out. However, you’d have to act fast because no one is around to monitor the storage tanks and top off the liquid nitrogen as it evaporates.

There are practical concerns to consider as well. The last man and woman, as well as their kids, would need to have large numbers of children and, unless one of the founders happens to be a doctor, it is hard to imagine many of these babies surviving in such a world. Even if they (and mom) survive childbirth, there are countless opportunities for them to perish in this type of environment before reaching childrearing age.

Considering the collective evidence, it seems virtually impossible that just two people could repopulate the planet. But that doesn’t make The Last Man on Earth any less fun to watch.

How many people are required to sustain a human population is an intriguing question that has not been settled. One study estimates that only 70 people who crossed the Bering land bridge 14,000 years ago successfully populated North America.

 

Contributed by:  Bill Sullivan
Follow Bill on Twitter.

Alkuraya FS (2012). Discovery of rare homozygous mutations from studies of consanguineous pedigrees. Current protocols in human genetics / editorial board, Jonathan L. Haines ... [et al.], Chapter 6 PMID: 23074070

Hey J (2005). On the number of New World founders: a population genetic portrait of the peopling of the Americas. PLoS biology, 3 (6) PMID: 15898833

2-7 Offsuit: Is Cancer Just "Bad Luck"?

There are many forms of cancer that ravage the body, but the key feature they share is uncontrolled cell growth. Virtually any cell type can suddenly go rogue and start reproducing itself again and again – this is what we call a tumor. Some of these rogue cells venture to other parts of the body where they don’t belong and establish a new colony there – this is called metastasis. As cancerous tumors grow and spread around, they can do a number of things that endanger the life of the patient, such as interfere with organ function and steal nutrients from other cells or tissues.



This cartoon illustrates a general model for the development of cancer. A "benign" tumor is not considered cancerous because they do not invade other parts of the body. In contrast, "malignant" tumors, like that ugly looking thing above, are cancerous because they invade nearby tissues. If cells from a malignant tumor get into the bloodstream, they can establish life-threatening satellite tumors elsewhere in the body, making them all the more challenging to eliminate.

Cancer is caused by a change, or mutation, in one of our cell’s DNA. Our DNA contains tens of thousands of genes that encode proteins that make our cells tick. Some of these proteins regulate cell division, but they are normally shut off after the job is done. A mutation that turns one (or more) of these regulatory proteins back on can turn that cell into a Xerox machine stuck on "copy". Since there are so many different types of genes that can mutate in a wide variety of cell types, a “one size fits all” cure is very difficult to conceive.

Scientists (and many pseudoscientists) have long been trying to identify things in our environment that cause mutations that lead to cancer. Others have argued that cancer is just “bad luck” and that our genes play a larger role. This is important to sort out:  should we invest more money to identify potential carcinogens in the environment or to find ways to repair “bad” genes?

Every now and then, someone gets lung cancer who never took a single puff on a cigarette. Why? To understand the answer, consider poker. You can study dozens of books on how to play to win, practice for 10,000 hours, pay hundreds of dollars to learn all the secrets from the professional players. But none of this will help you if the dealer gives you junk cards. To look at this another way, there are some people who start chain smoking at twelve and live to be 90 with no trace of cancer (perhaps breathing through a tube in their throat, but no cancer). That’s like a rookie at the poker table being dealt a straight flush. Long story short:  cancer is not always the patient’s fault, and a lack of cancer is not always indicative of a healthy lifestyle.

In Texas Hold’em poker, you begin with just two cards. Being dealt a 2 and 7, offsuit, is considered the worst possible hand you can get. In contrast, being dealt two aces is one of the best starting hands. The genes that combined to form your DNA are analogous to the cards you would be dealt at a poker table. Unlike the poker game, though, you can’t win by bluffing.

Researchers have found plenty of environmental agents that can mutate DNA. For example, exposure to UV radiation is one of the more notorious risk factors for skin cancer. But there are a few people who worship the sun and never get skin cancer. In addition, most children have not had extensive exposure to environmental carcinogens, yet, tragically, they can still get cancer. In 2014, it was estimated that 15,780 children and adolescents ages 0 to 19 years would be diagnosed with cancer and nearly 2,000 would not survive. Facts such as these support the notion that cancer is largely due to bad genes, not necessarily the environment.

Scientists at Johns Hopkins recently set out to tackle the question by constructing mathematical models of the disease. Their findings might take you by surprise:  in the majority of cases, the reason why a cell starts running all the red lights is due to a random mutation that occurs during cell division. In other words, lifestyle choices and even your genetic makeup play a lesser role in your chances in getting cancer. Let that sink in for a moment: RANDOM mutation - not mutation caused by UV light, engine exhaust, or some other carcinogen. Since the mutation appears to be a random mistake made by cell division enzymes, the authors dubbed this "bad luck".

DNA replication is a complex process in which the two strands are separated and used as a template to make a complementary second strand. But replication enzymes are not perfect (if they were, there'd be no evolution) and sometimes insert the wrong DNA base, causing a mutation.

 

This new study reminds us that every cell division contains an inherent risk that the daughter cell acquires a mutation that makes it divide like gangbusters. This doesn’t mean you should grab a carton of Marlboros to smoke as you suntan on the beach while devouring a couple extra-charred burgers for lunch.

Highlighted in this study was the finding that not all cell types give rise to cancer equally. Not surprisingly, tissues with a higher number of stem cell divisions are more prone to cancer, which explains why we don’t hear a lot about duodenum cancer. Importantly, the researchers identified several types of cancer that are influenced more by our lifestyle choices or inherited mutations: colon cancer, basal cell carcinoma, and lung cancer.

The findings essentially assert that since cells divide they are veritable time bombs. Somewhere down the line a mistake is going to happen regardless of environmental insults, and if that mistake occurs in the wrong gene, cancer can ensue. These are noncontroversial statements and not news to most people. However, the idea that "most" cancers are due to "bad luck" is a more controversial conclusion. A major limitation is that the model did not incorporate some of the most common cancers, such as breast and prostate cancer, because the frequency of stem cell divisions is unclear. Readers would be wise to check out this article by David Gorski at Science-Based Medicine, which provides detailed insight into the strengths and weaknesses of the experimental design. The World Health Organization was so opposed to the message this study sends that they issued a press release critical of the study.



Obi-Wan (Ben) Kenobi famously said, “In my experience, there’s no such thing as luck.” Some scientists who take issue with the Hopkins study would agree with Ben. 

 

At the end of the day, since we don’t yet know how all genes operate, much less which ones you might have in your DNA, it is wise to take common sense steps to minimize your exposure to known carcinogens and take advantage of tests designed to detect cancer at its earliest stage. Bad luck may be a major factor in cancer, but there are plenty of simple lifestyle changes you can make to try and beat the odds.

Contributed by: Bill Sullivan

Follow Bill on Twitter.





Tomasetti, C., & Vogelstein, B. (2015). Variation in cancer risk among tissues can be explained by the number of stem cell divisions Science, 347 (6217), 78-81 DOI: 10.1126/science.1260825

Ward E, DeSantis C, Robbins A, Kohler B, & Jemal A (2014). Childhood and adolescent cancer statistics, 2014. CA: a cancer journal for clinicians, 64 (2), 83-103 PMID: 24488779

The Friday Five

Highlighting some of the coolest science news we’ve seen lately.

1. Paleo, Atkins, raw, juice...diets, diets, diets! Sort the fact from the fiction with this excellent article, “10 Fad Diets, Debunked”, by Esther Inglis-Arkell.

2. The new film odyssey, Interstellar, blasted into theatres recently. Director Christopher Nolan went to great lengths to try and get the science right in the movie, which included consultation with theoretical physicist Kip Thorne. The video below details how they worked together to imagine a real black hole.

Unfortunately, not all of the science in the movie is accurate. 

3. In this week's episode of “The Big Question”, Craig Benzine explains why your voice gets higher when you inhale helium. Interestingly, it is not the pitch that changes…

4. Here, kitty kitty…what’s the difference between a wildcat and a domesticated one? Nothing – they both hate you. Jokes aside, scientists have recently performed a genetic comparison between the two and found a number of genes that were enriched due to domestication. These genes may explain why your housecat is less shy, tamer, and more responsive to a reward. Interpreted another way, they also explain why we can't really stroll through the woods with tigers.

5. Our ongoing coverage of new species named after celebrities converged with another subject that constantly fascinates us: Ozzy Osbourne. A new species of frog was recently found in Brazil and named Dendropsophus ozzyi. The males have a bat-like mating call, which reminded the researchers of the infamous concert when Ozzy bit the head off a bat during the show.

Scientists named this new species of frog after Ozzy because it makes a bat-like noise, which reminded them of Ozzy's strange stage diet in the 1980s.

Science quote of the week:

"Science fiction has become science fact today - Hollywood is good, but Rosetta is better" –Dr. David Parker, in reference to the first time humans have landed a probe on a comet.

Contributed by:  Bill Sullivan

Follow Bill on Twitter: @wjsullivan

ORRICO, V., PELOSO, P., STURARO, M., SILVA-FILHO, H., NECKEL-OLIVEIRA, S., GORDO, M., FAIVOVICH, J., & HADDAD, C. (2014). A new “Bat-Voiced” species of Dendropsophus Fitzinger, 1843 (Anura, Hylidae) from the Amazon Basin, Brazil Zootaxa, 3881 (4) DOI: 10.11646/zootaxa.3881.4.3

  Montague, M., Li, G., Gandolfi, B., Khan, R., Aken, B., Searle, S., Minx, P., Hillier, L., Koboldt, D., Davis, B., Driscoll, C., Barr, C., Blackistone, K., Quilez, J., Lorente-Galdos, B., Marques-Bonet, T., Alkan, C., Thomas, G., Hahn, M., Menotti-Raymond, M., O'Brien, S., Wilson, R., Lyons, L., Murphy, W., & Warren, W. (2014). Comparative analysis of the domestic cat genome reveals genetic signatures underlying feline biology and domestication Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1410083111