Bioluminescence- the immense diversity of organisms that glow

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Bioluminescence is a beautiful evolutionary phenomenon which has aided organisms in defending against predators, attracting mates, attracting prey, communicating, and even coping with metabolic stress. A ton of groups contain bioluminescent members (fungi, echinoderms, cnidarians, the list goes on and on) including some real evolutionary stand-outs.

In most cases (but not all!), bioluminescence results from enzyme-catalyzed oxidation of luciferins—light-emitting compounds—by luciferases. There can be many different luciferase compounds used even in closely related species.

New luciferin found in glowworms

A newly identified luciferin was discovered in caves in New Zealand (because of course it would be in a cave in New Zealand) in glowworms. This luciferin uses Xanthurenic acid and tyrosine as the two precursors to the glow. This particular glowworm is Arachnocampa luminosa, a species of fungus gnat that, in its larval stage, produces sticky threads by building a long muscousy tube and moving along the tube sort of vomiting up little sticky threads to trap insect prey. How disgustingly beautiful nature can be!

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Waitomo caves 

Glowworms are not really worms, but rather, larvae of several families of beetle and fungus gnat–however the bioluminescence is not homologous among the groups (so it’s arisen independently many times over). While it’s not always just the larvae that glows, the larvae emits the brightest blue-green glow. The glow helps the glowworms attract insects, attract mates, and protects them from predation (it also inspired James Cameron to make the blockbuster hit, Pocahontas with Glowworms Avatar).

Deep sea Cephalopods like to flash each other

An especially cool evolutionary example is of a deep-sea octopus, whose “suckers,” which still retain a sucker appearance and sucker-like traits, have had many of their muscle cells replaced with light producing cells. Researchers suspect this may have occurred as the result of once being a shallow-water bottom dwelling octopus, and moving to a deep open-ocean environment where suckers were less necessary.

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Deep sea octopus

Now it appears, the octopi use these glowing suckers for communicating to one another via visual signaling. They may also be using them for attracting a favorite prey item of theirs—copepods (small crustaceans). This is an unusual prey item for an octopus, but the copepods are attracted to the bioluminescence.

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Toyama Bay, Japan where firefly squid gather to spawn. Credit: Brian J. Skerry/Getty Images

For a more flashy light show, I’d recommend the firefly squid. Their deep blue lights (produced by photophores) are used for communicating with mates and perhaps rival squid. The light can also be used to break up the body pattern to confuse predators and attract small fish to prey on (because deep sea fish just cannot seem to learn which glowing lights mean danger). The really cool thing about firefly squid though, is not so much the light they produce, but the evolution of their eye that seemed to come with it. They are thought to be one of the only cephalopods to have color vision (by the way, cephalopod eyes: a fascinating topic). They have three visual pigments while other cephalopods only have one, and this may be so they can better distinguish ambient light from bioluminescent light, and perhaps because their light color is pretty unique from other bioluminescence emitted in the deep sea.

Symbiotic bacteria- lux operon helps flashlight fish and bobtail squid

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In perhaps the most of obvious function of bioluminescence: The flashlight fish (Anomalous katoptron in this case, though there are several species), produces light using symbiotic bacteria. The fish’s light organs are located under it’s eyes so it can turn the light on and off by blinking. These organs are packed with bioluminescent bacteria to produce a greenish-blue light. Researchers found that the fish blink less (meaning their organs are open) in the presence of their planktonic prey indicating they use their bioluminescence for finding prey.

 

Quorum sensing and a beautiful tale of symbiosis

One of my favorite bioluminescent evolutionary excerpts is that of Vibrio fischeri and Euprymna scollops (the Hawaiian bobtail squid). V. fischeri is a symbiotic bacterium that produces bioluminescence through the lux operon (which involves another luciferase oxidizing a compound to produce blue-green light). The Vibrio interact with the squid (using type IV pili) which starts the maturation of light organs in the squid. These bacteria help the squid conceal its shadow while its foraging for food under the moonlight. This protects the squid from predators while providing the bacteria with a stable home. 980x

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simplified diagram of lux operon at low and high cell density

What makes this bacterium especially notable, is that it was one of the first bacteria to be discovered to use quorum sensing. Quorum sensing is a gene expression regulation tool (often called “bacteria communication” and totally going to be on your exam tomorrow) where the Vibrio’s gene expression responds to changes in bacteria cell density. A signal molecule- N-acylhomoserine lactone (AHL), is synthesized by LuxI (a protein produced by the lux operon I mentioned earlier) and leaves the bacteria cells. LuxR forms a complex with AHL and binds the lux box causing the activation of luminescence genes. The bacteria colonize the squid’s light organ at a very high density producing lots of this AHL molecule.

Millipedes: Glow first used for coping with climate, co-opted for warning signal

 

If you’re ever in California, be on the lookout for the Motyxia millipedes. They’re pretty easy to spot as they emit a teal glow from their entire body. They also produce poison cyanide which many other millipedes do as well. Instead of concentrating their glow to one light organ and instead of emitting light from a luciferase reaction, they glow all over their exoskeleton using a photoprotein whose homology is unknown.

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M. sequoiae (left), M. bistipita (right)

But what’s REALLY cool about the Mytoxia is that for a while it was thought that bioluminescence evolved in the millipedes as a way to warn predators. However, when researchers discovered that another species (previously Xystocheir bistipita, now reclassified as Mytoxia bistipita) glows, but much more faintly, they looked more into it.

They found that Mytoxia may have actually evolved to cope with hot, dry climates (this species is found in the Sierra Nevada Mountains). The glow of M. bistipita is much less intense and they also have fewer predators than other species. Millepedes have difficulty metabolizing oxygen in hot, dry climates which creates toxic byproducts (like peroxide). Their bioluminescent photoprotein actually helps to neutralize these toxic byproducts. The researchers concluded that the millipedes colonized higher elevations more recently than the bioluminescence evolved, and that with that colonization came more predation. Only then did they co-opt the trait for warning predators of their poison cyanide production. The brighter the millipede, the more cyanide it contained!

Sources:

  1. Paul E. Marek, Wendy Moore. Discovery of a glowing millipede in California and the gradual evolution of bioluminescence in Diplopoda. Proceedings of the National Academy of Sciences, 2015.
  2. Jens Hellinger, Peter Jägers, Marcel Donner, Franziska Sutt, Melanie D. Mark, Budiono Senen, Ralph Tollrian, Stefan Herlitze. The Flashlight Fish Anomalops katoptron Uses Bioluminescent Light to Detect Prey in the Dark. PLOS ONE, 2017.
  3. Quorum Sensing in the Squid-Vibrio Symbiosis. Subhash C. Verma and  Tim Miyashiro. Int J Mol Sci. 2013 Aug.
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Cryptococcus — (r)evolutionary genius

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If you’re in the market for a particularly fun fungus to tell your friends about, I’d suggest the pathogenic yeast, Cryptococcus.

Cryptococcus is primarily known for causing Cryptococcal meningitis—a leading cause of death in AIDS patients. This is Cryptococcus neoformans, which, while more common really only infects the immunocompromised. Its cousin, Cryptococcus gatti, frequently infects immunocompetent, healthy individuals and is an important emerging pathogen. Weirdly enough, C. gatti infections all seem to come from trees. The fungal pathogen has dispersed all over the world surprisingly, scientists believe, due to continental drift. In the past few years, there’s been large outbreaks of the less common Cryptococcus gatti in the Pacific Northwest/Canada and South Africa.

Sex has something to do with it

Cryptococcus needs a lot of sugar to reproduce, particularly inositol which is all over the human brain and spinal cord which is probably why Cryptococcus is known for causing meningitis. Cryptococcus (depending on the species) has between about 6 to 12 genes involved in inositol transport while most fungi have only about two. While inositol is needed for reproduction and results in higher virulence, which mating type the fungus is may also play a large role in pathogenicity.

In yeasts there are two mating types, MATa and MATα. MATα strains are able to produce an extensive hyphen phase in the haploid state called monokaryotic fruiting. All the clonal C. gattii VGII (or C. deuterogatti) that have been infecting people have notably been from the same mating type: Matα. MATα strains are capable of same-sex mating which could be the origin for the outbreak around Vancouver. In C. neoformans, MATα strains which have their own genes specific to that mating type, are associated with higher virulence (but there’s no evidence of this with C. gattii). 

From Springer 2010, isolations of C. gatti from human clinical, veterinary, environmental sources. Underestimation of actual prevalence.
Hypermutator

Researchers found that several isolated C. deuterogatti strains contained mutations in MSH2, one of the genes involved in mismatch repair (genes that fix DNA replication errors). The human homologous MSH2 gene has the same effect where humans with mutations in MSH2 can have Lynch Syndrome, where they’re prone to various cancers.

The mutations in MSH2 in fungi were tested and resulted in their genomes mutating at a faster pace but growing at the same rate as wildtype Cryptococcus strains. When exposed to stressful conditions however, such as antifungals like rapamycin and FK506, the mutant strains rapidly took over as they could quickly acquire resistance to drugs. In the process of acquiring drug resistance the strains actually decreased in virulence and were significantly weakened compared to the outbreak strains.

So the hypermutators are much more fit in stressful conditions and can rapidly outcompete the wildtype strains, however they make a less fit pathogen without significant selective pressures present. This brings up an interesting question in host-pathogen evolution—what kind of genome makes for an ideal pathogen? Fungi in general are lousy pathogens, especially in people. Certainly white-nose syndrome in bats and chytrid fungus in amphibians have devastated populations but there aren’t a whole lot of “in between” fungal pathogens. Having a lower mutation rate than bacteria or viruses contributes to their easiness to treat. While in bacteria a hypermutator strain rapidly evolving drug resistance usually sounds like a bad thing, in this case hypermutator strains taking over would at least lower the virulence and fitness of the fungus as a pathogen.

It’s even worse in men

As is the case with many pathogens, there is an increased incidence of the disease in men and when it does appear mortality rates are significantly higher in men. C. neoformans isolates from females had a slower growth rate and released more capsular glucoronoxylomannan (GXM), (a virulence factor and immunosuppressant). Testosterone was associated with higher levels of GXM release while 17-β estradiol was associated with lower levels and slower growth rate. Furthermore, macrophages from females were better at fighting off C. neoformans than macrophages from males which were more damaged from the infection. This may explain why infections are more common in men.

While Cryptococcus neoformans has been a common issue for a while, Cryptococcus gatti has only recently emerging as a prominent pathogen, which scientists believe could be the result of climate change. These Cryptococcus infections infect healthy people and are fatal if left untreated.

Sources:

  1. The second STE12 homologue of Cryptococcus neoformans is MATa-specific and plays an important role in virulence. Y. Chang-L. Penoyer-K. Kwon-Chung – Proceedings of the National Academy of Sciences – 2001
  2. The Role of Host Gender in the Pathogenesis of Cryptococcus neoformans Infections. Erin Mcclelland-Letizia Hobbs-Johanna Rivera-Arturo Casadevall-Wayne Potts-Jennifer Smith-Jeramia Ory – PLoS ONE – 2013
  3. Highly Recombinant VGII Cryptococcus gattii Population Develops Clonal Outbreak Clusters through both Sexual Macroevolution and Asexual Microevolution. R. Billmyre-D. Croll-W. Li-P. Mieczkowski-D. Carter-C. Cuomo-J. Kronstad-J. Heitman – mBio – 2014

 

Salty Antarctic Lake Provides Clue to Viral Evolution

In the Vestfold Hills region of Antarctica, a team of researchers have discovered a unique method of genetic exchange happening in a deep lake—so salty it remains unfrozen down to minus 20 degrees. In it, lives members of Haloarchaea, a class of Euryarchaeota that thrive in high salt conditions. These extremophiles are able to thrive by having extremely high rates of horizontal gene transfer, swapping genes with other genera even, to more rapidly evolve. Despite the shockingly high rate of gene sharing, the lake still maintains distinct species with no one dominant species winning out. The lake is incredibly cold, providing very little energy, so the archaea in this area only produce about six generations a year because they have to metabolize and reproduce so slowly.

While people have been aware of the archaeal extremophiles there for some time, it was only recently that anyone noticed their plasmids.

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Haloarchaea colonies

The team discovered plasmids in one strain that were not behaving like regular plasmids. Plasmids are pieces of DNA independent of the host genome which replicate independently and are kept around usually only if they have some beneficial gene to the organism (e.g. antibiotic resistance). What differentiates a plasmid from a virus is the method of transmitting genetic information.

For background, a plasmid typically relies cell-cell contact or “conjugation”–when a sex pilus of a bacterial cell containing the plasmid will combine with another bacterium to transfer the plasmid, or is picked up as naked DNA. Other horizontal gene transfer methods would be transduction (genetic transfer via a virus) or transformation (the collecting of naked DNA from the environment).

Viruses travel, encased in a protein coat, and rely on lysing or budding off their host cell and finding new cells to attach to and enter (or, in the case of retroviruses and lysogens, integrating in the host DNA and being activated later in any of that host’s progeny).

But these plasmids are acting like viruses. The pR1SE (plasmid) encodes proteins that go into the host membrane and allow the membrane to bud into vesicles containing plasmid DNA. These vesicles could then infect more of the archaeal species, plasmid-less members. The plasmids could then replicate themselves in their new host cell.

Besides this just being cool because it’s a way we’ve never seen before of transmitting DNA (and because anything involving archaea in extreme environments is cool), it also may be representative of some early stage of viral evolution.

Viral evolution is a hugely debated topic with no one really agreeing on how viruses came about, whether they evolved many times or once then diversified, which came first in the evolution of life etc., but this does seem to support the idea that at least some groups of viruses may have started as plasmids.

Some people think the obvious answer to viral evolution is gene reduction (a lot of giant virus fans), others think it’s gene addition (what this supports). I think the answer is gradually proving to be both—that viruses have evolved independently many times throughout history, and you could probably find at least a few examples to best support each major theory.

Source:

  1. Susanne Erdmann, Bernhard Tschitschko, Ling Zhong, Mark J. Raftery, Ricardo Cavicchioli. A plasmid from an Antarctic haloarchaeon uses specialized membrane vesicles to disseminate and infect plasmid-free cells. Nature Microbiology, 2017

2. High level of intergenera gene exchange shapes the evolution of haloarchaea in an isolated Antarctic lake. Ricardo Cavicchioli et al. PNAS. 2013

Phytoestrogens aid flightless parrot- Soy will still not give men breasts

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Phytoestrogens are simply plant-derived xenoestrogens (mimics of estrogen). They’re abundant in legumes (soy, notably), but also present in many other plants. Despite their presence in certain plants being touted as scary, I’d say they’re pretty misunderstood.

Phytoestrogens help breeding success of kākāpō, the flightless nocturnal parrot

The darling flightless parrot of New Zealand has a struggling population partly because it only breeds once every few years. They seem to only breed during mast years (when plants produce a ton of edible fruit/seeds) and seek out fruit from the native rimu tree, which suggested the birds breeding success may rely on the presence of phytoestrogens found in the native plants. The hypothesis is that kākāpō don’t produce enough estrogen to make a fertile egg but the phytoestrogens act as supplements.qjjo1oz4sopgxokvcjww.jpg

The scientist conducting this study tested the native plants for estrogenic content and found high levels of phytoestrogens. They looked at the ligand binding region of the progesterone receptor, the androgen receptor, the estrogen receptor 1, and estrogen receptor 2 in four native parrot species, and non-native parrots and compared them with chicken receptors. They found that in most receptors there was more then 90% homology except in the estrogen 1 receptor. Parrot estrogen receptors are actually genetically different, containing an extra 8 amino acids in the hormone binding region, which changes the binding strength to estrogen.

Soy probably pretty good for you

Screen Shot 2017-07-30 at 11.12.05 AM.pngActive compounds of soy include isoflavones- daidzein, genistein, and glycitein. They act as phytoestrogens, a word which seems to frighten some people. A popular belief amongst anti-soy people is that men who ingest too much soy are going to re-enter puberty and turn into estrogen-filled feminized men (heaven forbid).

But this is not really what’s going on.

Phytoestrogens are structurally similar to estradiol so they have the ability to cause either estrogenic or antiestrogenic effects by blocking estrogen receptors. Phytoestrogens have weak estrogen activity in your body, so they may also bind weakly to estrogen receptors. They don’t displace estrogen, they supplement it.

Plants like soy have evolved phytoestrogens to protect from harmful microbes and to help form nitrogen-fixing root nodules.

So the expectation is that they act like antiestrogens in high estrogen concentration environments, and act like estrogen in low estrogen environments.

There are actually a lot of different estrogen receptors in the human body so the same chemical could be and agonist on one estrogen receptor type and an antagonist on another. So the phytoestrogens in plants could trigger an increase or decrease in endogenous estrogen through feedback loops.

It is almost certainly a serious oversimplification to say phytoestrogens are estrogen mimetics. Lots of compounds have partial agonist activity, meaning that at one concentration they are agonists and at a different concentration they are antagonists. It is possible they could affect the different receptors differently.

Reasons to even get excited over Phytoestrogens

Certain hormonal cancer (uterine, prostate, breast etc.) risks could possibly be lowered with phytoestrogen consumption. If they do actually compete with and block estrogen (an antagonist) at estrogen receptors in the breasts, cervix, or uterus, or if they depress estrogen production, they could tend to inhibit estrogen dependent tumors.

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Phytoestrogens may even provide some sort of benefit to women undergoing menopause and experiencing hot flashes, and post-menopausal women at risk for developing osteoporosis and issues in cognitive function which can sometimes be experienced due to dramatic hormone changes.

If phytoestrogens are agonists at estrogen receptors on osteoblasts and osteoclasts they will help reduce osteoporosis. These estrogen receptors are quite different from the receptors on breast tissue.

From a meta analysis on the effects of isoflavones on bone mineral density in menopausal women: “Isoflavone intervention significantly attenuates bone loss of the spine in menopausal women. These favorable effects become more significant when more than 90 mg/day of isoflavones are consumed. And soy isoflavone consumption for 6 months can be enough to exert beneficial effects on bone in menopausal women.”

Soy phytoestrogens are associated with much less negative effects than synthetic endocrine disruptors. And while results of most of these soy studies are dubious—varying with age, level of consumption, and the composition of the individual’s intestinal microflora, soy studies are just as well supported as pretty much any other “eat/drink more of (vegetable, tea, “superfood” etc) and (some health benefit) happens” claims.

While most people tend to be skeptical of simply ingesting a plant and deriving benefits (especially when they are comparing the plant to the highly powerful and concentrated drugs we’ve developed), underestimating and understudying plants has brought about, and continues to bring about, death and illness to plenty of people. So maybe it provides small benefits, but at the very least research does seem to have debunked the myth that soy is dangerous for people.

Constituents of soy if you’re still worried

 

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  • Protein
  • Oil- large amounts of polyunsaturated fatty acids (i.e. linoleic acid (omega 3))
  • Carbohydrates
  • Vitamins and minerals- K, P, Ca, Mg, Fe, B-Vitamins, antioxidants
  • Isoflavones
  • Phytosterols– Disogenin –sterol- is converted to progesterone in body
  • Phospholipids
  • Saponins*- being looked at especially
  • Ferritins- Soybean is a good source of iron
  • Phytic acid
  • Glyceollins- antiestrogen activity
  • Lunasin- peptide

Sources:

  1. Unique oestrogen receptor ligand-binding domain sequence of native parrots: a possible link between phytoestrogens and breeding success. Catherine E. J. Davis A , Adrian H. Bibby A , Kevin M. Buckley A , Kenneth P. McNatty A and Janet L. Pitman. 11 July 2017.
  2. 2014 Jul 7. Do soy isoflavones improve cognitive function in postmenopausal women? A meta-analysis. Cheng PF1, Chen JJ, Zhou XY, Ren YF, Huang W, Zhou JJ, Xie P.
  3. 2003 Jan-Feb. Effects of soy and other natural products on LDL:HDL ratio and other lipid parameters: a literature review. Hermansen K1, Dinesen B, Hoie LH, Morgenstern E, Gruenwald J. .
  4. Soy intake and risk of endocrine-related gynaecological cancer: a meta-analysis. Myung SK, Ju W, Choi HJ, Kim SC; Korean Meta-Analysis (KORMA) Study Group.
  5. Soy intake and cancer risk: a review of the in vitro and in vivo data. Messina MJ1, Persky V, Setchell KD, Barnes S.
  6. 2014 May 28. Effects of isoflavones and amino acid therapies for hot flashes and co-occurring symptoms during the menopausal transition and early postmenopause: A systematic review. Thomas Ismail, Taylor-Swanson Cray, Schnall, Mitchell, Woods
  7. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Hamilton-Reeves JM1, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ.
  8. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomized controlled trials. Ma DF1, Qin LQ, Wang PY, Katoh R.
  9. Soy isoflavones for osteoporosis: an evidence-based approach. Taku K1, Melby MK, Nishi N, Omori T, Kurzer MS.

Platypus venom- weird and unique, as expected

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“Do you think God gets stoned? I think so — look at the platypus.”

-Robin Williams

Venom isn’t very special in the animal kingdom, but our anthropocentric mindsets tend to focus more on large mammals than anything else, so to us it seems pretty mystical. Only a dozen or so mammals deliver venom, almost all of which deliver it via a bite for defense or predation. The platypus is unique in that it is so far the only animal known to use venom for a purpose other than defense or predation.

Only the male platypus has venom. And the male platypus only seems to have potent venom seasonally. The season when they have a lot of venom is unsurprisingly mating season, as the males actually use their venom, injected via venomous spurs on their hind legs, for intraspecific competition with other platypus males to keep territories and mates. While technically the echidna has venom, it can’t erect it’s spurs, and simply excretes a milky secretion.

platypus-spur-png.pngTheir venom, though nonlethal, causes excruciating pain for hours or days and is essentially nonresponsive to morphine. Only nerve-blocking agents (or antivenom) can provide relief.

A 2010 study found 83 peptides in platypus venom, many of which resemble venom genes from snakes, sea stars, and spiders. The platypus and reptiles have independently co-opted the same genes for venom usage making the platypus venom a cool example of molecular convergent evolution.

And just so the monotremes can continue to follow their pattern of general nonconformity and being surprisingly different from each other, the echidna venom gland transcriptome looks very different from the platypus one. You can read this post on their weird sex chromosomes for more.

The venom induces Ca2+ influx in cells, which results in neurotransmitter release. Defensin-Like peptides (defensins being immune proteins that usually defend the host from microbes), C-type natriuretic peptides (OvCNPs), nerve growth factor (OvNGF), and hyaluronidase have also been found. These peptides cause muscle relaxation, inflammation by promoting histamine release, and form ion channels in the lipid membranes of cells. The venom also contains a D-amino acid (as opposed to just all L-amino acids, which is the isomer previously thought to be the only conformation manufactured by cells).

First venomous animals were mammals
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Artist interpretation of Euchambersia mirabilis

The platypus having venom and laying eggs isn’t even that weird, as it seems to be that that was the norm for the ancestors of mammals. Euchambersia mirabilis, a therocephalian therapsid from the end of the Permian (~255 mya), which were some of the “almost-mammals” (the term “mammal-like reptile” is horribly outdated and silly but for some reason people still use it), was determined to have venom glands. Venom glands which appeared way before snakes and lizards evolved them, and actually millions of years before any snakes even existed.

bk9781849736633-00001-f1_hi-resSo while venom in mammals is very rare now, it may actually be an ancestral characteristic. Venom relatively expensive to have as it requires some method of injection into another animal, a gland, and then the making of proteins. It’s also suspected to be expensive because the loss of venom in animals that are no longer under pressure to produce any, is very common. Venom has a weak phylogenetic signal—similar types of venom are not necessarily found near each other on a phylogenetic tree, so genetically it seems not very “difficult” for various venoms to arise.

Monotreme venom as diabetes treatment?

The hormone, glucagon-like peptide-1 (GLP-1), is secreted in the gut, stimulating the release of insulin to lower blood glucose. But GLP-1 typically degrades within minutes in humans.

People with type 2 diabetes can’t maintain a normal blood sugar balance, but maybe they could if they had a less rapidly degrading GLP-1.

However in the platypus, there’s conflicting functions of the GLP-1. Not only is it a regulator of blood glucose in the gut, it is also in their venom. This conflict between the two different functions has resulted in the evolution of a dramatically changed GLP-1 system. GLP-1 in monotremes is resistant to the rapid degradation that occurs in other animals, and degrades by a completely different mechanism.

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GLP-1 and diabetes relationship

The function of GLP-1 in the venom seems to have resulted in the evolution of a stable form of GLP-1 in monotremes. Stable GLP-1 molecules can potentially be used as a type 2 diabetes treatment.

Both platypus and echidnas have evolved the same long-lasting form of the hormone GLP-1 despite echidnas not having spurs.

 

Sources:

  1. Kita, Masaki, David Stc. Black, Osamu Ohno, Kaoru Yamada, Hideo Kigoshi, and Daisuke Uemura. “Duck-Billed Platypus Venom Peptides Induce Ca2 Influx in Neuroblastoma Cells.” Journal of the American Chemical Society50 (2009)
  2. Enkhjargal Tsend-Ayush, Chuan He, Mark A. Myers, Sof Andrikopoulos, Nicole Wong, Patrick M. Sexton, Denise Wootten, Briony E. Forbes, Frank Grutzner. Monotreme glucagon-like peptide-1 in venom and gut: one gene – two very different functions. Scientific Reports, 2016
  3. Julien Benoit, Luke A. Norton, Paul R. Manger, Bruce S. Rubidge. Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using μCT-scanning techniques. PLOS ONE, 2017

Cholera- it’s all about the phage

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If, like me, you’ve been reading about the unusually horrific outbreak of cholera in Yemen, you may be wondering how it got so bad. While an understanding of ecology is central to fighting any disease, if feels especially important when discussing cholera, as the current Yemen outbreak is being almost entirely blamed on war resulting in collapsing infrastructure resulting in millions of people losing access to clean drinking water. On top of that, the malnutrition of many children in the area results in them being more susceptible to Vibrio infection.

To add to that, access to rehydration therapy (the common treatment for cholera when intravenous fluids and antibiotics aren’t an option) is low, and the vaccine campaign has been dropped with the justification being that the limited amounts of vaccine would not be as effective in Yemen as they would in areas where less people are infected.

The varieties of Vibrio

The most important vibrio species to human disease are Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio cholerae. Vibrio species have flagella and pili which are important virulence factors–notably the toxin co-regulated pilus. The cell walls of Vibrio contain lipopolysaccharides consisting of lipid A (endotoxin), core polysaccharide, and an O polysaccharide side chain. Vibrio can then be divided into serogroups based on this O polysaccharide (200 serogroups in V. cholerae’s case).

V. cholerae O1 and V. cholerae O139 both produce cholera toxin (which causes a rise in cAMP resulting in the cell losing nutrients, which is why you not only need tons of water, but also need to replenish lost electrolytes). These are the serogroups associated with cholera epidemics. Many strains of V. cholerae do not have this toxin and do not cause epidemics though they may still cause illness.

The O1 serogroup is further subdivided into three serotypes: Inaba, Ogawa, and Hikojima. There are then two “biotypes” of V. cholera O1: Classical and El Tor. These biotypes can be further subdivided but let’s just stop here.

The cholera that were famous for killing lots of people in the 1800s were all of the Classical type. The cholera that is responsible for today’s pandemic is of the El Tor biotype.

The CTXφ phage

V. cholerae secretes cholera toxin. This is the toxin that causes the “rice-water” stool (not diarrhea really, as it’s just mucus and water), resulting in dehydration of the host. Colonization of the small intestine required the toxin co-regulated pilus (coded by the vibrio pathogenicity island).PMC3282888_TOMICROJ-6-14_F2.png

The genes for cholera toxin are not in the Vibrio genome unless the bacteria has been infected by a CTXphi (CTXφ) filamentous phage, which inserts it’s genome into the V. cholerae genome. The CTXφ can transmit cholera toxin genes from one V. cholerae sstrain to another (via horizontal gene transfer).

Infectious CTXφ particles are produced when V. cholerae infects humans. Phages are then secreted from the infected bacteria without lysing the cell.

Seasonal epidemics inversely correlated with environmental cholera phage presence

Cholera seasons usually make sense as they tend to coincide with monsoon season. But perhaps less obvious (or totally obvious if you’re into viruses) cholera phages have a very dramatic influence on seasonality.

The presence of viruses infecting V. cholerae O1 or O139 inversely correlates with the occurrence of viable V. cholerae in the environment and the number of cholera cases. Both epidemic and nonepidemic serogroups have been shown to sometimes carry lysogenic phages which reproduce and kill epidemic strains. Lysogenic phages integrate into the genome so it replicates with every reproduction of the bacteria

One common O1 phage can use several V. cholerae non-O1/non-O139 strains as alternative hosts.

Having alternative hosts present combined with the lysogenic V. cholerae strains can result in a cholera phage “bloom,” thus lowering the transmission of phage-sensitive, more virulent cholerae strains.

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Concentration of lytic vibriophages in the aquatic environment of Dhaka, Bangladesh, and the estimated number of cholera cases. From Faruque et al 2004.
Phage and Vibrio waves controlling epidemics

 Cholera outbreaks occur in waves with different serogroups dominating at different times.

The absence of one phage specific for one cholerae type provides an opportunity for that serogroup to begin the seasonal epidemic. However, the phages for that serotype will eventually amplify in the environment and attack this serogroup, ending that epidemic.

A different cholerae serogroup would then be resistant to that first phage or carry it as a prophage in the genome—so it isn’t killing that bacteria. A second epidemic wave from the new dominating serogroup can now occur until phages specific to this new serotype bloom, thus ending the epidemic.

Some serotypes will be resistant to all the phages that were killing the virulent phages in the environment, and these serotypes will occupy the interepidemic periods. These strains usually lack typical virulence factors that would make them particularly good pathogens, but are instead more environmentally adapted than the other more virulent strains.

These resistant serotypes may ALSO harbor prophages—phages integrated in the genome—which kill virulent serogroups and may pick up virulence factors via horizontal gene transfer.
If this happens, new serotypes that were previously not very virulent may emerge as the new epidemic serotype.

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Vibriophage
Self-limiting seasonal epidemic also probably caused by phage

A naturally occurring lytic phage, JSF4 (lytic meaning it simply lyses the cell), infects and kills Vibrio that are sensitive to it.

In a study from 2005, it was shown that the peak of cholera season was preceded by a peak in V. cholerae presence which was then followed with a peak in JSF4 phage presence as the epidemic ended. JSF4 phages would then also be excreted in the diarrhea of sick cholera patients. So the patients at the end of the epidemic end up ingesting both a lot of V. cholerae as well as JSF4 phage which kills the bacteria. The increase of phage results in the decrease of V. cholerae and the epidemic ends.

This is likely why outbreaks are self-limiting.

V. cholerae O139 spread by turtles

Screen Shot 2017-07-14 at 12.56.28 AM.pngWhile O1 causes the majority of outbreaks, O139 is confined to Southeast Asia. Recently however, it’s been discovered that soft-shelled turtles in China are big carriers of O139. While a lot of aquatic animals spread cholera, the soft-shelled turtles have been definitively linked to human disease and make excellent hosts as they are unaffected by the bacteria which clings to many of their surfaces and intestines. These turtles are then consumed by people, to spread more cholera to new unsuspecting hosts. If turtles being cute wasn’t a good enough reason to stop eating them, maybe this is?

Sources:

Jiazheng Wang, Meiying Yan, He Gao, Xin Lu, Biao Kan. Colonization of Vibrio cholerae on the Soft-shelled Turtle. Applied and Environmental Microbiology, 2017

Faruque, S. M., I. B. Naser, M. J. Islam, A. S. G. Faruque, A. N. Ghosh, G. B. Nair, D. A. Sack, and J. J. Mekalanos. “Seasonal epidemics of cholera inversely correlate with the prevalence of environmental cholera phages.” Proceedings of the National Academy of Sciences 102.5 (2005)

Faruque, S. M., M. J. Islam, Q. S. Ahmad, A. S. G. Faruque, D. A. Sack, G. B. Nair, and J. J. Mekalanos. “Self-limiting nature of seasonal cholera epidemics: Role of host-mediated amplification of phage.” Proceedings of the National Academy of Sciences 102.17 (2005)

Murray, Patrick R., Ken S. Rosenthal, and Michael A. Pfaller. Medical microbiology. Philadelphia, PA: Mosby/Elsevier, 2016.

A microbe manipulating sex- and how it can fight Zika

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A fan favorite, and probably the most successful genus on the planet–at least on land—the bacteria, Wolbachia infects an estimate between 40 to 65% of all arthropod and nematode species. This microbe is constantly drifting across the line between mutualist and parasite to it’s host. Some hosts are unable to survive and reproduce without a Wolbachia infection whilst others are killed by it.

Wolbachia as a mutualist

Plenty of species have become reliant on this microbe. The caterpillar of the spotted tentiform leaf miner uses Wolbachia to create green islands on yellowing leaves which remain fresh for munching on.

It provides a benefit to certain nematode worms, such as Brugia malayi and Wuchereria bancrofti which cause elephantiasis, and which cannot survive without a Wolbachia infection. Image1.jpg

Some Wolbachia bacteria provide metabolic advantages to their hosts such as in bed bugs who use it to synthesize B-vitamins that are absent in their blood meals. Wolbachia can even mediate iron metabolism in Drosophila.

But most exciting given the recent explosion of flavivirus infections (Zika traveling farther and farther north every summer for example), Wolbachia provides flies with resistance to many RNA viruses.

Wolbachia as a sex-determinator

In leafhoppers, Zyginidia pullula, females have two X chromosomes while males have only one X chromosome, yet when infected with Wolbachia, the X0 genetic males appeared to be female.

Some females of the Japanese butterfly, Eurema mandarin have a sex chromosome system where the males are (ZZ) and the females are (ZWEurema_blanda_on_flower_by_kadavoor.JPG). This incongruence between chromosomal and phenotypic sex can be explained by feminization of genetic males induced by Wolbachia. Two strains of WolbachiawCI and wFem, have been found in E. mandarina and the females having male chromosomes (ZZ) are consistently infected with both wCI and wFem. However females with only wCI are true females (ZW). Despite having male chromosomes, ZZ females are physically female and fully fertile.

A similar thing happens in woodlice (pillbugs? Roly-polys?), where all the ZZ males infected with Wolbachia develop as female. The W chromosome is sometimes lost entirely in these populations and sex is entirely determined by presence or absence of Wolbachia.

Who needs males?

Wolbachia has evolved into an intracellular parasite, and while it can infect many different organs, it is most famous for infecting the testes and the ovaries. Wolbachia are too large for sperm, but fit nicely into mature eggs so the infection is inherited maternally through the eggs.

So now the evolutionary dilemma that keeps Wolbachia on the balance between parasite and mutualist is, if males are an evolutionary dead-end, how does this intracellular parasite that needs its host to survive and reproduce, and it’s host species to continue thriving, evolve to both spread throughout populations but not allow evolutionary cheater strains to ruin everything? Wolbachia has developed numerous ways of targeting males to help itself spread such as:

  • Male killing- infected male larvae die, so more infected females are born
  • Feminization- where infected males develop as females or infertile pseudofemales
  • Parthenogenesis- when females reproduce without males
  • Cytoplasmic incompatibility (CI)- when Wolbachia-infected males can’t successfully reproduce with uninfected females or females infected with another Wolbachia strain.CI-causing Wolbachia interferes with the chromosomes during mitosis so they no longer divide in sync.
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Warren et al, 2008, Nature Reviews, Microbiology.

Mosquitos carrying Wolbachia have a higher reproductive success when present in a population with mosquitoes not carrying Wolbachia. When a male mosquito carrying Wolbachia tries to mate with a female who is not carrying Wolbachia, the female’s eggs won’t hatch. However females with Wolbachia do not have this issue and produce perfectly healthy offspring which are all also carriers of Wolbachia. So you can imagine how the Wolbachia is able to sweep through a mosquito population. The female Wolbachia carriers have a much higher fitness than the non-carriers.

Scientists have taken advantage of this evolutionary strategy in fighting mosquito-transmitted viruses such as Dengue and Zika. The Aedes aegypti, a black-and-white striped species of mosquito infects people with Dengue virus which has no vaccine or real treatment and causes pains, fevers, rashes, and headaches. A plan (credited to evolution/ecology biologist, Scott O’Neill) to release Wolbachia infected mosquitos into the wild to lower dengue spread is becoming more and more popular. Wolbachia stops Aedes mosquitoes from carrying degue virus. Wolbachia carrying females have a selective advantage and should sweep through the population.

Wolbachia to rescue us from Dengue (and others?)- The original plan

An unusually virulent strain of Wolbachia the ‘popcorn’ strain, essentially halves the mosquito lifespand (it’s pretty gruesome, the bacteria essentially reproduce like crazy in the brains, eyes, and muscles filling up neurons). Dengue takes a long time to be able to reproduce and make it to the salivary glands so it can be transmitted, so only older mosquitos can transmit it.

Unfortunately Aedes (and Anopheles which transmist malaria) are not natural hosts of Wolbachia infections, so Scott O’Neill carefully developed a new symbiosis by injecting eggs. This took forever to work, until one lucky grad student was able to make it a success. Finally, an egg was stably infected and a line of Wolbachia-carrying Aedes was created. But after all that work, the strain was too virulent and the females did NOT have a selective advantage and actually had lower numbers of eggs with lower viabilities (honestly, they should have seen this coming really).

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But none of that even ended up mattering because some other scientists figured out that Wolbachia stops dengue virus from replicating. Simply the presence of a nonvirulent strain of Wolbachia in the population was enough to stop the spread of dengue. So the team switched to a less virulent strain, wMel, and successfully started a line of wMel carrying Aedes mosquitos.

wMelwAlbB update and some critiques of the data

As amazing as Wolbachia is, it will take at least a few years to get significant results and many years to eliminate any particular mosquito-transmitted pathogen with this method. Some papers say Wolbachia was able to make not only dengue, but also Plasmodium and other flaviviruses less able to replicate, but others said the opposite. It would be kind of important to figure out how the Wolbachia is inhibiting the virus because no one seems to agree if it’s general or specific. But now with significant selective pressures on all these diseases I’d suspect it’d be easy to switch host vectors because mosquitos bite hosts, geographically spreading infections very far (potentially) exposing the virus to a wide variety of new potential hosts vector species.

In a 2016 paper looking at the progress of the wMel strains, they collected Ae. Aegypti after one year and it continued to have low levels of dengue (which implies it was passed around through tons of mosquitos and not a whole lot of apparent evolution has taken place in terms of resistance) but what if the some of the dengue viruses switched species?

They could try passing the dengue through many mosquitos perhaps in a mixed host population. Or basically just try to provide the virus with as many opportunities as you can for it to evolve in the hopes that you may understand potential mechanisms the different diseases may have to get around this one Wolbachia infected species of vectors.

While the paper shows wMelwAlbB, the superinfection, that strain of Wolbachia actually doesn’t appear to inhibit DENV very much. But later in the paper, it’s very dramatically different so it’s difficult to say if the data is actually supporting that the superinfection sweep will work.

As far as how they ensure the right strain is dominating all the time given selective pressures towards different mosquito sex alteration methods, that remains unanswered. Combined male-killing CI strains readily become extinct following invasion so CI strains are more selected for but sex-ratio distortion decreases male infection and therefore reduces the occurrence of CI meaning that you might expect selective pressure for evolution from CI to CI/sex ratio distortion to sex ratio distortion only.

Wolbachia’s potential

Using Wolbachia’s ability to stop mosquitos from carrying Zika, Dengue, Chikungunya, Plasmodium—the parasite responsible for malaria, may mean the eventual elimination of these diseases in humans.

It may even be used to someday stop nematode worms from causing blindness, disability, elephantitis etc in many millions of people every year.

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Particularly inspiring about this story is how ecologists and evolutionary biologists ended up being the ones to figure out a way to eliminate viral infections. Yet more evidence that pre-med students or medical researcher hopefuls shouldn’t blow off biodiversity/ecology/evolution classes in college.

Sources

Kageyama, Daisuke, Satoko Narita, and Masaya Watanabe. “Insect Sex Determination Manipulated by Their Endosymbionts: Incidences, Mechanisms and Implications.” Insects 3.4 (2012): 161-99.

I contain multitudes: the microbes within us and a grander view of life. Ed Yong, 2016

Bacterium offers way to control dengue fever. Natasha Gilbert – Nature – 2011.

Establishment of a Wolbachia Superinfection in Aedes aegypti Mosquitoes as a Potential Approach for Future Resistance Management. D. Joubert-Thomas Walker-Lauren Carrington-Jyotika Bruyne-Duong Kien-Nhat Hoang-Nguyen Chau-Iñaki Iturbe-Ormaetxe-Cameron Simmons-Scott O’Neill – PLOS Pathogens – 2016