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[QUOTE]Originally posted by DD'eDeN: [QB] Placental mammals: subterranean origins? Modulation of DNA Repair Systems in Blind Cavefish during Evolution in Constant Darkness Haiyu Zhao, Giuseppe Di Mauro, Sebastian Lungu-Mitea, Daniela Vallone, Cristiano Bertolucci & Nicholas Foulkes 2018 Curr.Biol. doi https://doi.org/10.1016/j.cub.2018.08.039 €During evolution, the cavefish P.andruzzii has lost photo-reactivation DNA repair. €Only P.andruzzii & placental mammals are known to lack photo-reactivation. €The D-box enhancer coordinates DNA repair in response to ROS, UV & visible light. €Loss of D-box function in P.andruzzii underlies the lack of photo.reactivation. How does the environment shape the function & evolution of DNA repair systems? In a comparative study using zebrafish & the Somalian blind cavefish Phreatichthys andruzzii, we reveal: - during evolution for millions of years in continuous darkness, photo-reactivation DNA repair function has been lost in P.andruzzii, - this loss results in part from loss-of-function mutations in pivotal DNA-repair genes: C-terminal truncations in P.andruzzii DASH & 6-4 photolyase render these proteins predominantly cytoplasmic, with consequent loss in their functionality, - a general absence of light-, UV- & ROS-induced expression of P.andruzzii DNA-repair genes. This results from a loss of function of the D-box enhancer element, which coordinates & enhances DNA-repair in response to sunlight. Apart from Placentalia, P.andruzzii iq the only species described that lacks the highly evolutionary conserved photo-reactivation function. In the DNA repair systems of P.andruzzii, we may be witnessing the first stages in a process that previously occurred in the ancestors of placental mammals during the Mesozoic era. _____ http://blogs.discovermagazine.com/deadthings/2018/10/11/cavefish-mammal-evo lution/#.W8f-pqZLiUl Dead Things Gemma Tarlach 11.10.18 Clues to the earliest days of mammal evolution may lie in the genome of the Somalian blind cavefish, Phreatichthys andruzzii. If you're trying to understand the earliest days of mammal evolution, the genome of a blind cave-fish might not strike you as the most obvious place to hunt for clues. A study out today, however, suggests that's exactly where you can glimpse our distant ‹ and very dark ‹ past. Animals that live exclusively in lightless caves often share a suite of traits (troglomorphisms): - loss of pigmentation, - loss of vision & of the eyes entirely. The genomes of cave-dwellers have other (not always obvious) mutations & changes in genetic expression (genes remain unchanged, but are turned on or off), showing how the animal adapted to its environment over time. Researchers discovered: the Somalian blind cavefish shares a particular genetic quirk with nearly all mammals that led to the loss of a light-dependent DNA repair system. This particular DNA repair system is "highly conserved": once it evolved, very early in the history of life, it was a trait that was retained by virtually every living thing, from plants to animals to even bacteria. But not in the Somalian blind cavefish or placental mammals (every mammal aside from 5 monotreme spp (e.g. platypus) & a couple 100 marsupials: kangaroos, koalas, wombats, Tasmanian devils etc.). Unlike placentals, marsupials retained a crucial DNA repair kit known as photo-reactivation. Lights Out If you're a living thing on Earth exposed to sunlight (whether microbe or megafauna), you get hit with UV radiation, which can damage your DNA: sunlight is the most common & significant factor that leads to mutations & other errors in the genetic code. To promote DNA stability, organisms have DNA repair systems. The most common DNA repair kit is "photo-reactivation", which depends on light, and activates photolyases, that then go to town patching up an organism's genetic blueprint. Photoreactivation is an awesome thing ‹ and it is completely absent in placentals & 1 little cavefish. Placentals evolved a different DNA repair mechanism, not as efficient as photo-reactivation, but +- gets the job done. It's long been a puzzle in the study of mammal evolution over why & how this large diverse group of animals lost photoreactivation, so crucial a trait for most of the rest of life on Earth. Enter the "nocturnal bottleneck" concept. It was originally proposed decades ago, before genome decoding was a thing, to explain physical & behavioral traits common to mammals, e.g. certain thermo-regulation methods & acute hearing. These traits could be explained, went the theory, if the earliest mammals were generally nocturnal (most mammals today still are). And when you look at the ebb & flow of dominant animals over millions of years, it makes sense. There is some disagreement over where to draw the line between true mammals & ancestors that were on the evolutionary course to become true mammals, but mammals go back >160 Ma, possibly >200 Ma. Consider what else was 160-200 Ma: dinosaurs. Not just harmless herbivores, but a lot of swift & bitey predators. The fossil record suggests that the earliest/nearly-mammals were small shrew-like creatures. Burrowing into the ground (another typical mammalian trait) & limiting activity to night-time would have afforded some protection from hungry dinosaurs. Once the dinosaurs (except birds) bit the dust at the end-Cretaceous mass extinction 65 Ma, mammals were able to diversify & evolve to occupy all those emptied ecological niches, incl. daytime activity. Bereft of photo-reactivation for millions & millions of years, these later mammals evolved a different, less-efficient way to repair DNA damage. It's not yet known exactly when placentals developed their unique DNA repair kit. Going To Ground What if, however, the earliest placental mammals weren't just nocturnal? What if they went a step further and, instead of venturing out of their burrows only at night, didn't venture out at all? What if they stayed safe by staying subterranean? Let's hop back to P.andruzzii for a moment. Unlike many other cavefish, which occasionally may be exposed to sunlight or which have been shut off from it for merely 1000s of years, researchers believe the Somalian blind cave-fish has lived in the absence of sunlight for millions of years ‹ long enough, the authors say, to lose the ability to use sunlight to repair DNA damaged by sunlight: did P.andruzzii share a loss of photo-reactivation with placentals, because it shares the same lightless lifestyle as the animals at the very dawn of mammal evolution. It's not a conclusive finding: e.g. there's no evidence that the earliest placentals had the troglomorphisms you'd expect to find in an animal living without light. It's a tantalizing clue, however, that suggests maybe our very distant forebears went through not just a nocturnal bottleneck, but a subterranean one. <https://www.cell.com/current-biology/fulltext/S0960-9822 [/QB][/QUOTE]
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