Genome complexity: puzzling epigentics (Introduction)

by David Turell , Tuesday, April 19, 2022, 20:23 (731 days ago) @ David Turell

Studies in C. elegans worms raise issues:

https://www.quantamagazine.org/in-worms-inheritance-beyond-genes-can-help-evolution-202...

"As a mostly hermaphroditic species (with a few males thrown in for variety), a C. elegans worm usually self-fertilizes its eggs until its sperm stash is depleted late in life; only then does it produce a pheromone to attract males and stay in the reproductive game. But when environmental conditions become stressful, the worms become sexually attractive much sooner. For them, sex is the equivalent of a Hail Mary pass — a desperate gamble that if their offspring are more genetically diverse, some will fare better under the new, rougher conditions.

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"As a mostly hermaphroditic species (with a few males thrown in for variety), a C. elegans worm usually self-fertilizes its eggs until its sperm stash is depleted late in life; only then does it produce a pheromone to attract males and stay in the reproductive game. But when environmental conditions become stressful, the worms become sexually attractive much sooner. For them, sex is the equivalent of a Hail Mary pass — a desperate gamble that if their offspring are more genetically diverse, some will fare better under the new, rougher conditions.

“'Epigenetics has this double function: They’re inheritance systems, but they’re also response systems. You respond to the environment not by waiting for a mutation but by changing the way you express your genes,” said Eva Jablonka, a geneticist and philosopher of science at Tel Aviv University, who was not involved in the study. (my bold)

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"If DNA sequences are the words in the book of life, then epigenetics is the punctuation, marking the difference between “Let’s eat, Grandma!” and “Let’s eat Grandma!” By telling the cell’s protein translation machinery where to work and how active to be, epigenetic signals can work either in addition to existing genetic signals or as the manifestation of these genetic commands. This layer of regulation provides an interface between the genome (which rarely changes) and the environment (which is always in flux), explained Eric Greer, an epigeneticist at Harvard Medical School.

“'Epigenetics exists at the interface between the environment and the genetics,” he said.

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"In the years since, researchers have found evidence in invertebrates for the RNA-based epigenetic inheritance of pathogen avoidance, response to olfactory cues, longevity and more. Precisely how the gene silencing is inherited is still under study, but somehow either RNAi molecules or the instructions for making them pass from parents to offspring.

“'Only in the last few years have we started being able to really provide some evidence that small RNAs themselves can be the vessel that is actually transferring information from one generation to the next,” said Lamia Wahba, a postdoctoral fellow in the Fire lab at Stanford.

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"Toker and Rechavi first needed to confirm that small RNAs were responsible for the inherited mating effect. This proved to be true: When the researchers prevented short interfering RNAs from being transmitted between generations, the offspring didn’t express the pheromone to attract mates prematurely.

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"Nevertheless, Coleen Murphy of Princeton University, who also studies transgenerational epigenetic inheritance in worms, isn’t yet convinced that the observed effect is relevant to the real world. For 10 generations (roughly a month), Rechavi and Toker had to keep the worms in a narrow temperature range between 25 C and 27 C: hot enough to see the precocious mating effect become heritable but not so hot that it triggered the worms to enter what’s called a “dauer” state of stasis instead of reproducing.

“'I don’t know how to interpret that significance,” she said.

"Toker agrees that designing laboratory experiments to reflect real-world phenomena more accurately is a big issue for the field, and one that scientists have yet to resolve. Showing that this epigenetic inheritance can happen in the lab is only the first step toward understanding its overall significance, he said.

"Another major question that biologists are beginning to face is whether multigenerational epigenetic inheritance happens in vertebrates. Although scientists have found tantalizing clues that this might be happening in mammals, and even in humans, many scientists remain unconvinced.

Comment: This won't satisfy dhw's desires for bacteria's DNA editing ability to be widespread in advanced animals or plants. All we are sure of is epigenetics by methylation or small RNA's offers temporary adaptations within species, nothing more.