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Monday, October 21, 2019



Since Mendel's time, we have accepted that the main means of information and transmission of biological information in complex organisms is mainly carried out by encrypted DNA. However, today, thanks to epigenetics we are learning that at least in less complex animals, certain adaptive responses are not necessarily transmitted to the new generations by gene sequences, that is, that some learned behaviors and certain physiological responses can be epigenetically inherited.  Lately, the:A. Weismann barrier (1892, "the traits acquired by the somatic cells of complex organisms -by exposure to the environment- are not transmitted to oocytes and sperm and from there to the next generation"), is losing its rigidity. Epigenetically speaking and according to Oded Rechavi (Tel Aviv University), the information learned by somatic tissues is communicated and incorporated into the germ line, by means of small RNA molecules and / or perhaps by hormone like-peptides, being the nervous system able to promote inheritable adaptive responses. Proposals that have required epigenetic researchers  to ask themselves: a) If learned adaptive behaviors can be passed on to the next generation, that would seem to eliminate the necessity for certain standard evolved changes to the genome. b) why not incorporate these adaptive changes to the genome so that they could be more stable? O. Rechavi, thinks in this regard arguing that, although more studies are lacking, it is real the existence of 2 inheritance mechanisms (RNA-DNA), being the DNA, the most recent. In 1950 R. Alexander Brink, achieved  under different environmental conditions, that corn plants with identical genomes, had different expressions in the form of heritable grains of different colors, inferring the existence of different production mechanisms: chemical modifications of proteins and DNA or  the existence of small RNA molecules that, when transiting to the germ cells, interacted with the DNA, affecting genetic regulation.   In another experiment, the germ cells producing sperm and oocytes of the C. elegans worm were marked with a green fluorescent protein and the neurons with red, proving that the adaptive responses learned by these worms caused changes in the neural system that induced changes in the germ cells, allowing the progeny of worms to exhibit the same adaptive behavior to cope  with  stress. Rechavi said that this was possible due to the emergence of small RNA-RNA transmitter molecules, which performed different functions from the usual peptide production. 10 years ago, at Columbia University, Rechavi showed that C. elegans virus-infected worms could defend themselves by generating small RNAs that neutralized viruses and that their subsequent progeny also produced these protective RNAs, even if they were not exposed to viruses (Cell, 2011), and that the stress could induce the production of small inheritable molecules of RNAs that helped adaptive response. In Cell (June 13,2019), Rechavi, investigated the inheritance of chemotaxis, concluding that, in these cases, the worms inherited siRNA molecules produced in their parents' neurons, adding Peter Sarkies, that the information mediated by the siRNAs could also be transmitted transgenerationally. According to Sarkies, C. elegans worms also have some ability to take double stranded RNA from the environment and use it to silence endogenous genes, inducing adaptive responses. In this regard and according to G. Bosco (Dartmouth College), it is necessary to answer certain questions: a) why does the neural signals reach the germ tissue and change the information contained in the oocyte? b) What need induces the brain to perform these actions in the germ tissue? c) If a worm ingesting an environmental chemist manages to change the epigenome of oocytes and spermatozoa, why can't we make our brain to generate a similar molecule? In a paper (2017/Nature Cell Biology), Burton exposed C. elegans worms to high levels of salt, inducing a state called osmotic stress, against which the worm's brain responded by secreting insulin-like peptides that changed  oocytes, inducing in them epigenetic changes, making the worm's progeny produce more protective glycerol against osmotic stress. For Burton, the hormone-like peptides secreted by worm brains induce epigenetic changes in oocyte-forming cells, further achieving that their progeny solves the problem of high environmental salt levels. A characteristic of the epigenetic inheritance is that it only lasts a few generations and then ceases, denominating it for that reason: adaptive plasticity.

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