Biological complexity: inheriting actions and instinct (Introduction)

by David Turell , Wednesday, October 16, 2019, 19:36 (1865 days ago) @ David Turell

Epigenetic research does not really tell us how this happens, especially related to the preceding discussion of insect/larval behavior and trees changing water controls:

https://www.quantamagazine.org/inherited-learning-it-happens-but-how-is-uncertain-20191...

"Most recently, some researchers have found evidence that even some learned behaviors and physiological responses can be epigenetically inherited. None of the new studies fully address exactly how information learned or acquired in the somatic tissues is communicated and incorporated into the germline. But mechanisms centering around small RNA molecules and forms of hormonal communication are actively being investigated.

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"The evolutionary “why” for epigenetic inheritance is also an area of active investigation because it’s paradoxical. 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. On the other hand, if epigenetically transmitted traits are adaptive, why not hardwire them into the genome so that they can be inherited more stably?

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“'The possibility that the nervous system could generate heritable responses was especially intriguing, because the nervous system is a very unique system in its ability to organize information about the environment,” said Oded Rechavi, a neurobiologist at Tel Aviv University who studies inheritance and evolution. “It has [a] unique capability of planning.” (my bold)

"Rechavi is one of the scientists behind a series of studies published in the past few months that point to small noncoding RNAs — RNA molecules that serve functions other than the production of peptides — as key mediators of some epigenetic effects in a simple model animal, the roundworm Caenorhabditis elegans. Different mechanisms, probably including ones as yet unknown, may govern other instances of inherited learning in the worms and in more complex organisms. Indeed, some scientists don’t agree that small noncoding RNAs have a singularly important role in the phenomenon at all. And in general, researchers are cautious in approaching the argument that epigenetics enables the transmission of specific, adaptive traits to new generations.

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"Rechavi investigated the inheritance of a learned behavior: chemotaxis, the ability to orient and move toward food sources. He wanted to know whether small RNAs made specifically in nerve cells could somehow communicate with the germline and generate heritable behavioral responses.

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"Further work determined that the worms’ chemotaxis depended mainly on so-called small interfering RNAs (siRNAs) that muted the effects of a particular gene, saeg-2, in the neurons.

"The surprise came when they looked at the worms’ gonads: More than 1,000 siRNAs had changed in abundance relative to those in worms that lacked rde-4 entirely. And although none of the progeny’s cells carried a working rde-4 gene, the worms could still perform chemotaxis. Somehow, their germ cells still had siRNAs targeting saeg-2. Rechavi and his colleagues concluded that these worms had inherited the siRNAs from their parents — evidence that the production of specific small RNAs in the parental neurons could generate a heritable response that showed up in the worms’ progeny.

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“'What I would suggest is that our brains are our pharmacies,” Bosco said. “Our brains are making chemicals all the time,” such as neuropeptides and other neuromodulatory molecules with diverse functions. Some of those functions impinge directly on processes in other organs, including the reproductive system. “If we can ingest a chemical from our environment that changes the epigenomes of the egg or sperm, why couldn’t our brain make a similar molecule that does the same thing?” he said.

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"One of the outstanding questions in the field is why epigenetic inheritance only lasts for a handful of generations and then stops, said Eric Greer, an epigeneticist at Harvard Medical School and Boston Children’s Hospital who studies the epigenetic inheritance of longevity and fertility in C. elegans. It appears to be a regulated process, in part because the effect persists at the same magnitude from one generation to the next, and then abruptly disappears. Moreover, in a paper published in Cell in 2016, Rechavi and colleagues described dedicated cell machinery and specific genes that control the duration of the epigenetically inherited response. “So it’s an evolved mechanism that likely serves many important functions,” Rechavi said.

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"Greer concurs that there could generally be a cost to deploying an adaptive response permanently. For example, deploying antiviral defenses when pathogens aren’t around is a waste of resources that could be used instead for growth and reproduction."

Comment: We just do not know how instinctual behavior is generated, but it exists. That animals are adaptable is obvious as is the role of the brain. (note my bold).