Lamarck, Darwin, and a bunch of scared mice

For better or worse, I look a lot like my mother. I have her eyes and we share the same smile. Putting aside our collective fears of one day turning into our parents, the question of how it is that I physically resemble my mother is pretty well settled. In fact, the mechanism behind inheritance of traits from our parents had already become dogmatic by the mid twentieth century. DNA had been discovered, our genetic alphabet revealed.   We learned that these letters code for proteins, or words in this metaphor; and by the time that I was conceived, it was clear that I would be born with a genetic book of instruction, half of the words from each of my parents.

More than a century before the structure of DNA was uncovered, a battle raged about the fundamental questions and nature of evolution. Two competing theories divided the scientific community. At the center were Lamarck and Darwin, both agreed that physical characteristics were not random, but rather heritable through generations. The pressing question was how traits were conferred. Could an animal adapt over the course of its life and give these adaptations to its offspring? For example, if someone living in southern California developed a deep tan in response to prolonged sun exposure, is it possible that his/her offspring would be born with darker skin as a result? Lamarck thought that it was precisely these kinds of pressures from the environment that could lead to physical traits passing from one generation to the next. On the other side of things was Darwin; his proposition was that physical traits of the next generation were predetermined. If adaptations arose over the life of an animal, they will not be passed along. Darwin and his like-minded colleagues didn’t know about DNA, but they argued that there was something intrinsic in the animal responsible for the physical characteristics of one’s offspring. They suggested that there was something deeper than just the physical condition of the parent at the time of conception responsible for heritability.

In the end, Darwin’s theory won out: but maybe that’s not the end of the story. What if we get more from our parents? To stick with my book metaphor; what if we get the letters, but also the notes they wrote in the margins, things that might change the meaning of the ‘book’ itself? Although features such as eye color follow classic genetic patterns, some of our traits and predispositions might be influenced by our parents’ choices and experiences. With mounting data to this effect, most of us have now shifted over to an “its complicated” relationship status with genetic inheritance.

A paper published in 2013 does a beautiful job illustrating exactly how complicated it really is1. In this study, male mice were exposed to acetophenone, a sweet-smelling aromatic ketone, which was paired with a mild shock. Over time the mice became fearful around this smell, freezing in place and startling more readily. The truly remarkable finding was the impact that this experience had on the sons of these mice: When the authors tested the next generation of mice, they seemed to have an implicit negative reaction to the smell. This suggests that adults can pass along information acquired though direct experiences, in this case the information that this new smell is something to be avoided.

It seems that changes in development are responsible for this inherited fear. Sensory neurons, able to detect a particular odor were more abundant in these mice, being borne with an enhanced ability to detect acetophenone. The authors of this paper found that the fearful mice were genetically identical to their non-fearful counterparts. Letter for letter, the DNA was the same between the sons of scared and not-scared mice, but chemical augmentation accessorizing the DNA was different; effectively changing the genetic instructions without directly modifying the words.

Not surprisingly, the more scientist probe this topic, the less cut-and-dry things become. In some cases parents can pass along acquired adaptations or information about environmental conditions to their children. It remains unclear how wide spread this is, but metabolic disorders and odor predispositions are on the running list of confirmed cases1,2,3. Whether it was serendipitous or inspired insight, Lamarck is having another say on the nature of inheritance.

 

  1. Dias, Brian G., and Kerry J. Ressler. “Parental olfactory experience influences behavior and neural structure in subsequent generations.” Nature neuroscience 17.1 (2014): 89-96.
  2. Kaati, Gunnar, Lars O. Bygren, and Soren Edvinsson. “Cardiovascular and diabetes mortality determined by nutrition during parents’ and grandparents’ slow growth period.” European journal of human genetics: EJHG 10.11 (2002): 682.
  3. Rechavi, Oded, Gregory Minevich, and Oliver Hobert. “Transgenerational inheritance of an acquired small RNA-based antiviral response in C. elegans.” Cell 147.6 (2011): 1248-1256.